Study of Photoneutron Production for the 18 MV Photon Beam of the Siemens Medical linac by Monte Carlo Simulation

Background:

Considering the importance of photoneutron production in linear accelerators, it is necessary to describe and measure the photoneutrons produced around modern linear accelerators.

Objective:

The aim of the present research is to study photoneutron production for the 18 MV photon beam of a Siemens Primus Plus medical linear accelerator.

Material and Methods:

This study is an experimental study. The main components of the head of Siemens Primus Plus linac were simulated using MCNPX 2.7.0 code. The contribution of different components of the linac in photoneutron production, neutron source strength, neutron source strength and photon and electron spectra were calculated for the flattening filter and flattening filter free cases for the 18 MV photon beam, and was scored for three fields of 5 × 5 cm², 10 × 10 cm² and 20 × 20 cm² in size.

Results:

The results show that the primary collimator has the largest contribution to production of neutrons. Moreover, the photon fluence for the flattening filter free case is 8.62, 6.51 and 4.62 times higher than the flattening filter case for the three fields, respectively. The electron fluences for the flattening filter free case are 4.62, 2.93 and 2.79 times higher than with flattening filter case for the three fields under study, respectively. In addition to these cases, by increasing the field size, the contribution of neutron production related to the jaws is reduced, so that when the field size increases from 5 × 5 cm² to 20 × 20 cm², a 17.93% decrease in photoneutron production was observed.

Conclusion:

In all of the accelerators, the neutron strength also increases with increasing energy. The calculated neutron strength was equal to 0.83×10¹² neutron Gy ⁻¹ at the isocenter.

Evaluation of MLC Thickness and Composite Effects on Collimation Parameters using EGSnrc and IAEA Phase Space Data

Background

Recently, multileaf collimators (MLC) have become an important part of any LINAC collimation system because they reduce the treatment planning time and improve the conformity. Important factors that affect MLCs collimation performance are leaves material composition and their thickness.

Objective

In this study, we investigate main dosimetric parameters of a typical MLC including dose in the buildup point, physical penumbra as well as average and end leaf leakages. Effects of the leaves geometry and density on these parameters are evaluated.

Materials and Methods

In this analytical study, calculations were performed by using phase space data for Varian ix just above MLC and BEAMnrc/DOSXYZnrc for SSD=100cm and in a water phantom.

Results

Based on the results, a new MLC with improved dosimetric parameters is proposed. The physical penumbra for proposed MLC is 4.7mm was compared to 5.16 mm for Millennium 120 leaf. Average leakage in our design is reduced to 1.16% compared to 1.73% for Millennium 120 leaf, the end leaf leakage suggested design also reduced to 4.86% compared to 7.26% for Millennium 120 leaf.

Conclusion

The results show that the proposed MLC could improve the dosimetric parameters and conformity of treatment planning.