United States Patent
`
`[191
`
`[11] Patent Number:
`
`5,233,990
`
`Barnea
`
`[45] Date of Patent:
`
`Aug. 10, 1993
`
`||||||ll||||||||lllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`Usoo523399oA
`
`[54] METHOD AND APPARATUS FOR
`DIAGNOSTIC IMAGING IN RADIATION
`THERAPY
`
`[76]
`
`Inventor: Gideon Barnea, 7887 E. Uhl St., No.
`410, Tucson, Ariz. 85710
`
`[2]] App]. No.: 819,957
`
`[22] Filed:
`
`Jun. 13, 1992
`
`Int. Cl.5 .............................................. .. A61B 5/05
`[51]
`[52] U.S. Cl. .................................. .. 128/653.1; 378/65
`[58] Field of Search ...................... .. 128/653.1; 378/65
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`l/I974 Pavltovich .......................... .. 378/65
`3,783,251
`4,123,660 l0/l978 Horwitz .............................. .. 378/65
`4,930,509 6/1990 Brisson ........................... .. 128/653.1
`
`OTHER PUBLICATIONS
`
`Droege, R. T. et al., “Influence of Metal Screens on
`Contrast in Megavoltage X—Ray Imaging,“ Med. Phys.
`6, 487-492, 1979.
`Lutz, W. R. et al., “A Test Object for Evaluation of
`Portal Film" Int. J. Radiat. Oncol. Biol. Phys. 11,
`631-634, 1985.
`Munro. P. et al., “Therapy Imaging: A Signal-to-Noise
`Analysis of Metal Plate/Film Detectors,” Med. Phys.
`14, 975-984, 1987.
`Mark, J. E. et al., “The Value of Frequent Treatment
`Verification Films in Reducing Localization Error in
`the Irradiation of Complex Fields," Cancer,
`37,
`2755-2761, 1976.
`I-luizenga et al. “Accuracy in Radiation Field Align-
`ment in Head and Neck Cancer: A Prospective Study,"
`Rad. Oncol. 11, 181-187, 1988.
`Pearcy et al., “The Impact of Treatment Errors on
`Post—0perative Radiotherapy for Testicular Tumors,”
`Br. J. Radiol. 58, 1003-1005, l985.
`Rabinowitz et al. “Accuracy of Radiation Field Align-
`ment in Clinical Practice," Int. J. Radiat. Oncol. Biol.
`Phys. 11, 1857-1867, 1985.
`
`Lam et al. “On-Line Measurement of Field Placement
`Errors in External Beam Radiotherapy," Br. J. Radiol.
`60, 361-367, 1987.
`Baily et al. “Fluoroscopic Visualization of Megavoltage
`Therapeutic X—Ray Beams," Int. J. Radiat. Oncol. Biol.
`Phys. 6, 935-939, 1980.
`Herk et al., “A Digital Imaging System for Portal Veri-
`fication,” in The Use of Computers in Radiation Ther-
`apy, I. Brunvis Ed., North Holland, 371-373, 1987.
`(List continued on next page.)
`
`Primary Examiner—Lee S. Cohen
`Assistant Examiner—Samuel Gilbert
`Attorney, Agent, or Firm—Ant0nio R. Durando; Harry
`M. Weiss
`
`[57]
`
`ABSTRACT
`
`An apparatus for diagnostic and verification imaging in
`radiation therapy that consists of attachments for stan-
`dard radiotherapy equipment comprising an x-ray tube
`and an x-ray detector placed on opposite sides of a
`patient along the main axis of the beam produced by the
`treatment unit. The detector is placed on a plane or-
`thogonal to the axis of the treatment beam and between
`the beam source and the patient, while the x-ray tube is
`placed on the other side of the patient, coaxially with
`the treatment beam and facing the detector. As a result
`of this configuration, the radiographic View of the x-ray
`beam, as seen on the detector, is equivalent to the view
`produced on the same detector by the therapeutic beam.
