Document Type: Original Paper
Department of Medical Radiation Engineering Science and Research Branch, Islamic Azad University, Tehran, Iran
Introduction: Utilizing high-energy photons in the medical linear accelerator can lead to photoneutron production. An analyzation on influences of physical components of the linac head including FF and MLC, and also the dependence of therapeutic field size on the photoneutron spectrum, dose, and flux was carried out in this essay.
Materials and Methods: Fundamental components of the Varian Clinac 2100 linac’s head, performing in x-ray mode with 18MV energy were simulated by FLUKA. In order to ensure the simulation validity, PDD and lateral dose profile were measured using a PTW thimble chamber.
Results: Photoneutron spectrum analysis indicates that neutrons with highest RBE are delivered to the phantom surface, and opening the field from 0×0 to 40×40〖cm〗^2 shifts this spectrum by 24.545% to the higher energies. Target and the vicinity parts play the most prominent roles in neutron contamination. The relationship between the field size and the photoneutron dose is not linear, and it reaches a peak for 20×20cm^2. Furthermore, although using small field sizes formed by the MLC contribute to the lower dose compared to those shaped by the jaws, MLC-equipped machines result in 21.98% higher dose. Moreover, the FF removal unexpectedly increases the isocenter photoneutron dose by 11.63%. This unwanted dose can be up to 2.54 mSv/Gy for the reference field size at the isocenter, while the out-of-field dose is about 0.5 mSv/Gy for most of the field dimensions.
Conclusion: As a result, it is critical to consider this unwanted absorbed dose which is seriously influenced by implemented therapeutic conditions.