Dose GRID increase the unwanted photoneutron dose to the patients in GRID therapy?

Introduction: GRID therapy is a treatment modality in which a high dose of radiation in a single fraction is given to the tumor. This technique previously was used to treat bulky tumors with orthovoltage beams. Recently, studies have been conducted to suggest that GRID therapy with megavoltage photons can be useful in palliative treatments. On the other hand, there has always been concern about the unwanted production of photoneutrons by LINAC head in treating with energies above 10 MeV. This problem is more worrying in modern radiotherapy techniques in which more monitor units are used, such as Intensity Modulated Radiotherapy (IMRT) and GRID therapy. So far, the effect of the GRID on photoneutron contamination has not been completely studied. While in terms of radiation protection, it is important to evaluate the GRID effect on photoneutron dose to the patients. In addition, one of the limitations of neutron dosimeters is that even with the most precise tools, one cannot achieve uncertainty less than 10% in measurements, while by Monte Carlo (MC) simulations, these limitations can be removed very much and a more accurate estimate of neutron dose can be provided. Materials and Methods: In this study, using the Monte Carlo code MCNPX, the main components of the Varian Clinac 2100 C/D were simulated. For 4×4, 10×10, and 20×20 cm2 field sizes, the Percentage Depth Dose (PDD) and profile curves were calculated and benchmarked with the measured data. After the model tuning, the neutron source strength of the LINAC (Q), the distributions of ambient dose equivalent (H*(10)) of neutrons were calculated on the treatment table for 10×10 cm2 field size, in both cases: with/without a typical GRID. Results: Our results indicate that, the GRID increases the neutron source strength by only 0.3% and also, one can see that the GRID reduces the ambient dose equivalent of neutrons at the isocenter (IC) 48.9%. In addition the average reduction in the ambient dose equivalent of neutrons is 38.5% at the intervals of less than 30 cm from the IC. Conclusion: It was found that in GRID therapy technique compared to conventional radiotherapy, the neutron dose on the treatment table is dramatically reduced. Therefore; there will be no concern about the increase of unwanted neutron dose in GRID therapy.