IRREGULAR FIELD MODELING OF AN 18 MV X-RAY BEAM USING THE PROGRAM CBEAM.

Computer modeling of irregularly shaped photon fields is an important problem, especially with the advent of CT-assisted treatment planning. Beam profiles from an 18 MV linear accelerator were measured with an ion chamber for a series of square and irregular fields. Beam profile constants were determined from four different depths from the largest square field studied (15 multiplied by 15 cm**2). The best set of these constants was used in modeling the irregular fields. The block side of the irregular field was calculated by using either a subtraction, bar wedge, or shifting center technique. Distances from the central axis of the beam to the edge of the blocked and normal side were determined for both the calculated and measured fields.