EUSONesteu
`S(ONCOLOGY
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`JOURNAL OF THE EUROPEAN SOCIETY FOR
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`THERAPEUTIC RADIOLOGY AND ONCOLOGY
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`Radiotherapy and Oncology, 1999, Volume 50, Number 3, March, pp. 247-378
`
`CONTENTS
`
`Cited in: Chemical Abstracts, Excerpta Medica (EMBASE), Current Contents (Clinical Medicine; Life Sciences), Index Medicus (MEDLINE), Current
`Awareness in Biological Sciences (CABS)
`
`Review article
`Altered fractionation: limited by mucosa] reactions?
`J.H.A.M, Kaanders, A.J. van der Kogel, K. Kian Ang (The Netherlands, USA)
`
`Original articles
`Modulation of accelerated repupulation in mouse skin during daily irradiation
`K-R.Trott, A. Shirazi, F. Heasman (UK)
`
`‘Loco-regional recurrences after mastectomy in breast cancer: prognostic factors and implications for postoperative irradiation
`J.J. Jager, L. Volovics, L.J. Schouten, J.M.A. de Jong, P.S.G.J. Hupperets, M.F. von Meyenfeldt, B. Schutte, G.H. Blijham (The Netherlands)
`
`Acute and late morbidity of using a breast positioning ring in women with large pendulous breasts
`G.C. Bentel, L.B. Marks, C.S. Whiddon, L.R. Prosnitz (USA)
`
`Evaluation of predictive factors for local tumour contro! after electron-beam-rotation irradiation of the chest wall in locally advanced breast cancer
`T. Hehr, W. Budach, F. Paulsen, C. Gromoll, G. Christ, M. Bamberg (Germany)
`
`The use of compensators to optimise the three dimensional dose distribution in radiotherapy of the intact breast
`L.J. Carruthers, A.T. Redpath, IH. Kunkler (UK)
`
`Clinical delivery of intensity modulated conformal radiotherapy for relapsed or second-primary head and neck cancerusing a multileaf collimator
`with dynamic control
`W.De Neve, W. De Gersem, S. Derycke, G. De Meerleer, M. Moerman, M.-T. Bate, B. Van Duyse, L. Vakaet, Y. De Deene, B. Mersseman, C. De
`Waeter (Belgium)
`
`A conformal technique for a ring shaped conjunctive lymphoma treatment (Technical note)
`R. Arrans, S$. Alonso, F. SAnchez-Doblado, J.A. Sanchez-Calzado, A. Leal, M. Perucha (Spain)
`
`A single-variable method for the derivation of collimator scatter correction factors in symmetrical and asymmetrical X-ray beams (Technical note)
`J.L.M. Venselaar, N. Beckers (The Netherlands)
`
`Commissioning of a micro mullti-lcaf collimator and planning system for stereotactic radiosurgery
`V.P. Cosgrove, U. Jahn, M. Pfaender, S. Bauer, V. Budach, R.E. Wurm (Germany)
`
`Daily positioning accuracy of frameless stereotactic radiation therapy with a fusion of computed tomography and linear accelerator (focal) unit:
`evaluation of z-axis with a z-marker (Technical note)
`M. Uematsu, M. Sonderegger, A. Shioda, K. Tahara, T. Fukui, Y. Hama, T. Kojima, J.R. Wong, S. Kusano Japan, USA)
`
`Small-field fractionated radiotherapy with or withoutstereotactic boost for vestibular schwannoma
`K. Kagei, H. Shirato, K. Suzuki, T. Isu, Y. Sawamura, T. Sakamoto, S. Fukuda, T. Nishioka, $. Hashimoto, K. Miyasaka (Japan)
`
`Seminomaof thetestis: is scrotal shielding necessary when radiotherapy is limited to the para-aortic nodes?
