US 20030086530A1
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`(19) United States
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`(12) Patent Application Publication (10) Pub. No.: US 2003/0086530 A1
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`Otto
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`May 8, 2003
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`(54) METHODS AND APPARATUS FOR
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`PLANNING AND DELIVERING INTENSITY
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`MODULATED RADIATION FIELDS WITH A
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`ROTATING MULTILEAF COLLIMATOR
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`(76)
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`Inventor: Karl Otto, Vancouver (CA)
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`Correspondence Address:
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`OYEN, WIGGS, GREEN & MUTALA
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`480 - THE STATION
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`601 WEST CORDOVA STREET
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`VANCOUVER, BC V6B 1G1 (CA)
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`(21) Appl. No.:
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`10/253,781
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`(22)
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`Filed:
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`Sep. 25, 2002
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`Related U.S. Application Data
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`(60) Provisional application No. 60/324,266, filed on Sep.
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`25, 2001.
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`Publication Classification
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`(51)
`Int. Cl.7 ..................................................... ..A61N 5/10
`(52) U.S.Cl.
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`(57)
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`ABSTRACT
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`A method and system for controlling the spatial distribution
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`of radiation produced by a radiation device having a multi-
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`leaf collimator can generate arbitrary intensity-modulated
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`radiation fields. The methods control both angles and leaf
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`configuration of the multi-leaf collimator for each of mul-
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`tiple sub-fields. The leaf positions, collimator angles, and
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`individual sub-field contributions may be derived by opti-
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`mization techniques.
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`10
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`Elekta Exhibit 1004
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`Elekta Exhibit 1004
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`Patent Application Publication May 8, 2003 Sheet 1 of 6
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`US 2003/0086530 A1
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`FIGURE 1
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`Patent Application Publication May 8, 2003 Sheet 2 of 6
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`FIGURE 2B
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`Patent Application Publication May 8, 2003 Sheet 3 of 6
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`US 2003/0086530 A1
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`FIGURE 3
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`Patent Application Publication May 8, 2003 Sheet 4 of 6
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`US 2003/0086530 A1
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`C)
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`C)
`COV‘
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`®1
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`CollimatorAngle(deg)
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`FIGURE4
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`Patent Application Publication May 8, 2003 Sheet 5 of 6
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`US 2003/0086530 A1
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`Input desired spatial
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`distribution of
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`radiation field, physical
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`constraints,
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`optimization method
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`and termination criteria
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`Select fixed treatment
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`parameters (e.g.d ynamic or
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`static mode, range ofrotation)
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`contributions using the chosen
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`optimization method
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`Evaluate discrepancies
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`between desired and optimized
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`fields
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`the termination
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`criteria been
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`Attained? ‘
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`Has
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`t YES
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`Transfer optimized parameters _f— 109
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`to radiation device
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`FIGURE 5
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`Patent Application Publication May 8, 2003 Sheet 6 of 6
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`120
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`Randomly selectl _eafa nd leaf
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`movement, rotation angle or sub-
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`field contribution and modify
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`126
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`Is the
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`modification within
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`physical and
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`mechanical
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`limits?
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`l YES
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`Calculate the change in the
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`spatial distribution that would —/_
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`result from the modification
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`Calculate difference between the
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`prescribed spatial distribution
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`and calculated distribution of
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`Has
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`the difference
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`decreased or is the location
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`of the modification still within
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`an acceptable
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`range?
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`V YES
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`Acceptt he modification and
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`calculate the termination criteria
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`_f- 130
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`NO
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`NO
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`FIGURE 6
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`US 2003/0086530 A1
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`May 8, 2003
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`METHODS AND APPARATUS FOR PLANNING
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`AND DELIVERING INTENSITY MODULATED
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`RADIATION FIELDS WITH A ROTATING
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`MULTILEAF COLLIMATOR
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`CROSS-REFERENCE TO RELATED
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`APPLICATION
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`[0001] The benefit of the filing date of U.S. provisional
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`patent application No. 60/324,266 filed on Sep. 25, 2001 and
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`entitled INTENSITY MODULATION USING A ROTAT-
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`ING MULTILEAF COLLIMATOR is claimed herein. The
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`text and drawings of U.S. provisional patent application No.