`varied only by parallax deviations that can be corrected
`by geometrical calculations. Accordingly, x-ray expo-
`sures and real-time verification of the position of a pa-
`tient can be obtained with the same unit used for treat-
`ment and without requiring movement of either patient
`or equipment. In addition, the apparatus enables a user
`to produce diagnostic images that can be used directly
`to manufacture shielding blocks in conventional shield-
`ing-block cutters.
`
`10 Claims, 2 Drawing Sheets
`
`4
`
`/20
`
`Page 1 of 10
`
`Elekta Exhibit 1012
`
`

`
`5,233,990
`
`Page 2
`
`OTHER PUBLICATIONS
`
`Shalev et al. “Video Techniques for On—Line Portal
`Imaging," Comp. Med. Imag. Graph. 13, 2l7—226, 1989.
`Munro et al. "A Digital Fluoroscopic Imaging Device
`for Radiotherapy Localization,” Int. J. Radiat. Oncol.
`Biol. Phys. 18, 641-649, 1990.
`Durham et al. “Portal Film Quality: A Multiple Institu—.
`tional Study,” Med. Phys. 11, 555-557, 1984.
`Biggs et al. “A Diagnostic X-Ray Field Verification
`Device for a 10 MV Accelerator," Int. J. Radiat. Oncol.
`Biol. Phys. 11, 635-643. 1985.
`Marks J. E. et al. “Localization in the Radiotherapy of
`
`I-Iodgkins Disease and Malignant Lymphoma with Ex-
`tended Mantle Fields,” Cancer 34, 83-90, 1974.
`Marks J. E. et al. “Dose—Response Analysis for Nasc-
`pharyngeal Carcinoma: An Historical Perspective,”
`Cancer 50, 1042-1050 1982.
`
`White J. E. et al. “The Influence of Radiation Therapy
`Quality Control on Survival, Response and Sites of
`Relapse in Oat Cell Carcinoma of the Lung, ” Cancer
`50, 1084-1090, 1982.
`
`Kinzie J. J. et al. “Pattems of Care Study: Hodgkins
`Disease Relapse Rates and Adequacy of Portals, " Can-
`cer 52, 2223-2226, 1983.
`
`Page 2 of 10
`
`

`
`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 1 of 2
`
`5,233,990
`
`(PRIOR ART)
`
`Page 3 of 10
`
`

`
`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 2 of 2
`
`5,233,990
`
`2
`
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`Page 4 of 10
`
`

`
`1
`
`5,233,990
`
`l0
`
`15
`
`35
`
`45
`
`50
`
`METHOD AND APPARATUS FOR DIAGNOSTIC
`IMAGING IN RADIATION THERAPY
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention is related to the general field of radia-
`tion imaging for medical applications. In particular, the
`invention provides a new method and apparatus for
`producing a diagnostic image of the portion of the body
`affected by a tumor, so that the required dosage of
`radiation can be accurately delivered to the prescribed
`target volume.
`2. Description of the Prior Art
`The main object of radiotherapy is to deliver the
`prescribed dosage of radiation to a tumor in a patient
`while minimizing the damage to surrounding, healthy,
`tissue. Since very high energy radiation (produced at 4
`to 25 million volts, typically generated by a linear accel-
`erator) is normally used to destroy tumors in radiother-
`apy, the high energy is also destructive to the normal
`tissue surrounding the tumor. Therefore, it is essential
`that the delivery of radiation be limited precisely to the
`prescribed target volume (i.e., the tumor plus adequate
`margins), which is accomplished by placing appropri-
`ately constructed shielding blocks in the path of the
`radiation beam. Thus, the goal is to accurately identify
`the malignancy within the body of the patient and to
`target the prescribed dosage of radiation to the desired
`region on the immobilized patient.