`S. Bieri, M. Rouzaud, R. Miralbell (Switzerland)
`
`Characteristics and clinical application of a treatment simulator with Ct-option
`D. Verellen, V. Vinh-Hung, P. Bijdekerke, F. Nijs, N. Linthout, A. Bel, G. Storme (Belgium)
`
`Three-dimensional movementof a liver tumor detected by high-speed magnetic resonance imaging
`S. Shimizu, H. Shirato, B. Xo, K. Kagei, T. Nishioka, S. Hashimoto, K. Tsuchiya, H. Aoyama, K. Miyasaka (Japan)
`Errata
`ESTRO Courses
`ESTRO Meetings
`Calendarof events
`
`247
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`261
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`267
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`277
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`283
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`291
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`301
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`315
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`319
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`325
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`337
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`341
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`349
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`355
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`367
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`371
`375
`375
`376
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`—————,4-—4————+—————~ available on the Elsevier Science website at www.elsevier.nl or www.elsevier.com or www.clsevier.co.jp
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`This material may be protected by Copyrightlaw (Title 17 U.S. Code)
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`
`RADIOTHERAPY
`& ONCOLOGY
`JOUANAL OF THE LULOPEAN SOCIETY FOR
`THEAAPELTIC RADIOLOCT AND ONCOLDGT
`
`Radiotherapy and Oncology 50 (1999) 325-336
`
`Commissioning of a micro multi-leaf collimator and planning system for
`stereotactic radiosurgery
`Vivian P. Cosgrove*, Ulrich Jahn, Mathias Pfaender, Susanne Bauer, Volker Budach,
`Reinhard E. Wurm
`
`Klinik fiir Strahlentherapie, Universitdtsklinikam Charité, Schumann StraBe 20-21, 10117 Berlin, Germany
`
`Received 15 July 1998; received in revised form 9 November 1998; accepted 28 December 1998
`
`
`Abstract
`
`Purpose: A computer controlled micro multi-leaf collimator, m3 mMLC,has been commissioned for conformal, fixed-field radiosurgery
`applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam
`penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5
`stereotactic planning system.
`Materials and methods: The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmis-
`sion, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data
`were collected using a diamonddetector in a scanning water tank and planned dosedistributions were verified using LiF TLDsandfilm. A
`small, shaped phantom wasalso constructed to confirm field shaping accuracy using portal images.
`Results: Mean transmission through the closed multi-leaves was 1.9 + 0.1% and leakage between leaves was 2.8 + 0.15%. Between
`opposing leaves abutting along the central beam-axis transmission was ~ 15 + 3%, but was reduced to a mean of 4.5 + 0.6% by moving the
`abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing squarefield size and asymmetric
`fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR andrelative output beam data measurements of
`circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were
`calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1-2 mm
`and to + 3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm.
`Conclusions: The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery
`applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters. © 1999 Elsevier
`Science Ireland Ltd. All rights reserved.
`
`Keywords: Micro multi-leaf collimator, MLC; Conformal therapy; Dosimetry; Radiosurgery planning
`
`
`1. Introduction
`
`From the earliest clinical applications of stereotactic
`radiosurgery and radiotherapy for the treatment of intra-
`cranial pathological processes it has been the primary
`goal to improve therapeutic ratio. This involves maximis-
`ing the radiation dose to damage or destroy a lesion while
`minimising the dose to nearby, uninvolved, normal tissue.
`Stereotactic
`radiosurgery using linear
`accelerators
`is
`conventionally carried out with single or multiple isocen-
`tre, non-coplanar, arcing techniques for radiation delivery.
`As an alternative, non-coplanar,static, conformally shaped
`beams can be employed [2,21,22,29,32,33,39,41,42].
`
`* Corresponding author. Joint Department of Physics, Royal Marsden
`NHSTrust, Downs Road, Sutton, Surrey SM2 5PT, UK.
`
`Conformal radiosurgery or radiotherapy involves accu-
`rately tailoring a dose of radiation to match the shape of a
`target volume in three dimensions. Conformal beam shap-
`ing has been shown to offer a number of advantages over
`conventional, arcing radiosurgery using single and multiple
`isocentres. This includes improved normaltissue sparing
`(especially for larger, irregularly shaped target volumes),
`more homogeneousdosedistributions throughoutthe target
`volume and reduced treatment times [2,22,29,32,39]. A
`report by Nedziet al. [30] indicated that radiosurgery treat-
`ment complications were associated with multiple isocen-
`tre treatments, particularly for larger target volumes, due to
`tumour dose inhomogeneity.
`Multi-leaf collimators (MLC) are currently regarded as
`the state-of-the-art device for producing arbitrary, irregu-
`larly shaped radiation fields. MLC design and use are now
`
`0167-8140/99/§ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved.