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`60/324,266 are hereby incorporated by reference herein.
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`TECHNICAL FIELD
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`[0002] The invention relates to the field of radiotherapy
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`and in particular to the delivery of radiotherapy by way of
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`a radiotherapy device equipped with a multi-leaf collimator.
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`The invention relates to radiotherapy devices and to systems
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`and methods for controlling radiotherapy devices to deliver
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`radiation treatments.
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`BACKGROUND
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`In radiation therapy a radiotherapy device is used
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`to generate a source of radiation for the treatment of patients.
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`The device may comprise a linear accelerator, for example.
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`A typical radiotherapy device is mounted on a rotating
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`gantry that allows radiation beams focused on a target to
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`intersect the patient at varying orientations. Radiation to
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`healthy tissue and organs must be restricted to avoid detri-
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`mental effects to the patient. The amount of radiation that
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`can be concentrated on the target is limited by the need to
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`limit the radiation dosage received by normal tissue sur-
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`rounding the target.
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`[0004] A beam-shielding device modifies the spatial dis-
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`tribution of the radiation beam by selectively blocking areas
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`where lower amounts of radiation are desired. A multileaf
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`collimator is commonly provided in the path of the radiation
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`beam for this purpose. The multileaf collimator shapes the
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`radiation beam. The multileaf collimator has two opposing
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`banks of adjacent blocking leaves. The leaves can each be
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`moved in and out of the radiation beam to define arbitrary
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`field shapes. The multileaf collimator can be used to shape
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`the radiation beam so that it roughly matches the shape of
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`the target area.
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`[0005] A method known as intensity modulation may be
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`used to tailor a radiation field to further reduce the amount
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`of radiation received by healthy tissue. This method pro-
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`vides a radiation field which has a non-uniform intensity
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`over its spatial extent. A complete treatment may comprise
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`the delivery of an different intensity modulated radiation
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`field from each of a plurality of gantry angles.
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`[0006] Anon-uniform field may be delivered by delivering
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`radiation in each of a set of uniform sub-fields, each having
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`a different multileaf collimator configuration.
`Intensity
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`modulated fields may be delivered using static or dynamic
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`methods. In static methods each sub-field is shaped while the
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`radiation beam is off and then a radiation sub-field is
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`delivered once the leaves are in position. In dynamic meth-
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`ods the leaves are moved while the beam is on.
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`[0007] The multileaf collimator has certain characteristics
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`that limit its ability to protect healthy tissue from exposure
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`Page 8 of 14
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`to radiation. Because each leaf has a finite width,
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`precision at which the radiation beam can be spatially
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`controlled is limited to that width in that dimension. The
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`multileaf collimator is constructed such that each leaf is as
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`close as possible to its adjacent leaf to avoid the leakage of
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`radiation in between them. Still, the leaves are not perfectly
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`in contact at all times and there is some radiation that will
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`leak through the gap between them. To avoid this problem,
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`adjacent leaves can be constructed with a tongue and groove
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`shape on each side. Although the amount of leakage radia-
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`tion is reduced some radiation still leaks through, damaging
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`healthy tissue. Also, the tongue and groove creates unwanted
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`under-dosing effects to the target for some intensity modu-
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`lated fields.
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`[0008] When delivering intensity modulated fields it is in
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`general necessary to have the radiation device produce more
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`radiation and for a longer period of time than for an
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`un-modulated field. The amount of time it takes to treat a
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`patient is increased, reducing the number of patients that can
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`be treated per day. This also has implications on treatment
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`room radiation shielding requirements, necessitating addi-
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`tional shielding for new and existing rooms.
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`[0009] Multileaf collimators are constructed with enough
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`leaves to cover a given length (e.g. 40 cm). Due to mechani-
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`cal limitations, the range of leaf motion is restricted to a
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`fraction of that length. The maximum intensity modulated
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`field size is therefore limited to a rectangle whose width is
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`given by the range of leaf motion.
