Monte Carlo studies for the optimization of hardware used in conformal radiation therapy (in English)

Abstract

The Monte Carlo system GEANT was applied to characterize and develop devices used in conformal radiation therapy. First, the Siemens Primus accelerator head was modeled. Second, two alternative techniques for intensity modulation were investigated: (1) Two multileaf collimators were modeled with respect to their penumbra and leakage radiation. To reduce the penumbra, an optimized shape of the leaf face was determined. The effectiveness of a new collimator design for leakage reduction was confirmed, showing a decrease in the average leakage from 4.7% to 2.6%. (2) The feasibility of a scanned narrow photon beam was examined for accelerators with an electron energy of about 20 MeV. The simulated photon intensity distributions were verified by comparison with both data and the theoretical prediction. With a thin beryllium bremsstrahlung target and a purging magnet, a photon beamwidth of about 40 mm at 100 cm source-to-surface distance can be achieved. The cost of the narrow distribution is a lower yield, which increases the irradiation time. To narrow down the beamwidth, a design for a fixed collimator with parallel holes was developed that yields a lateral resolution of about 1.5 mm at 100 cm source-to-surface distance and a penumbra of less than 1 mm. The use of this collimator requires that the scanned incident electron beam is always perpendicular to the target. This approach has the potential to improve both the cost-effectiveness and the beam characteristics of intensity modulated radiation therapy. © 1999, American Association of Physicists in Medicine. All rights reserved.