Abstract
The Monte Carlo computer program has been used to analyze the characteristics of scattered radiation in a phantom irradiated by 10 MV X-rays. The program simulated an irradiation by 10 MV X-rays with a 0 area field upon a center of 30×30×30 cm^3 water phantom. The study distinguished energy distribution of the secondary electron which was set in motion by interactions in the ring with radius R and depth d. Using these results, spectrums of the secondary electron on the beam axis of the circular fields were calculated by means of the superposition principle. Ratios of the average collision mass restricted stopping power (water/air, acrylic/air, polystyrene/air) on the beam axis of circular fields were calculated based on the electron spectrum data. Stopping power ratios calculated on spectrum of electron, which were set in motion only by first interactions of the primary photon beam, coincided with the data in AAPM protocol (1983) within 0.5%. However, stopping power ratios, which were calculated on electron spectrums with full interaction, increased with the field radius, and differed from AAPM data beyond 2% at the maximum. For precise measurement or calculation of the absorbed dose for high energy radiation, it is better to apply the mass restricted stopping power based on the electron spectrum with full interactions.