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`[0010] There is a need for methods and apparatus for
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`delivering radiation which minimize the dosage delivered to
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`healthy tissue. There is a particular need for such methods
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`and apparatus which can be practiced in existing radio-
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`therapy devices without
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`modifications.
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`SUMMARY OF THE INVENTION
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`[0011] The invention relates to methods and apparatus for
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`controlling radiotherapy devices. One aspect of the inven-
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`tion provides a method for determining a set of configura-
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`tions for a multileaf collimator in a radiotherapy device for
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`production of a desired radiation field. The method com-
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`prises varying a parameter of an initial set comprising three
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`or more multileaf collimator configurations to provide a
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`varied set of multileaf collimator configurations. Typically
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`each configuration in the initial set comprises a different
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`angle of rotation for the collimator. The method then deter-
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`mines a calculated radiation field resulting from the varied
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`set of multileaf collimator configurations. Based upon the
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`calculated radiation field, the method determines whether
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`one or more acceptance criteria are satisfied. If the accep-
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`tance criteria are satisfied,
`the method makes a further
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`variation to the varied set of multileaf collimator configu-
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`rations. This is repeated until the varied set of multileaf
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`collimator configurations satisfies one or more termination
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`criteria.
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`[0012] Other aspects of the invention provide apparatus
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`for determining a set of configurations for a multileaf
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`collimator in a radiotherapy device. The apparatus com-
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`prises a data processor and computer software which, when
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`executed by the data processor causes the data processor to
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`execute a method of the invention.
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`[0013] The invention may also be embodied in a computer
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`readable medium which comprises
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`instructions which, when executed by a data processor,
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`cause the data processor to execute a method of the inven-
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`tion.
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`[0014] Afurther aspect of the invention provides a method
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`for controlling a radiation device to deliver a radiation field
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`having a desired spatial distribution of radiation. The
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`method comprises delivering in succession at least three
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`radiation sub-fields, each of the sub-fields shaped by a
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`multileaf collimator and rotating the collimator to a different
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`angular position for the delivery of each of the sub-fields.
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`[0015] Further aspects of the invention and features of
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`specific embodiments of the invention are described below.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`In drawings which illustrate non-limiting embodi-
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`ments of the invention:
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`[0017] FIG. 1 is a simplified schematic diagram of the
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`radiation emitting portion of a radiation treatment device
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`with a rotating multileaf collimator;
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`[0018] FIGS. 2A and 2B are simplified top view illustra-
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`tions of a multileaf collimator at two separate angulations
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`and leaf configurations;
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`[0019] FIG. 3 illustrates a method for generating intensity
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`modulated radiation fields with arbitrary spatial distributions
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`according to the invention;
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`[0020] FIG. 4 is a plot illustrating the trajectory of points
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`on a multileaf collimator which correspond to points in a
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`treatment field as the multileaf collimator is rotated;
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`[0021] FIG. 5 is a flowchart illustrating a method for
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`deriving rotated multileaf collimator configurations accord-
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`ing to one embodiment of the invention; and,
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`[0022] FIG. 6 is a flowchart illustrating an optimization
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`process for a particular embodiment of the invention.
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`DESCRIPTION
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`the following description, specific
`[0023] Throughout
`details are set forth in order to provide a more thorough
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`understanding of the invention. However, the invention may
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`be practiced without these particulars. In other instances,
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`well known elements have not been shown or described in
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`detail
`to avoid unnecessarily obscuring the invention.
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`Accordingly,
`the specification and drawings are to be
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`regarded in an illustrative, rather than a restrictive, sense.
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`[0024] This invention provides a method for controlling a
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`radiotherapy device to deliver a desired radiation field in a
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`treatment area within a patient. The method involves creat-
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`ing the desired radiation field by sequentially exposing the
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`patient to a number of sub-fields. Each sub field has a shape
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`defined by a multileaf collimator. The multi-leaf collimator
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`is rotated between the different sub fields.
