Radiation survey around a Liac mobile electron linear accelerator for intraoperative radiation therapy

Abstract

The aim of this study was to perform a detailed analysis of the air kerma values around a Liac mobile linac working in a conventional operating room (OR) for IORT. Liac delivers electron beams at 4-6-8-10 MeV. Radiation survey to determine photon leakage and scatter consisted of air kerma measurements on a spherical surface of 1.5 m radius, centered on the titanium exit window of the accelerating structure. Measurements were taken using a 30 cc calibrated cylindrical ion chamber in three orthogonal planes, at the maximum electron energy. For each point, 10 Gy were delivered. In selected points the quality of x-ray radiation was determined and additional measurements were performed for all energies. The contribution on photon scatter due to metallic internal shielding in IORT was also evaluated. Kerma values outside the OR were calculated in seven locations and compared to measurements performed using a survey meter. In the two vertical planes, kerma values ranged from approximately 6 (upper and rear sides of Liac) to 320 microGy (laterally to beam stopper), while in the horizontal plane values lower than 18 microGy were found. At 10 MeV, transmission behind 1 cm lead shield was 42%. The use of internal shielding appeared only to slightly increase photon scatter. Kerma values outside the OR were generally lower than 1 mGy for an annual workload of 200 patients. Liac can safely work in a conventional OR, while need of added shielding mainly depends on patient workload. Our data can be useful for Centers planning to implement an IORT program using a mobile linac, to estimate in advance shielding requirements and admitted workload.