Radiation protection measurements around a 12 MeV mobile dedicated IORT accelerator

The aim of this study is to investigate radioprotection issues that must be addressed when dedicated accelerators for intraoperative radiotherapy (IORT) are used in operating rooms. Recently, a new version of a mobile IORT accelerator (LIAC Sordina SpA, Italy) with 12 MeV electron beam has been implemented. This energy is necessary in some specific pathology treatments to allow a better coverage of thick lesions. At an electron energy of 10 MeV, leakage and scattered x-ray radiation (stray radiation) coming from the accelerator device and patient must be considered. If the energy is greater than 10 MeV, the x-ray component will increase; however, the most meaningful change should be the addition of neutron background. Therefore, radiation exposure of personnel during the IORT procedure needs to be carefully evaluated.

In this study, stray x-ray radiation was measured and characterized in a series of spherical projections by means of an ion chamber survey meter. To simulate the patient during all measurements, a polymethylmethacrylate (PMMA) slab phantom with volume 30 x 30 x 15 cm3 and density 1.19 g / cm3 was used. The PMMA phantom was placed along the central axis of the beam in order to absorb the electron beams and the tenth value layer (TVL) and half value layer (HVL) of scattered radiation (at 0 degrees, 90 degrees, and 180 degrees scattering angles) were also measured at 1 m of distance from the phantom center. Neutron measurements were performed using passive bubble dosimeters and a neutron probe, specially designed to evaluate ambient dose equivalent H*(10).

The x-ray equivalent dose measured at 1 m along the beam axis at 12 MeV was 260 microSv/Gy. The value measured at 1 m at 90 degrees scattering angle was 25 microSv/Gy. The HVL and TVL values were 1.1 and 3.5 cm of lead at 0 degrees, and 0.4 and 1 cm at 90 degrees, respectively. The highest equivalent dose of fast neutrons was found to be at the surface of the phantom on the central beam axis (2.9 +/- 0.6 microSv/Gy), while a lower value was observed below the phantom (1.6 +/- 0.3 microSv/Gy). The neutron dose equivalent at 90 degrees scattering angle and on the floor plane on the beam axis below the beam stopper was negligible.

Our data confirm that neutron exposure levels around the new dedicated IORT accelerator are very low. Mobile shielding panels can be used to reduce x-ray levels to below regulatory levels without necessarily providing permanent shielding in the operating room.