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`[0025] FIG. 1 illustrates a patient P positioned to receive
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`radiation from a radiotherapy device 10. Radiation is emit-
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`ted from a source (not shown) in a portion 11 of the
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`radiotherapy device. Radiation from the source exits through
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`a collimator 12 and impinges onto patient P. A multi-leaf
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`collimator 14 is housed in collimator 12. Multi-leaf colli-
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`mator 14 has multiple movable leaves 15. Leaves 15 are
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`shown in an exemplary configuration in FIG. 1. Radio-
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`therapy device 10 includes a mechanism (not shown) for
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`rotating multi-leaf collimator 14 about an axis in the plane
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`of leaves 15 as indicated by arrow 16.
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`[0026] Radiotherapy device 10 comprises a control system
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`which is coupled to control mechanisms which move leaves
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`15 and rotate multi-leaf collimator 14. The control system
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`can place multileaf collimator 14 in any allowable configu-
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`ration. The control system typically comprises a computer
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`processor which receives parameters specifying the leaf
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`positions and rotation angle for a sub field and actuates the
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`mechanisms to cause the leaves to move to the desired
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`positions and to cause the multi-leaf collimator to be rotated
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`to the desired angle.
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`the controller operates the
`[0027] For each sub-field,
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`radiation source to produce radiation. The radiation passes
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`through collimator 12 and is shaped by multileaf collimator
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`14. The shaped radiation impinges onto the patient P. The
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`total radiation dosage delivered at a point in the patient from
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`several sub fields is the sum of the radiation dosage deliv-
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`ered by each sub field individually. Therefore a radiation
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`field which closely approximates an ideal radiation field can
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`be delivered by delivering several appropriately configured
`sub-fields at different times.
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`[0028] FIGS. 2A and 2B illustrate a simplified schematic
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`view of multileaf collimator 14 at two configurations. Each
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`of the two configurations has a different rotation angle.
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`Leaves 15 are shown in an exemplary configuration and are
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`not limited to the number or shape shown in FIGS. 2A and
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`2B. A typical multileaf collimator has many more leaves
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`than illustrated. Leaves 15 may all have equal widths, W, as
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`illustrated or may vary in width. Some multi-leaf collimators
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`have narrower leaves in their central portions and wider
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`leaves in their outer portions.
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`[0029] Leaves 15 may be adjusted to block radiation
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`which would not pass through an area to which it is desired
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`to deliver radiation. Leaves 15 are movable longitudinally so
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`that the shape of the treatment area 18 can be modified. The
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`leaf configurations of FIGS. 2A and 2B each define a
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`sub-field. These sub-fields can be added to each other and to
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`other sub-fields to build up an arbitrary spatial distribution
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`of radiation.
`In a typical
`intensity modulated radiation
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`treatment a number of sub-fields are configured to produce
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`an overall spatial distribution of radiation which matches a
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`desired radiation distribution very closely.
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`[0030] Each of leaves 15 can be moved longitudinally (i.e.
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`in the x-direction 20) but not
`transversely (i.e. not
`in
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`y-direction 21). Because individual leaves 15 are only able
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`to move longitudinally, the maximum spatial resolution with
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`which the shape of any sub-field can be specified in y-di-
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`rection 21 is limited by the width of each individual leaf 15.
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`Leaves 15 typically have widths such that each leaf blocks
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`a strip of radiation about 0.5 cm wide to about 1 cm wide at
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`the treatment area. Typically some radiation is transmitted
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`between adjacent
`leaves 15 outside of the desired field
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`shape. This “leakage” results in some radiation being deliv-
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`ered along edges 17 of leaves 15 where, ideally, all radiation
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`would be blocked. Where adjacent leaves interlock with one
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`another to minimize radiation leakage, for example, by
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`providing a tongue on one leaf and a complementary groove
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`on a neighboring leaf, the amount of radiation delivered at
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`Page 9 of 14
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`Page 9 of 14
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`US 2003/0086530 A1
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`May 8, 2003
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`[0039]
`It
`is relatively straightforward to add together
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`radiation doses delivered by each of a number of given
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`sub-fields to yield an overall radiation field. In practice,
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`however, the desired overall radiation field is the starting
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`point. The desired overall radiation field is derived by a
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`treatment planning system in response to a prescription
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`specified by a physician.