`PII: §0167-8140(99)00020-1
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`326
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`V.P. Cosgrove et al. / Radiotherapy and Oncology 50 (1999) 325-336
`Table 1
`Design.specifications of the m3 micro multi-leaf collimator
`
`Numberof leaves
`Leaf width(at isocentre)
`
`26 Pairs
`14 x 3.0 mm
`6x45 mm
`6x 5.5 mm
`10.2 x 10cm
`Maximum field size
`5cm
`Maximum leaf over-travel
`31 cm
`Clearance from isocentre
`1.5 cm/s
`Maximum leaf speed
`
`Weight 30 kg
`
`leaves have this shaft
`inserted into each leaf. Adjacent
`inserted at vertical increments to permit optimum position-
`ing of each leaf’s driving motor. Single, 10 mm diameter
`motors are used to drive each micro-leaf. Leaf ends are
`milled to three angled straight edges, each. covering a
`third of the leaf edge length. These edges correspond ta
`the divergence of the beam whenthe leaf is fully extended
`(—5 cm), centred (0 cm) and fully retracted (+5 cm). The
`vertical central edges allow opposing leaves to meet when
`closed. Each leaf is also shaped in cross-section to match
`the change in divergence of the beam across thefield area.
`The maximum square field area that can be defined at
`isocentre is 10.2 x 10 cm’.
`Monitoring of the position of each leaf is carried out
`using two independent methods. Primary positional feed-
`back is derived from the rotation of the motor shaft to each
`leaf, with the numberof turns of each motor being related
`to leaf displacement. Secondary feedback derives from two
`mechanical brushes physically mounted on and along the
`longitudinal plane of each leaf. Together these ensure
`precise (to 0.1 mm) leaf positioning.
`the m3 is manually
`With the gantry turned to 180°,
`transferred to the accessory mount of the linac by means
`of a specially designed trolley. The mounting andinitiali-
`sation procedure
`is
`straightforward and is usually
`
`+———— 12 em
`
`
`
`\ ~ 3° — +1.9mm
`
`— +—2.1'mm
`
`(b)
`
`6cm
`
`|
`
`|||
`
`|
`
`}
`
`b
`
`t
`
`Ee
`
`(a)
`
`Fig. 1. Schematic diagram of a multi-leaf in (a) plane viewand (b) cross-
`section.
`
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`
`well established [4,9,13,14,19,20,23,25,34]. The advantage
`of using automatic field shaping devices over conventional
`lead alloy shielding blocks has also been documented
`(10,11,16,26]. More recently, MLCs have been shown to
`have the additional potential for modulating the intensity of
`radiation across a field [1,6,7,15,24,40,44].
`Accurate conformation of dose to small
`intracranial
`targets (up to 5 cm in cross-section) using conventional
`multi-leaf collimators is impeded by the relatively large
`leaf width at isocentre (usually ~ 1 cm). Linear accelera-
`tor based stereotactic radiosurgery and radiotherapy of
`these small targets have generally been carried out using
`lead alloy blocks or manually shaped miniature multi-
`leaves [2,12,27,37]. Between four and eight static beams
`are generally used, shaped with 3D ‘beams-eye view’
`image displays. The process of treatment delivery can be
`very time consuming and labourintensive in terms of block
`construction or manual field shaping, verification andtreat-
`ment delivery. There has, therefore, been much interest in
`developing an automated, miniature multi-leaf collimator
`to improve dose conformation to small target volumes as
`well as take advantage of the known labour and time
`saving attributes of a computer controlled field shaping
`device.
`
`Here we report on the dosimetric measurements,initial
`acceptance testing and commissioning of the m3 computer
`controlled micro multi-leaf collimator (m3). The measure-
`ments were principally used to verify that
`the unique
`features of the m3, such as leaf design, positioning and
`monitoring,
`fulfilled the demands
`for high precision,
`stereotactic radiosurgery applications. Beam data were
`then collected and transferred to a BrainSCAN v3.5 stereo-
`tactic planning system and used to calculate conformal,
`multiple-field,
`non-coplanar
`treatment plans. Further
`measurements were carried out
`in phantom to compare
`and verify the calculated and delivered dose distributions.
`
`2. Materials and methods
`
`2.1. m3 Design features
`
`The m3 micro multi-leaf collimator investigated in this
`work (a joint development project between BrainLAB
`GmbH, Germany, and Varian Inc., USA) is based upon
`the architecture of a standard Varian MLC [3,14,20]. It
`has 52 tungsten leaves (26 pairs), which move perpendicu-
`larly to the beam central axis (i.e. unfocussed). However,
`unlike the standard Varian MLC that has a constant leaf
`width of 1 cm at isocentre, the m3 has variable leaf widths
`(see Table 1). The finer, 3 mm wide leaves, located in the
`centre of the field area, allow improved shaping around
`smaller targets, of the size treated by radiosurgery.
