Purpose: In Spatially Fractionated Grid Radiation Therapy (SFGRT), the grid block in the collimation system modifies the neutron production, photon scattering, and electron contamination in and out of the radiation field. The main purpose of this study is to investigate the photoneutron production and the secondary cancer risk in SFGRT with 18-MV photons.
Methods: Using the MCNPX code, three types of grid made of brass, cerrobend, and lead, located at the tray of an 18-MV medical linear accelerator, were simulated. Plans with different field sizes were generated to compare the neutron and electron contaminations. A revised female MIRD phantom with an 8-cm spherical tumor inside the liver was simulated to study the lifetime risks of fatal cancer due to neutrons and scattered photons using a 20-Gy SFGRT plan and a 40-Gy CFRT plan with a 9 × 9 cm2 field size.
Results: The neutron fluences per electron-history in SFGRT plans with brass, cerrobend and lead blocks are averagely 55%, 31% and 31% less than that of the CFRT plan, respectively. In both SFGRT and CFRT plans with a 9 × 9 cm2 field size, the contribution of the primary and scattered photons to the fatal cancer risk is 2 times or more than the photoneutrons. The total risks from photons in SFGRT with brass, cerrobend, and lead blocks are 1.733, 1.374, and 1.260%, respectively. These values are significantly lower than the total photon-risk in CFRT (5.827%).
Conclusion: In the brass, cerrobend, and lead grids, the attenuation of photoneutrons outweighs its photoneutron production in SFGRT. The total cancer risks in the SFGRT plan were less than the risk in the CFRT plan. The grids also decrease the amount of electron contamination.