Method and apparatus for calibration of radiation therapy equipment and verification of radiation treatment

#1. Field of the Invention

#This invention relates to radiation therapy equipment for the treatment of tumors or the like and specifically to an improved method of characterizing the radiation beam of such systems and confirming the dose received by the patient using a portal image of radiation exiting the patient.

#2. Background of the Invention

#The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration several preferred embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims herein for interpreting the scope of the invention.

#FIG. 1 is a perspective view of a radiation therapy system such as may be used to practice the present invention having a portal imaging device for receiving radiation transmitted through the patient and providing portal image data to an attached computer system;

#FIG. 2 is a schematic representation of the radiation therapy system of FIG. 1 showing the path of the radiation through a collimator, attenuating blocks and a phantom to a portal imaging device and showing the beam fluence profile and portal image;

#Referring to FIG. 1, a radiation therapy system 10, suitable for use in the present invention, includes a radiation source 12 producing a radiation beam 14 directed across a patient support table 16 that may hold a patient (not shown) or a calibration phantom 32 (shown in FIG. 2). The radiation beam 14 may be a megavoltage x-ray beam (e.g., 6-10 megavolts) or other photon source suitable for radiation therapy. The radiation beam 14, as shown, is collimated to a fan shape; however, the invention is equally applicable to “cone beams” of radiation.

#After passing through the patient support table 16 and either the patient or a calibration phantom 32, the beam 14 is received by an electronic portal imaging device (EPID) 18 having a detector surface 19 corresponding to the cross-sectional area of the beam 14. EPID 18 produces electronic signals providing measurements of the dose of the radiation received at a detector surface 19 at regularly spaced positions over the detector surface 19. Thus, the portal imaging device 18 generally provides the data of a projection image through the patient or phantom 32 along an axis of the radiation beam 14 at an arbitrary angle.

#The EPID 18 may be a liquid-filled ionization chamber matrix such as is manufactured by Varian & Associates, San Jose, Calif., which generally provides a detector surface 19 composed of two circuit boards (not shown) containing 256 wires arranged at right angles to each other. The circuit boards are separated by a one millimeter thick layer of trimethylpentane which serves as an ionization medium. The circuit boards and ionization medium effectively create matrix ion chambers with 256 rows and 256 columns. Each such chamber has dimensions of 1.27 mm×1.27 mm×1 mm.