Detailed Monte Carlo analysis of the secondary photons coming out of the therapeutic X-ray beam of linear accelerator

External photon beam radiotherapy is often used in tumor treatment. The photons are generated from the target which had stricken by the primary electron beam (incident particles). The photon beam contains the primary photons coming directly from the target and secondary photons coming from the photon interactions with head component materials (scattered photons). Altogether is thereafter used in radiotherapy treatment. This Monte Carlo study aims to investigate and evaluate the secondary radiations (photons) in terms of fluence, energy fluence, spectral distribution, mean energy and angular spread distribution.

The secondary photons, which contributed in radiotherapy treatment, are examined and evaluated in number (fluence) and energy. At the phantom surface, the secondary photons originated in the whole linac head are mainly coming from the primary collimator. In 0.45% of secondary photons coming from the whole linac head, the primary collimator contributes by 86% and they are more energetic. However, the flattening filter and the secondary collimator contribute together by less than 14% and their photons are less energetic and then can deteriorate the beam dosimetry quality. To improve the radiotherapy treatment quality, the number of photons of low energy should be as low as possible in the clinical beam. Our work can be a basic investigation to use in the improvement of linac head configuration and specially the beam modifiers.