`Leaf design differs from the standard Varian MLC (see
`Fig. lab). A more complicated ‘tongue and groove’ leaf
`cross-section was necessary to allow drive shafts to be
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`
`327
`
`completed within 5-10 min. The linac used during this
`work has a standard 52 leaf Varian MLC already built
`into the gantry head. For all measurements the MLC leaves
`were fully retracted so as not to interfere with the primary
`beam.
`
`2.2. Dosimetry tools
`
`In common with other work, radiotherapy verification
`film (X-OMAT V2, Kodak, Inc., Rochester, NY, USA)
`was used for dosimetry measurements [3,13,14,19,20].
`All
`irradiated films were scanned using a computer
`controlled digital densitometer (FIPS Plus laser’ Scanner,
`PTW GmbH,Freiburg, Germany). The minimum resolu-
`tion of this laser densitometer is 0.3 mm. Film optical
`density was calibrated to absolute dose using a set of
`films irradiated in a standard calibration geometry (10 x
`10 cm square field, 6 MV X-rays, films at isocentre and
`SSD = 98.5 cm). This was repeated wheneverfilms from a
`new batch of 50 were used. All measurements were carried
`out using 6 MV X-rays.
`Measurements of beam data parameters used to calculate
`stereotactic radiosurgery dose distributions requires cham-
`bers or detectors with dimensions small enough to resolve
`steep dose gradients and avoid the problems associated
`with the lack of lateral electronic equilibrium of very
`narrow beams
`[35,38]. Therefore, beam data were
`measured in a scanning water tank using a diamond detec-
`tor (PTW GmbH,Freiburg, Germany). The properties of
`the diamond detector makeit ideal for high precision small
`field dosimetry [17,36].
`Similarly, measurements of absolute dose were made
`using small, 0.5 X 6 mm LiF thermoluminescent detectors
`(TLDs). Irradiated TLDs were read out on a Rialto (NE
`Technology, UK) automatic reader. Before use, TLDs were
`individually calibrated in a Co therapy beam using a
`traceably calibrated 0.3 cm? PTW ionisation chamber.
`TLDs with a reproducibility of =3% were selected for
`the measurements.
`
`2.3. Leaf transmission, leakage and penumbra
`
`Measurements of leaf leakage and transmission were
`carried out using films positioned perpendicularly to the
`beam central axis in Solid Water (Gammex, RMT, USA)
`at a depth of 1.5 cm (d,,,), SSD = 98.5 cm and with the
`primary jaws of the linac set to 10 x 10 cm. The leaves
`were fully closed so that opposing leaves abutted either
`along the central beam axis or 4.5 cm off-axis (i.e. a
`bank of leaves was set to over-travel
`the central beam
`
`axis by 4.5 cm). Films were scanned with the densitometer
`to produce profiles across the closed leaves and at the leaf
`ends. Scans were normalised to the output measured in a
`10 x 10 cm? square field without the m3 attached to the
`linac. Ten times the monitor unit settings (500 MU) were
`used compared to openfield films to increase the intensity
`and distribution statistics of the transmitted X-rays. (Film
`
`was calibrated over a large dose range (0-3 Gy) to ensure
`all optical density values would be accountedfor.)
`80% to 20% beam penumbra was first measured as a
`function of square field size ranging from 2 x 2 cm? to
`10 X 10 cm? in steps of ~1 cm. (The exact size of each
`square field was defined taking into account the combina-
`tions of the variable leaf widths.) Films were also placed at
`max, SSD = 98.5 cm, in solid water and profiles across the
`irradiated films were scanned both perpendicular and paral-
`lel
`to leaf motion (i.e. across leaf sides and Icaf ends,
`respectively).
`Beam penumbra as a function of asymmetric leaf posi-
`tioning was investigated by defining a 2 x 10 cm rectan-
`gular field shape with the m3 leaves. (The 2 cm field width
`was parallel to the direction of motion of the leaves). Three
`films were simultaneously exposed, positioned at dy, 5
`cm and 10 cm deep in a solid water phantom, SSD = 95
`cm. The field shape was then moved asymmetrically in 1-
`cm steps away from the beam central axis so that the final,
`fifth field shape was centred 4 cm from the central axis,
`with oneset of leaves fully retracted (—5 cm), the other set
`extended by +3 cm.A total of five sets of three films were
`irradiated and beam profiles were measured on each film,
`parallel to the direction of motion of the leaves.