`To that end, the ideal procedure requires the identifi-
`cation of the exact anatomical location of the tumor and
`
`the corresponding accurate positioning of the radiation
`field during treatment. This could be easily achieved if
`it were possible to locate and treat the tumor at the same
`time. In practice, though, this is not possible because the
`equipment used to identify the tumor (x-ray machine,
`computed tomography equipment, or the like) is sepa-
`rate from the equipment used for the therapeutical irra-
`diation of the patient, requiring the movement and repo-
`sitioning of the patient from one piece of equipment to
`the other. As illustrated in schematic form in FIG. 1, a
`conventional treatment unit 10 consists of a linear accel-
`erator (linac) head 2 mounted on a gantry 4 so that its
`collimated high-energy emissions HR irradiate a patient
`P lying on a gumey 6 directly below through shielding
`blocks 8 attached to the head. A bracket 12 supporting
`a detector 14 may be mounted on the opposite side of
`the head within the field of radiation in order to take
`
`radiographs of the patient being treated. The gantry 4 is
`movable around a pivot 16 to permit the rotation of the
`head (and of the detector) around the patient to afford
`different views of the area to be treated (“multiple
`fields” treatment). The normal procedure involves the
`use of a diagnostic simulator, which is a diagnostic x-ray
`machine with the same physical characteristics of the
`radiation therapy machine (schematically also repre-
`sented by FIG. 1, where a diagnostic x-ray head re-
`places the linac head 2), so that the field of view of the
`low-energy x rays emitted in the simulator is the same as
`that of the high-energy radiation emitted in the radia-
`tion therapy machine. Prior to treatment, the patient is
`radiographed using the simulator and an image of the
`target area is obtained with low-energy radiation (in the
`order of 100 kVp), which yields good image quality.
`The exact target volume is then delineated on the radio-
`graph by a physician and matching shielding blocks are
`constructed to limit the field of view of the irradiating
`
`Page 5 of 10
`
`2
`machine to the region so delineated. A mold of the
`shielding blocks is first cut out of plastic material (nor-
`mally polystyrene) with a shielding-block cutter, a ma-
`chine that reproduces exactly the relative positions of
`the linac head, the shielding blocks and the detector as
`they stand in the treatment unit. By using mechanical
`means, the shielding blocks are cut so that the field of
`irradiation from the treatment unit will corresponds
`exactly to the area delineated by the physician on the
`diagnostic radiograph. The final shielding blocks are
`then made from the mold with lead alloys that attenuate
`considerably the propagation of radiation. Thus, the
`shielding blocks function as templets that limit the radi-
`ation treatment to the areas left open within the contour
`of the shielding blocks. In addition, it is common prac-
`tice to mark the skin of a patient with reference mark-
`ings that are used in aligning the position of the patient
`with the field of emission of the radiation therapy ma-
`chine.
`
`These apparently sound procedures in fact suffer
`from serious practical shortcomings. Errors in position-
`ing the shielding blocks between the radiating source
`and the patient, as well as incorrect beam alignment and
`patient movement, all have a cumulative effect reducing
`the accuracy of the procedure. Even the markings on
`the skin of the patient may be the cause of alignment
`problems because of shifting of the skin with respect to
`the patient‘s internal anatomy as a result of body motion
`or, over a period of time, even of body changes. Thus,
`the area actually irradiated during the therapeutic ses-
`sion often does not correspond to the area delineated in
`the radiograph generated by the simulator.
`Positioning errors during irradiation have been found
`to have very serious consequences for the successful
`prognosis of the treatment. For example, researchers
`have been able to correlate the recurrence of lymphoma
`to such positioning errors ( J. E. Marks, A. G. Haus, H.
`G. Sutton and M. L. Griem, “Localization Error in the
`Radiotherapy of Hodgkin‘s Disease and Malignant
`Lymphoma with Extended Mantle Fields," Cancer 34,
`83-90, 1974); and it has been found that improved tumor
`control of nasopharingeal carcinomas can be related to
`greater accuracy in the delivery of calculated dosages
`of radiation (J. E. Marks, J. M. Bedwinek, F. Lee, J. A.
