Analysis and evaluation of secondary photons originated in jaws as contamination particles at the phantom surface

Abstract

The present work is focused on the study of the geometry and material of a beam modifier (jaws), which is crucial for Linac improvement by reducing secondary photons emergent from it. They have negative effects when the cancer was treated and especially in the deep tumor because their energy is deposited in shallower depth and could destroy the healthy cells that surround the treatment volume. The purpose of this study is to investigate and evaluate the characterizations of secondary photons originated in jaws in terms of fluence profile, energy fluence profile, energy fluence distribution, spectral distribution, and angular spread distribution. This work was performed by using the BEAMnrc Monte Carlo and BEAMDP codes. The jaws are a potential source of secondary photons nearest to the phantom. The number of secondary photons emergent from X-jaw (in-plan jaw) is higher and they are more energetic in comparison to secondary photons of Y-jaw (cross-plan jaw). Therefore, the most significant result is on the angular spread distribution of secondary photons for each pair jaw. For Y-jaw, the majority of photons are scattered and spread with high angle degree that means these photons can fall out of the irradiation field and affect the healthy cells but for the X-jaw, most of the secondary photons are within the irradiation field and they can affect healthy cells mainly only at the entrance of treatment volume.

Keywords

• secondary collimator
• monte carlo simulation
• secondary photons
• beamnrc code
• beamdp code.