`The variation in the effective beam penumbra wasalso
`investigated as a function of straight but diagonal field
`edges. Two irregular field shapes were created by fitting
`leaves to a number of diagonal, straight field edges with
`differing angles to the horizontal (leaves werefitted so that
`the mid-point of each leaf-end intersected with the diago-
`nal field edge). Films were irradiated at d,,,,, SSD = 98.5
`cm and 80, 50 and 20% isodose distributions measured. For
`all penumbra measurements the primary jaws were fixed at
`a field size of 10 x 10 cm’.
`
`2.4. Mechanical stability of the linac with the m3 attached.
`
`Measurements were madc to investigate whether the 30
`kg m3 causes any additional gantry or collimator sagging
`whenattached to the linac gantry head. To do this, a test
`identical in principle to the isocentre verification procedure
`described by Winston and Lutz for stereotactic radiosur-
`gery applications was used [43]. A 3 mm diameter tungsten
`ball, supported on a rigid, Perspex rod, was fixed at the
`radiation isocentre as defined by the treatment room lasers
`(this reference point is not strictly a true radiation isocen-
`tre, which can move due to mechanical instability of the
`linac. It is instead a mean point defined after taking into
`account observed field displacements as a function of
`gantry, collimator and table angle). The gantry was then
`moved in steps of 45° and at each angle three separate port
`films were exposed with the collimator turned to angles of
`0°, 90° and 270°. On developmentofthe films the displa-
`cement of the field-edges relative to the tungsten ball was
`measured both with and without the m3 installed. Fields
`were either a 3 cm diameter circular field defined by the
`
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`V.P. Cosgrove et al. / Radiotherapy and Oncology 50 (1999) 325-336
`
`m3 (primary jaws at 5 X 5 cm’) or a 3 X 3. cm? squarefield
`defined by the primary jaws alone. The light-field cross-
`hairs were marked on all film exposures and were used
`togcther with the images of the field edges to determine
`a mean displacement.
`
`2.5. Stereotactic beam data measurements
`
`A commercial hardware and software system (BrainScan
`v3.5, BrainLAB GmbH, Germany) is used for conven-
`tional, stereotactic arc planning of intracranial
`lesions.
`Circular collimators are utilised which are diverging, 9
`cm deep blocks constructed from lead and positioned so
`that the base of the collimator is 27 cm from the isocentre.
`
`For dose calculation, a standard algorithm based upon
`measurements of tissue maximum ratios (TMR), off-axis
`ratios (OAR) andrelative output factors is used [28,35]. A
`software module has been added to this system for non-
`coplanar,
`irregularly shaped, conformal fixed-field plan-
`ning and dose calculation with the m3. The calculation
`model also uses the above beam data parameters but
`these are instead measured with circular fields defined by
`the m3. To allow dose distributions and relative output
`factors to be calculated for irregularly shaped fields a
`Clarkson sector integration algorithm [8] has been imple-
`mented. An irregular field shape is divided into a finite
`number of sectors of differing radial extent and the beam
`data parameters relating to each sector are then used to
`calculate dose.
`
`Beam data for twenty-one circular fields defined by the
`m3 (from 10 to 50 mm diameter in 2 mm steps) were
`measured using a scanning water tank and a diamond
`detector. Percentage depth-dose measurements were first
`collected with an SSD = 98.5 cm. These were converted
`to tissue maximum ratios using a standard computation[5].
`Relative output factors were measured for each circular
`field at a depth of 5 cm in water, was normalised to dose
`measured in a 10 X 10 cm? field. Off-axis ratios were
`measured in a direction parallel to the leaf motion (i.e.
`the leaf ends defined the field edges, not their sides) at
`isocentre and SSD = 92.5 cm. All
`the beam data were
`measured with a fixed primary jaw setting of 5 x 5 cm’.
`Fixed jaws were used to avoid undertaking more complex
`calculations for beam output, which will vary both as a
`function of the irregular m3 field area and primary jaw
`field size.
`
`2.6. Quality assurance ofthe m3 and treatment planning
`system
`
`After the beam data had been transferred to the treatment
`planning system various quality assurance procedures were
`carried out to verify that dose calculation and other aspects
`of the planning and treatment process are performed to the
`required accuracy. An initial test set out to verify a number
`of parameters simultaneously,
`including the accuracy of
`image localisation, three-dimensional shape reconstruction,
`
`target set-up and irradiation. To do this an irregularly
`shaped phantom wasconstructed from a hollow aluminium
`form and Optosil® P dental
`impression material. This
`material combination was used so that the phantom could
`be visualised without distortion both on CT scans and high-
`energy X-ray portal images, acquired on the linear accel-
`erator. The phantom dimensions were approximately 8 cm
`high x 4 cm wide X 1.5 cm thick.