`Purdy and C. A. Perez, “Dose-Response Analysis for
`Nasopharyngeal Carcinoma: An Historical Perspec-
`tive," Cancer 50, 1042-1050, 1982). Similarly,
`it has
`been found that shielding inaccuracies have resulted in
`significantly lower primary tumor control and survival
`of patients of oat cell lung cancer (J. E. White, T. Chen,
`J. McCracken, P. Kennedy, H. G. Seydel, G. Hartman,
`J. Mira, M. Khan, F. Y. Durrance and 0. Skinner, “The
`Influence of Radiation Therapy Quality Control on
`Survival, Response and Sites of Relapse in Oat Cell
`Carcinoma of the Lung." Cancer 50, 1084-1090, 1982);
`and that the local recurrence of Hodgkin's disease was
`significantly higher when the radiation field did not
`adequately cover the tumor (J. J. Kinzie, G. E. Hanks,
`C. J. Maclean and S. Kramer, “Patterns of Care Study:
`Hodgkin’s Disease Relapse Rates and Adequacy of
`Portals,” Cancer 52, 2223-2226, 1983).
`The only technique widely used today to check the
`accuracy of the radiation field is by imaging with the
`radiotherapy beam itself at the time of treatment. Prior
`to treatment, a “portal" image is obtained by using the
`therapy beam (at high energy) and the resulting expo-
`sure is visually compared with that taken with the simu-
`
`

`
`3
`lator (at low energy). This technique is therefore known
`as "portal imaging” or “therapy verification," and is
`repeated periodically during the period of radiation
`treatment. Unfortunately. though, because of the high-
`energy radiation emitted by the treatment beam (pro-
`duced at 4-25 million volts), the resulting portal images
`have poor resolution and show very poor contrast be-
`tween soft tissues and bones, often making the images
`totally unsuited for verification by comparison with the
`low-energy images produced by the simulator. See, for
`example, R. T. Droege and B. J. Bjarngard, “Influence
`of Metal Screens on Contrast in Megavoltage X-Ray
`Imaging," Med. Phys. 6, 487-492, 1979; L. E. Reinstein,
`M. Durham, M. Tefft, A. Yu and A. S. Glicksman,
`“Portal Film Quality: A Multiple Institutional Study,”
`Med Phys. 11, 555-557, 1984; W. R. Lutz and B. E.
`Bjarngard, “A Test Object for Evaluation of Portal
`Film,” Int. J. Radiat. Oncol. Biol. Phys. 11, 631-634,
`1985; and P. Munro, J. A. Rawlinson and A. Fenster,
`“Therapy Imaging: A Signal-to-Noise Analysis of 20
`Metal Plate/Film Detectors," Med. Phys. 14, 975-984,
`1987. Indeed, positioning errors occur very frequently
`in spite of the use of portal images. See J. E. Marks, A.
`G. Hans, H. G. Sutton and M. L. Griem, “The Value of
`Frequent Treatment Verification Films in Reducing 25
`Localization Error
`in the Irradiation of Complex
`Fields,” Cancer 37, 2755-2761, 1976; R. W. Byhardt, J.
`D. Cox, A. Homburgh and G. Lierrnann, “Weekly
`Localization Films and Detection of Field Placement
`Errors," Int. J. Radiat. Oncol. Biol. Phys. 4, 881-887,
`1978; Huizenga, P. C. Lenendag, P. M. Z. R. De Porre
`and A. G. Visser, “Accuracy in Radiation Field Align-
`ment in Head and Neck Cancer: A Prospective Study,"
`Rad. Oncol. 11, 181-187, 1988; R. G. Pearcy and S. E.
`Griffiths, “The Impact of Treatment Errors on Post-
`Operative Radiotherapy for Testicular Tumors," Br. J.
`Radiol. 58, 1003-1005, 1985; I. Rabinowitz, J. Broom-
`berg, M. Goitein, K. McCarthy and J. Leong. “Accu-
`racy of Radiation Field Alignment in Clinical Prac-
`tice," Int. J. Radiat. Oncol. Biol. Phys. 11, 1857-1867, 40
`1985; and W. C. Lam, M. Partowmah, D. J. Lee, M. D.
