Use of bubble detectors to characterize neutron dose distribution in a radiotherapy treatment room used for IMRT treatments

Neutrons are an undesirable by-product of megavoltage linear accelerator (linac) units operating at or above 10 MV. This work characterizes the distribution of neutrons produced by a medical radiotherapy treatment unit delivering 10 MV photons. Neutron levels were measured at various locations in a linac treatment room for a newly installed Varian Clinac iX (Varian Medical Systems, Palo Alto, USA) with 6 and 10 MV capabilities. Measurements were made using fast neutron dosimeters (BD-PND) calibrated to Am-Be with sensitivities of ∼ 0.07 bubbles/μSv and ∼ 1.8 bubbles/μSv (Bubble Technology Industries (BTI), Chalk River, Canada). Because of the potential advantages of using 10 MV for intensity modulated radiotherapy (IMRT) treatments of the prostate and other tumours at depths greater than 10 cm, neutron contamination contributing to the patient dose is very relevant. The results of this work, therefore, have direct clinical implications and neutron contribution to patient dose and neutron shielding were investigated. Patient neutron doses for a 10 MV IMRT fraction ranged between 0.06 and 0.6% of the prescribed X-ray dose at iso centre for closed and open fields, respectively. At 1 m from iso centre, the dose in the patient plane was found to be 0.036% of the prescribed X-ray dose. Neutron survey results gave neutron doses of about 1.5 μSv per 200 cGy IMRT fraction at the entrance door to the vault and a projected annual neutron dose of 6 mSv. Use of 10 MV beams for IMRT treatments is not expected to have significant adverse effect on either the patient or staff doses. (author)