`The phantom shape was solidly fixed to a BrainLAB
`stereotactic head frame and CT scanned with a fiducial
`localiser box in place. Images were acquired with scan
`steps and slice thickness of 1.5 mm. After the CT data
`wastransferred to the planning system, images were loca-
`lised relative to the stereotactic fiducial co-ordinate frame
`and a treatment isocentre was defined. The outer phantom
`surface was used to define a volume to which forty confor-
`mal m3 fields were shaped using various combinations of
`collimator, gantry and table angle. A 1 mm margin was
`arbitrarily added between the volumeandthe field edges to
`help with identifying any displacements.
`For plan verification, the head frame and attached phan-
`tom were fixed to the linac treatment table and positioned
`to the planned treatment isocentre using standard stereo-
`tactic patient positioning procedures. The planned field
`shapes were transferred from the planning system to the
`m3 workstation via a floppy disk. Portal Vision™ images
`of each field were then collected for the defined collimator,
`gantry and table angle combinations. Deviations between
`these images and the planned beams-eye views (BEV)
`were then measured.
`
`that the
`it is crucial
`treatment planning,
`For clinical
`planned isodose distributions accurately represent
`the
`actual delivered dose distributions. Therefore,
`relative
`dosimetry measurements were made to verify the spatial
`accuracy of the isodose calculations produced by the plan-
`ning system. Measurements were performed using film and
`a cubic, Solid Water phantom. This was constructed from
`14.8 x 14.8 cm? sheets of Solid Water, ranging in thickness
`from 1 mm to | cm. The cubic shape enabled films to be
`positioned in either the axial, sagittal or coronal planes by
`simply changing the orientation of the phantom. The phan-
`tom geometry was also considered to be more challenging
`than a spherically shaped phantom in terms of dose calcu-
`lation, particularly for beams that are incident obliquely to
`the phantom surface. The phantom:shape wasalso simple
`to handle, load films and position forirradiation.
`The cubic phantom was CT scanned within the fiducial
`localiser box, and the image data transferred to the treat-
`ment planning system. A treatment volume was drawn onto
`the images foruse as an irradiation target. Several volumes
`were used,
`including spheres and more irregular shapes,
`transcribed from actual patient data files. These were
`placed in various positions within the phantom and six-
`field, non-coplanar plans were
`calculated. Treatment
`gcometries were selected so that beams were well spaced
`in three dimensional space,
`to minimise dose overlap
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`Fig. 2. Transmission through a single bank of caves that abut with the opposing set 4.5 cm off-axis. Li ght grey line, A — 19.7 mm from central axis; grey
`line, B central axis; dark grey line, C + 25 mm from central axis; black line, D leaf ends,
`
`regions, while at the same time reflecting practical treat-
`ment arrangements.
`the treatment
`Before irradiation within the phantom,
`isocentre position was marked on each film. This was
`achieved by marking the film with a pin (distant from the
`irradiation area) using the treatment room lasers as a guide.
`The isocentre position was used as the dose normalisation
`pointafter the film was scanned with the densitometer and
`isodose boundaries calculated. Absolute dose calibration of
`the films. proved to be unreliable (measured dose was
`always lower than expected), with variations of up to —
`6% in the measured dose at isocentre This was most notice-
`able wheneverfilms were irradiated with oblique fields, or
`when fields were incident on the edge of a film, ie. the
`field central axis was parallel
`to the film surface. This
`phenomenon will be discussed later in more detail.
`Finally,
`dose was measured
`using
`an Alderson
`RANDO® anthropomorphic head phantom and TLDs.
`This phantom was also fixed inside a stereotactic head
`frame immobilised using self-penetrating head pins.
`It
`was CT scanned in the usual way and the images trans-
`ferred to the treatment planning system. Six-field, non-
`coplanar, conformal treatment plans were then calculated
`using arbitrary target volume shapes ranging in size from
`5-20 cm?. Several TLD chips were loaded inside the phan-
`tom at the isocentre position and were irradiated. Care was
`
`take, so that the planned dose distributions were homoge-
`neous over the length of the TLDs.
`
`3. Results
`
`3.1. Leaf transmission and leakage
`
`Fig. 2 plots the relative dose distributions obtained from
`scans across the closed muliti-leaves. In this example, one
`bank of leaves was moved to abut with the other 4.5 cm
`off-axis, i.e. one bank of leaves was set to over-travel the
`central beam axis by 4.5 cm. For the