Monte Carlo simulation of neutron dose equivalent by photoneutron production inside the primary barriers of a radiotherapy vault.

PURPOSE: To evaluate the neutron dose equivalent produced by photoneutrons inside the primary barriers of a radiotherapy vault. METHODS: Monte Carlo simulations were performed for investigating the production of photoneutrons as well as neutron shielding requirements. Two photon beams of 15 and 18 MV struck sheets of steel and lead, and the neutron doses were calculated at the isocenter (Piso) and at a distance of 50 cm from the inside wall (Pwall) while delivering 1 Gy to the patient. The proper thicknesses of borated polyethylene (BPE) and concrete were simulated to reduce neutron contamination. RESULTS: When the primary barrier consisted of a concrete alone, the neutron doses at Piso were 0.5 µSv/Gy and 12.8 µSv/Gy for 15- and 18-MV, respectively. At Pwall, the neutron doses were 15.8 µSv/Gy and 318.4 µSv/Gy for 15- and 18-MV, respectively. When 15 MV photons interacted with metal sheets, the neutron doses were 0.4-22.2 µSv/Gy at Piso and 15.8-812.5 µSv/Gy at Pwall, depending on the thickness and material of the metal sheets and neutron shielding. In the case of 18 MV photons with the same configuration, the neutron doses were 0.9-59.5 µSv/Gy and 73.9-5006.1 µSv/Gy for Piso and Pwall, respectively. The neutron dose delivered to the patient was reduced to the level of the dose delivered with a concrete barrier by including a 10-cm-thick BPE for each beam. CONCLUSIONS: When the primary barrier shielding is designed with a metal sheet inside for high energy, proper neutron shielding should be constructed to avoid undesirable photoneutron dose.