`It
`is not
`trivial
`to identify a
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`combination of sub-fields which will produce a desired
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`overall radiation field when collimator 14 is permitted to
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`rotate between sub-fields.
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`leaf edges which protrude into the field shape 18 can be
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`smaller than would normally be desired.
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`[0031] FIG. 2B illustrates multileaf collimator 14 in a
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`different configuration from that shown in FIG. 2A. Both
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`the leaf configuration and rotational angle differ between the
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`configurations of FIGS. 2A and 2B. In FIG. 2B, collimator
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`14 has been rotated through an angle 9 relative to the
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`configuration of FIG. 2A. The dotted lines in FIG. 2B are
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`an overlay which show the leaf positions of the configura-
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`tion of FIG. 2A. The configurations of FIGS. 2A and 2B are
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`examples of sub-fields that are rotated relative to one
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`another and could be used to generate an intensity modu-
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`lated field. More sub-fields would be used to generate a
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`typical intensity modulated field. For typical intensity modu-
`lated fields at least 5, and more typically 8 or more sub-fields
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`are used.
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`[0032] Because multileaf collimator 14 is rotated between
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`sub-fields, the limited spatial resolution resulting from the
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`finite width of leaves 15 is in a different direction for each
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`sub-field. By combining several appropriately configured
`sub-fields, each with a different rotation angle, the method of
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`the invention can generate spatial distributions of radiation
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`with a spatial resolution that is smaller than the leaf width in
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`all directions.
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`[0033] Since rotating multileaf collimator 14 moves the
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`positions of the edges of leaves 15, any radiation leakage
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`between the leaf edges occurs in different positions for each
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`sub-field. This leakage radiation is therefore distributed over
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`an area instead of being repeatedly delivered at the same set
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`of locations. This causes less damage to healthy tissue.
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`[0034] Furthermore, because the leaf edges move with
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`each sub-field it
`is possible to compensate for leakage
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`radiation by blocking areas which receive leakage radiation
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`in other sub-fields. Areas which may receive less radiation
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`than desired in one sub field because they are along leaf
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`edges can be exposed to more radiation in other sub fields to
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`compensate.
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`[0035] Radiation sub-fields may be delivered statically
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`(with the collimator not rotating while radiation is being
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`delivered) or dynamically (with the collimator rotating while
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`radiation is being delivered).
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`[0036] FIG. 3 illustrates several example configurations
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`for a multileaf collimator, the distribution of radiation 30 in
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`the sub-field delivered when each configuration is used and
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`the cumulative spatial distribution of radiation 32 that results
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`when that sub-field is added to the radiation dose delivered
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`by previous sub-fields.
`[0037] The first collimator shape 34A provides a first
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`sub-field, which is a 2-dimensional surface 36A of uniform
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`radiation intensity having a shape which matches shape
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`34A. The second multileaf collimator shape 34B is rotated
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`relative to shape 34A and has a different leaf configuration.
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`Second shape 34B also generates a uniform-intensity sub-
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`field 36B. The cumulative spatial distribution of radiation
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`32B for the first two sub-fields provides a simple spatial
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`distribution of radiation of limited complexity. Further sub-
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`fields are added until the desired intensity-modulated field
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`32C is achieved.
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`[0038] The invention permits building radiation fields in
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`which the overall field width is not limited by the range of
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`leaf movement.
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`Page 10 of 14
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`locations in the
`[0040] Each leaf will affect different
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`overall radiation field depending upon its location, position
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`and the angle of collimator 14. For a desired point (x, y) in
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`the overall radiation field, the leaf pair, L, and leaf position,
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`P, that intersect that point are given by:
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`L=x sin 6+y cos 6
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`P=x cos 6—y sin 6
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`(1)
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`(2)
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`[0041] where 0 is the rotation angle of multi-leaf collima-
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`tor 14. As collimator 14 is rotated the leaf pairs capable of
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`modulating the radiation delivered at a specific point
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`change. FIG. 4 is a sinogram showing the trajectories of
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`three points in the treatment field relative to the leaves in
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`collimator 14 as collimator 14 is rotated.