`Wharam and K. S. Lam, “On-Line Measurement of
`Field Placement Errors in External Beam Radiother-
`
`apy," Br. J. Radiol. 60, 361-367, 1987. Imaging devices
`other than X-ray film have been used in an attempt to
`improve the quality of the image produced during ther-
`apy verification. These include metal and fluorescent
`screens in contact with conventional film, and non-film
`imaging processes and devices such as xeroradiogra-
`phy, liquid ionization chambers, fluoroscopic imaging,
`linear diode arrays, photostimulable phosphors, and
`others. In addition, various image processing techniques
`(both analog and digital) have been used to enhance the
`quality of the final verification image; but all
`these
`methods and devices have resulted only in a limited
`success in yielding a good quality, and therefore useful,
`diagnostic image. Real-time portal imaging using video
`techniques has also been proposed, so that patient
`movement can be monitored during treatment. Because
`they all use the high-energy therapy beam as the source
`of radiation, though, the quality of the image remains
`poor. See N. A. Baily, R. A. Horn and T. D. Kampp,
`"Fluoroscopic Visualization of Megavoltage Therapeu-
`tic X-Ray Beams,” Int. J. Radiat. Oncol. Biol. Phys.6.
`935-939, 1980; M. V. Herk and H. Meertens, “A Digital
`Imaging System for Portal Verification,” in “The Use
`of Computers in Radiation Therapy," I. Brunvis Ed.,
`North Holland, 371-373, 1987; S. Shalev, T. Lee, K.
`
`Page 6 of 10
`
`5,233,990
`
`4
`Leszczynski, S. Cosby and T. Chu, “Video Techniques
`for On-Line Portal
`Imaging,” Comp. Med.
`Imag.
`Graph. 13, 217-226, 1989; and P. Munro, J. A. Rawlin-
`son and A. Fenster, “A Digital Fluoroscopic Imaging
`Device for Radiotherapy- Localization,” Int. J. Radiat.
`Oncol. Biol. Phys. 18, 641-649, 1990. A survey of 23
`different radiotherapy departments shows that at each
`of eight institutions (i.e., 35 percent of the 23 institutions
`sampled) more than i of the submitted portals were
`evaluated as poor in quality. Furthermore, it shows that
`approximately one-half of the institutions were produc-
`ing poor-quality films at a rate of at least 50 percent. See
`Reinstein, L. E., M. Durham, M. Tefft, A. Yu, and A. S.
`Glicksman, “Portal Film Quality: A Multiple Institu-
`tional Study," Med. Phys. 11, 555-557, 1984.
`Another, logical, approach to obtaining diagnostic
`quality portal films has been by mounting an x-ray tube
`on the head of the treatment unit as close to the linac
`
`gantry as possible. See P. J. Biggs, M. Goitein and M.
`D. Russell, “A Diagnostic X-Ray Field Verification
`Device for a 10 MV Accelerator," Int. J. Radiat. Oncol
`Biol. Phys. 11, 635-643, 1985. The x-ray tube is aligned
`with the linac emission field so that, to the extent possi-
`ble within the physical constraints of both devices, the
`x~ray emissions have the same field of view of the high-
`energy radiation. As a result. the image received on a
`film placed on a detector tray on the opposite side of the
`patient by exposure to either source of radiation is theo-
`retically almost exactly the same. In order to implement
`this approach, though, a special shielding-block holder
`coupled to the gantry has to be made, disabling the
`normal rotation of the linac’s collimator and limiting the
`adjustment capabilities of the equipment. Thus,
`the
`complexity of the procedure, the oblique view of the
`diagnostic beam and the increased time required for
`each treatment have prevented this technique from
`gaining widespread acceptance. Furthermore,
`this
`method is unsuited for real-time portal imaging.
`A similar approach has been followed by placing an
`x-ray tube at a fixed angle with respect to the axis of the
`therapeutic beam, so that the x-ray beam and the thera-
`peutic beam have coinciding isocenters corresponding
`to the location of the radiation target. By rotating the
`gantry of the radiation unit by that angle, the target can
`be irradiated from the same point either with a treat-
`ment beam or an x-ray beam, with every other variable
`remaining unchanged. Therefore, verification can be
`obtained simply by rotating the gantry and switching
`from one mode of operation to the other. The main
`problem with this approach is the inevitable angular
`error introduced during the rotation of the gantry. In
`addition, because of the alternative use of either mode
`of operation, this equipment is also not suitable for real
`time verification.
