Slit x-ray beam primary dose profiles determined by analytical transport of Compton recoil electrons

Accurate measurement of radiation beam penumbras is essential for conformal radiotherapy. For this purpose a detailed knowledge of the dosimeter's spa tial response is required. However, experimental determination of detector spatial response is cumbersome and restricted to the specific detector type and beam spectrum used. A model has therefore been developed to calculate in slit beam geometry both dose profiles and detector response profiles. Su mmations over representative photon beam spectra yield profiles for polyene rgetic beams. In the present study the model is described and resulting dos e profiles verified. The model combines Compton scattering of incident phot ons, transport of resulting electrons by Fermi-Eyges small-angle multiple s cattering theory, and functions to limit electron transport. This analytic model thus yields line spread kernels of primary dose in a water phantom. I t is shown that the spatial response of an ideal point detector to a primar y photon beam can be well described by the model; the calculations are veri fied by measurements with a diamond detector in a telescopic slit geometry in which all dose contributions except for the primary dose can be excluded . Effects of photon detector behavior, source size of the linear accelerato r (linac) and detector size are studied. Measurements show that slit dose p rofiles calculated by means of the kernel are accurate within 0.1 mm of the full-width at half-maximum. For a theoretical point source and point detec tor combined with a 0.2 mm wide slit, the full-width half-maximum values of the slit beam dose profiles are calculated as 0.37 mm and 0.42 mm in a 6 M V and 25 MV x-ray beam, respectively. The present study shows that the mode l is adequate to calculate local dose effects that are dominated by approxi mately mono-directional, primary photon fluence. The analytic model further provides directional electron fluence information and is designed to be ap plied to various detectors and linac beam spectra. (C) 2000 American Associ ation of Physicists in Medicine.