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`[0042] As collimator 14 is rotated, each point follows a
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`sinusoidal trajectory through the leaf pairs. The amplitude of
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`each trajectory depends upon a radial distance of the point
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`from a center of rotation of collimator 14. The phase of each
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`trajectory depends upon the initial position of the point
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`along collimator 14. In FIG. 4, points A and B are located
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`equal distances from the center of rotation of collimator 14
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`and trace trajectories with equal amplitudes but different
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`phases.
`[0043] Any desired overall radiation field can be made by
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`applying any of many possible combinations of sub-fields.
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`One can select from these a combination of sub fields that
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`will produce the desired overall intensity modulated field
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`with the smallest total output of the radiation source.
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`[0044] FIG. 5 is a flowchart which illustrates a method
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`100 which may be used to identify a set of sub-fields which
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`will produce a desired overall radiation field. Method 100
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`may be performed on a treatment planning computer system
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`or on another suitable programmed data processing device.
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`Method 100 is initialized in block 102. In block 102 the
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`desired overall radiation field is provided. The desired
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`overall radiation field may be specified in output from
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`treatment planning software.
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`[0045] Block 102 also provides any mechanical and physi-
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`cal constraints which must be observed. For example, in a
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`particular multileaf collimator:
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`the leaves may be movable over only limited
`[0046]
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`ranges,
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`[0047]
`adjacent leaves may be constrained to have
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`positions within certain distances from one another,
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`[0048]
`opposing leaves may be forbidden to overlap
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`with one another,
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`[0049]
`it may be necessary to maintain a minimum
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`gap between opposing leaves, and,
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`Page 10 of 14
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`US 2003/0086530 A1
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`May 8, 2003
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`in dynamic delivery the leaves may have a
`[0050]
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`maximum velocity, or the like.
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`[0051] Block 102 may also permit selection among a set of
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`available optimization routines and termination criteria.
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`Many different optimization methods or termination criteria
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`may be used.
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`[0052] Block 102 may permit an operator to optionally
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`specify values for certain parameters. Any parameters that
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`affect the leaf positions and the multileaf collimator rotation
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`may be selected as fixed. Some typical examples include,
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`but are not limited to, whether the radiation is to be delivered
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`statically or dynamically, the maximum range of rotation of
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`the collimator and the maximum number of sub-fields. As
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`
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`another example,
`the angle of rotation of the multileaf
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`collimator could be specified for one or more, or all,
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`sub-fields.
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`[0053] Block 104 determines a set of configurations for
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`delivering a number of sub-fields. Each configuration speci-
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`fies leaf positions, collimator angles and sub-field contribu-
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`tions. All the parameters that are not fixed may be varied
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`according to the chosen optimization method.
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`[0054] Block 106 evaluates any discrepancies between the
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`calculated spatial distribution of radiation resulting from the
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`configurations determined in block 104 and the desired
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`spatial distribution of radiation. Block 106 may do this by
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`summing the radiation contribution delivered to each point
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`in the radiation field over all of the sub-fields. Instead of
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`summing over all sub fields block 106 may calculate the
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`spatial distribution of radiation by identifying pixels in the
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`calculated field which are affected by a most recent change
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`in leaf positions and adding or subtracting an appropriate
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`amount to the total amount of radiation which the overall
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`radiation field will deliver to those pixels.
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`It is convenient to compute in advance the corre-
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`spondence between leaf position and points in the radiation
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`field for all possible angles of collimator 14. This informa-
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`tion may be provided in a lookup table. This can speed the
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`calculations since the pixels affected by a change in leaf
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`position can be identified with a lookup instead of a more
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`complicated calculation.
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`[0056] The discrepancies between the calculated and
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`desired radiation fields can be measured using any suitable
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`metric. For example, the sum of absolute differences (SAD)
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`between the calculated and desired radiation fields may be
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`used as a measure of the discrepancies. If the discrepancies
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`are acceptable and the termination criteria has been attained
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`then configuration information including leaf positions, col-
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`limator angles and individual sub-field contributions is
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`stored or transferred to radiation device 10 for patient
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`treatment. If the termination criteria has not been attained as
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`determined in block 108, then method 100 returns to block
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`104 for further optimization. Method 100 continues in this
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