`
`Therefore, it would be very desirable to have a sim-
`pler and more accurate verification imaging system for
`radiation therapy verification, especially for real time
`applications. This invention relates to the use of a con-
`ventional x-ray tube and conventional imaging devices
`in a novel geometric configuration to produce such an
`improved verification imaging system.
`
`BRIEF SUMMARY OF THE INVENTION
`
`One objective of this invention is the development of
`therapy verification apparatus that produces verifica-
`tion images of the same quality obtained with diagnostic
`apparatus.
`
`

`
`5,233,990
`
`6
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`5
`Another objective of the invention is an imaging
`apparatus that can be implemented as an accessory to
`existing radiation-therapy treatment units.
`A further goal of the invention is an imaging appara-
`tus and technique that are suitable for on-line, real time,
`applications in conjunction with radiation treatments.
`Still another objective of the invention is an apparatus
`that, as a result of the position of the x-ray emission
`source, produces an image corresponding to the same
`held of view of the linac beam used in conjunction with 10
`It.
`
`The heart of this invention lies in the recognition that
`5 verification of the correct position of a patient during
`treatment can be achieved by placing a detector be-
`tween the treatment beam source and the patient, and
`placing an x-ray tube along the axis of the treatment
`beam on the opposite side of the patient. Given the
`position of the various components, a fixed geometrical
`relationship exists that permits the direct construction
`of verification images for immediate use during treat-
`ment.
`
`A final objective of this invention is the realization of
`the above mentioned goals in an economical and com-
`mercially viable manner. This is done by utilizing com-
`ponents and methods of manufacture that are either
`already available in the open market or can be devel-
`oped at competitive prices.
`According to these and other objectives, the present
`invention consists of attachments for standard radio-
`therapy equipment comprising an x-ray tube and an
`x-ray detector placed on opposite sides of a patient
`along the main axis of the beam produced by the treat-
`ment unit. The detector is placed on a plane orthogonal
`to the axis of the treatment beam and between the beam
`
`source and the patient, while the x-ray tube is placed on
`the other side of the patient, coaxially with the treat-
`ment beam and facing the detector. As a result of this
`configuration, the radiographic view of the x-ray beam,
`as seen on the detector, is equivalent to the view pro-
`duced on the same detector by the therapeutic beam,
`varied only by parallax deviations that can be corrected
`by geometrical calculations. Accordingly, x-ray expo-
`sures and real-time verification of the position of a pa-
`tient can be obtained with the same unit used for treat-
`ment and without requiring movement of either patient
`or equipment.
`Various other purposes and advantages of the inven-
`tion will become clear from its description in the specifi-
`cation that follows and from the novel features particu-
`iarly pointed out in the appended claims. Therefore, to
`the accomplishment of the objectives described above,
`this invention consists of the features hereinafter illus-
`
`trated in the drawings, fully described in the detailed
`description of the preferred embodiment and particu-
`larly pointed out in the claims. However, such drawings
`and description disclose but one of the various ways in
`which the invention may be practiced.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is an elevational schematic representation of a so
`typical radiation therapy unit.
`FIG. 2 is an elevational schematic representation of
`the apparatus of the present invention, illustrated as an
`attachment to the typical radiation therapy unit shown
`in FIG. 1.
`
`FIG. 3 is a diagrammatic representation of the geo-
`metrical relationship of the various points of interest in
`the apparatus of FIG. 2.
`FIG. 4 shows, with reference to the geometry of the
`therapeutic unit represented in FIG. 3, the distance at
`which a diagnostic image must be positioned from the
`simulated radiation source’s position in a shielding-
`block cutter in order to have perfect correspondence
`with a picture produced with a simulator.
`FIG. 5 illustrates in schematic form the us of a fluo-
`rescent screen detector in conjunction with a mirror
`and a video camera to produce real-time verification
`images with the apparatus of the invention.
