Patent Number: 062087126
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to effective utilization of portal images within a virtual wedge treatment. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. In the following, the invention is described with primary reference to a system for delivering X-ray radiation to a field of a patient, and for delimiting the field using at least one movable plate in the beam path from a radiation source. This is by way of example. Thus, the present invention is not intended to be merely limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. FIG. 2 shows a portion of an illustrative radiation treatment device 2 and portions of a treatment processing unit in detail. An electron beam 1 is generated in an electron accelerator 20. Accelerator 20 comprises an electron gun 21, a wave guide 22, and an evacuated envelope or guide magnet 23. A trigger system 3 generates injector trigger signals and supplies them to injector 5. Based on these injector trigger signals, injector 5 generates injector pulses which are fed to electron gun 21 in accelerator 20 for generating electron beam 1. Electron beam 1 is accelerated and guided by wave guide 22. For this purpose, a high frequency (HF) source (not shown) is provided which supplies radio frequency (RF) signals for the generation of an electromagnetic field supplied to wave guide 22. The electrons injected by injector 5 and emitted by electron gun 21 are accelerated by this electromagnetic field in wave guide 22 and exit at the end opposite to electron gun 21 as electron beam 1. Electron beam 1 then enters a guide magnet 23, and from there is guided through a window 7 along axis 10. After passing through a first scattering foil 15, the beam goes through a passageway 51 of a shield block 50 and encounters a second scattering foil 17. Next, the beam is sent through a measuring chamber 60, in which the dose is ascertained. If the scattering foils are replaced by a target, the radiation beam is an X-ray beam. Plate arrangement 4 comprises a pair of aperture plates 41 and 42 and an additional pair of aperture plates (not shown) arranged perpendicular to plates 41 and 42. In order to change the size of the irradiated field, the aperture plates 41 and 42 can be moved with respect to axis 10 by a drive unit 43 which is indicated in FIG. 2 only with respect to plate 41. Drive unit 43 comprises an electric motor which is coupled to plates 41 and 42 and which is controlled by a motor controller 40. Position sensors 44 and 45 are also coupled to plates 41 and 42, respectively, for sensing their positions. The area of a patient that is irradiated is known as the field. As is well known, plates 4 are substantially impervious to the emitted radiation. They are mounted between the radiation source and patient in order to delimit the field. Areas of the body, for example, healthy tissue, are therefore subjected to as little radiation as possible, and preferably to none at all. As previously described, with at least one of the plates movable, the distribution of radiation over the field need not be uniform (one region can be given a higher dose than another); further, with the gantry able to be rotated, different beam angles and radiation distributions are allowed without having to move the patient around. A central treatment processing or control unit is usually located apart from radiation treatment device 2 in a different room to protect the therapist from radiation. Treatment processing unit includes an output device, such as at least one visual display unit or monitor 70, and an input device, such as a keyboard 19, although data can be input also through data carriers, such as data storage devices. The treatment processing unit is typically operated by the therapist who administers actual delivery of a radiation treatment as prescribed by an oncologist. By utilizing keyboard 19, or other input device, the therapist enters into a control unit 76 of the treatment processing unit the data that defines the radiation to be delivered to the patient. On the screen of a monitor 70, various data can be displayed before and during the treatment. Central processing unit 18, included in treatment processing unit, is connected with the input device, e.g., keyboard 19, for inputting the prescribed delivery of the radiation treatment and with a dose control unit 61 that generates the desired values of radiation for the controlling trigger system 3. Trigger system 3 suitably adapts the pulse repetition frequency or other parameters to change the radiation output. A digital dosimetry system is particularly advantageous in order to more easily control the digital output of central processing unit 18. Central processing unit 18 suitably includes a control unit 76 for controlling execution of the treatment program in conjunction with memory 77 and a combination circuit 78 which suitably receives signals from the control unit 76 and memory 77 for combination to produce a set signal, S, that identifies a dose rate for dose rate control unit 61. Further included in the treatment system is a portal imaging device 80. A portal imaging device, as previously described, allows an image of the patient to be taken to determine if the radiation being supplied is accurately reaching the target treatment area. The image, e.g., an EPI, is suitably processed via the processing unit 18 and displayed via display unit 70 for analysis by the therapist. FIG. 3 illustrates a flow diagram representative of a preferred method of portal imaging during virtual wedge treatment through the treatment processing unit in accordance with the present invention. In a similar manner to prior virtual wedge techniques, a total treatment dose to be provided during a dynamic portion and static portion of jaw positioning is determined (steps 120 and 122). Then, rather than positioning the jaws, e.g., 41 and 42, at their initial gap position following pretreatment jaw calibration, as is conventionally done, the dynamic jaw, e.g., 41, is positioned in the final gap position indecated at 43 (step 124) in the present invention, as represented by FIG. 4a. The portal image is then taken by delivering a dose, MU.sub.image, required for the imaging device 80 (FIG. 2), e.g., 10 MUs, (step 126). Once the image is taken, appropriate adjustment of the patient positioning may then occur, if necessary, (step 128). The jaws are then positioned at their initial gap position (step 130), as shown by FIG. 4b indecated at 45, e.g., by moving dynamic jaw 41 inward. The total dynamic dose, MU.sub.jaw, is then supplied (step 132) by moving the dynamic jaw 41 from the initial gap position to the final position at a constant average speed, as further represented by FIG. 4c indicated 47, and as is well understood by those skilled in the art. Once the final gap position is reached indicated 43, a remaining dose for the treatment, MU.sub.static, is supplied (step 134), as represented by FIG. 4d. Thus, through the present invention, a portion of the dose supplied during a static portion of a virtual wedge treatment is delivered as an initial dose to allow capture of a portal image. Since the portal image is advantageously produced prior to a dynamic portion of the virtual wedge treatment, better utilization of the portal image can be made to correct for patient positioning errors. FIGS. 5a and 5b present static MU calculations for two 50 MU (dose), 3 megavolt (MV) (energy of the beam), 60 degree (wedge angle) virtual wedge treatments to further illustrate that use of the present invention is capable even under worst case virtual wedge treatment conditions. What is meant by worst case in the context of the specification is a low amount of MU during the open field part of the treatment. For the calculations of FIG. 5a, a treatment field size of 50 mm (millimeter) is considered, while for FIG. 5b, a larger treatment field size of 200 mm is considered. Typically, a small treatment field such as 50 mm requires less than about 20 MUs for EPI. As illustrated, the 50 mm virtual wedge in FIG. 5a has 38.5 MU at Dmax and central axis, i.e., the static portion of the virtual wedge treatment. Accordingly, enough dosage is supplied at the static position to accommodate providing an image dose before the dynamic portion for portal imaging in accordance with the present invention. Accordingly, a large treatment area such as 200 MV shown at FIG. 5b typically requires less than about 10 MUs for portal imaging. The 200 mm virtual wedge treatment condition has 17.5 MU at Dmax and central axis, again clearly accommodating provision of an image dose before the dynamic portion for portal imaging. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.