Monte Carlo (PENELOPE code) study of the x-ray beams from SL Linacs (Elekta)

This study investigates the capabilities of the PENELOPE code for the simulation of three photon beams (6, 18 and 25 MV x-rays) from SL-Elekta medical linear accelerators (linacs). The radiation head has been separated in two parts, the target section and the collimator section. At the end of each section, phase space files containing all the par-tides and their parameters were stored. These generated files were validated according to a recent method [Med. Phys. 26 (8), 1999] already applied to EGS4 code. The sources of head scattered radiation have been identified and the proportion of contaminant electrons and positrons have been calculated for all the configuration beams. To check the validity of the photon beam descriptions, the characteristics (depth dose curves, cross profiles, output factors) have been determined for several field sizes ranging from 4x4 cm to 40x40 cm for a SSD of 100 cm and compared to measurements. Calculated linear attenuation coefficients in water for on axis and off axis positions (5, 10, 15 cm from beam axis) were compared to measured values. Primary and scatter dose separation in water was investigated, and the scattering functions and peak scatter factors for the studied field sizes have been evaluated. Incident electron mean energies on the target have been determined to 6, 17 and 21 MeV for 6, 18 and 25 MV x-ray beams respectively. The mean energy photon beam spectrum varies with field size by 26%, 33% and 18% for the three beam qualities between 4x4 cm and 40x40 cm square fields. Measured and calculated linear attenuation coefficients agree within 1.2%. Dose distributions in water and output factors are correlated with experimental values.