`
`Page 7 of 10
`
`Referring to the drawings, wherein like parts are
`identified with like symbols and numerals throughout
`this‘ specification, FIG. 2 illustrates in schematic eleva-
`tional representation the verification apparatus 20 of
`this invention, shown as an attachment to a standard
`radiation treatment unit (as illustrated in FIG. 1). An
`x-ray tube 22 is mounted on a support bracket 24 on the
`bottom side of the gantry 4, possibly replacing the de-
`tector 14 shown in FIG. 1. The x-ray tube is positioned
`facing up along the axis A of the beam radiated by the
`linac head 2, so that the axis of the x-ray beam is coaxial
`with that of the treatment beam. At the same time, a
`bracket 25 supporting a detector 26 is mounted on the
`gantry (or otherwise placed in the same position) be-
`tween the patient P and the linac head, as close to the
`patient as practicable. Thus, the detector 26 may be
`exposed either to the high-energy beam HR produced
`by the linear accelerator head 2 or to the low-energy
`beam LR (x rays) produced by the x-ray tube 22, or to
`both beams at the same time.
`
`FIG. 3 is a diagrammatic representation of the geo-
`metrical relationship of the various points of interest in
`the apparatus of FIG. 2. Point 2' represents the location
`of the source of high-energy radiation HR emitted by
`the linac head 2 and contained by the boundary 8’ of the
`shielding blocks 8; similarly, point 22’ corresponds to
`the location of the coaxial source of low-energy radia-
`tion LR emitted by the x-ray tube 22; and the line A’
`corresponds to the axis common to the two beams. Line
`26' represents the location of the detector 26 and point
`P’ represents a point at the boundary of the target vol-
`ume. S, and S4 are the distances of the high-energy
`radiation source (therapeutic) and the low-energy radia-
`tion source (diagnostic), respectively, from the detec-
`tor. Obviously, the view of point P’ on the detector 26',
`as projected by the x-ray beam,
`is different from the
`view seen on the same plane by the treatment beam.
`Point P’ is projected a distance Xdfrom the axis by the
`diagnostic beam (LR), but it is seen (back-projected) at
`a distance X; from the axis by the treatment beam (HR).
`The relationship between Xdand X, is a function of the
`distance d between the detector 26' and the point P’, the
`two being obviously the same for d=0. Simple trigo-
`nometry permits the determination of the following
`general relationship between these variables:
`
`Xd=IXg(I+d/S1)/(1 -d/SJ).
`
`(l)
`
`where S, and S4 are the distances of the high-energy
`radiation source and the low-energy radiation source,
`respectively, from thedetector.
`Therefore, for a given physical configuration of the
`equipment (i.e., for given values of S4, S1 and d), the
`relationship between any point in the image created by
`the x-ray tube and the location of the same point in the
`
`

`
`7
`corresponding image seen on the detector by the thera-
`peutic beam is linear and fixed. That is, one can be
`obtained from the other by a simple parallax correction
`according to the equation given above. Therefore, the
`field of view of the therapeutic beam on the detector
`can be correlated to the image projected on the plane by
`the diagnostic beam by a simple computation, and the
`two fields of view can be compared for verification
`purposes.
`In practice, shielding blocks will have been manufac-
`tured as outlined above and the verification task before
`
`the treatment
`is directed at ensuring that
`treatment
`beam, as attenuated by the blocks,
`is irradiating the
`target area delineated in the original diagnostic picture.
`If that picture was taken using a conventional simulator
`(i.e., an x-ray machine with the exact same geometry of
`the treatment unit). the shielding blocks are developed
`by conventional techniques so that the outline of the
`permitted field of view of the treatment beam coincides
`with the area delineated by the physician on the diag-
`nostic image. If, for example, point P’ corresponds to a
`point in the diagnostic image outlined by the physician
`as a boundary for radiation, the shielding blocks are
`shaped to permit the treatment beam to irradiate the
`area between the axis A’ and point P’ only. and the
`object of verification is to see whether that point is
`indeed at the boundary of the therapeutic beam during
`treatment. As mentioned above, current practice in-
`volves taking a picture with the high-energy treatment
`beam and a detector below the patient, so that the new
`image can in theory be compared with the diagnostic
`image. Unfortunately, though, the poor quality of the
`image obtained with the high-energy treatment beam
`renders it, in many cases, nearly useless in practice.
`The present invention addresses this problem by pro-
`ducing a coaxial image taken with an x-ray machine. By
`creating an x-ray image on the detector 26 through
`exposure to a low-energy beam from the tube 22, a good
`quality image of a patient‘s anatomy is obtained that can
`be corrected by the parallax relationship given above to
`produce an exact replica of the corresponding image
`seen by the therapeutic beam. Therefore, the resulting
`corrected picture is in the same scale of. and can be
`directly compared with, the original diagnostic image
`outlined for targeting the treatment area. As this can be
`done just before treatment, when the patient is posi-
`tioned for therapeutic irradiation, it can be an extremely
`useful
`tool for therapy verification. In addition, the
`verification image can be superimposed on an image
`obtained by exposing the detector on the plane 26’ to
`the shielded therapeutic beam, which in practice will
`only show the contour of the shield (i.e., the boundary
`of the radiation field) because of the high-energy radia-
`tion. These two pictures combined correspond exactly
`to the field of radiation currently targeted by the treat-
`ment beam. Therefore, they provide the radiotherapist
`with a current verification of the area targeted for treat-
`ment.
`
`According to another method of use of the apparatus
`of this invention, it is possible to use the x-ray tube 22
`also for the initial diagnostic image (normally taken on
`film); that is, it may be used as a substitute for the con-
`ventional simulator. In that case,
`the position of the
`diagnostic film in the shielding-block cutter can be ad-
`justed to permit its use to cut the shielding blocks as if
`the diagnostic image had been produced by a simulator.
`As would be obvious to one skilled in the art, FIG. 4
`shows that for a given geometry of the therapeutic unit,
`
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`8
`
`taken for example as illustrated in FIG. 3, there exists a
`distance Y at which the film must be positioned from
`the simulated high-energy radiation source's position in
`the shielding-block cutter in order to have perfect cor-
`respondence with a picture produced with a simulator.
`Again, simple trigonometry shows that distance is given
`by the following equation:
`
`Y= S,(l + d/S,)/( I —d/S4).
`
`(2)
`
`Thus, this procedure eliminates the need for a simulator
`as a separate piece of equipment. Moreover, since both
`diagnostic and verification images are taken with the
`same equipment, the x-ray source 22, no correction of
`the verification images is required for comparison with
`the target area in the diagnostic image. The two sets of
`pictures are automatically available in the same scale.
`taken from the same point and perspective, and of the
`same acceptable quality. The only requirement is the
`parallax correction of the radiation field boundary pro-
`duced by the treatment beam HR as a superimposed
`image on the detector, so that it is converted to the same
`scale of the verification image. Equation 1 above is used
`for this correction.
`
`Finally, because this invention does not require
`movement of the patient or of the treatment unit, it is
`suitable for on-line, real-time, application, which ren-
`ders it particularly valuable for radiation therapy. Many
`detectors exist that produce a real-time image. either
`directly or through computerized image enhancement
`processes. For example, FIG. 5 illustrates in schematic
`form the use of a fluorescent screen detector in conjunc-
`tion with a mirror and a video camera to produce real-
`tirne verification images with the apparatus of the in-
`vention. By appropriately positioning the mirror (such
`as, for example, at a 45 degree angle with the detector),
`the video camera can be placed outside the therapeutic
`field of radiation (at a 90 degree angle with the detec-
`tor), so that the image created on the fluorescent screen
`detector by the diagnostic beam is received by the video
`camera during the treatment session without interfer-
`ence. If the patient moves, the image on the detector
`will immediately reveal any change in the area being
`irradiated, so that immediate steps can be taken to mini-
`mize any localization error.
`Note that radiation dosages during therapy are usu-
`ally within the range of 3-10 cGy (rads), while a normal
`dose for diagnostic x-ray imaging purposes is usually
`within the range of 0.0]-0.1 cGy (rads). Therefore,
`detector sensitivity at diagnostic energies is about two
`orders of magnitude higher th