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

Mario Ciocca, Guido Pedroli, Roberto Orecchia, Andrea Guido, Federica Cattani, Raffaella Cambria, Umberto Veronesi, Mario Ciocca, Guido Pedroli, Roberto Orecchia, Andrea Guido, Federica Cattani, Raffaella Cambria, Umberto Veronesi

Abstract

The aim of this study was to perform a detailed analysis of the air kerma values around a Liac mobile linear accelerator working in a conventional operating room (OR) for IORT. The Liac delivers electron beams at 4, 6, 8 and 10 MeV. A 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 cm3 calibrated cylindrical ion chamber in three orthogonal planes, at the maximum electron energy. For each point, 10 Gy was delivered. At selected points, the quality of x-ray radiation was determined by using lead sheets, and measurements were performed for all energies to investigate the energy dependence of stray radiation. The photon scatter contribution from the metallic internal patient-shielding in IORT, used to protect normal tissues underlying the target, was also evaluated. At seven locations outside the OR, the air kerma values derived from in-room measurements were compared to measurements directly performed using a survey meter. The results, for a delivered dose of 10 Gy, showed that the air kerma values ranged from approximately 6 microGy (upper and rear sides of the Liac) to 320 microGy (lateral to beam stopper) in the two orthogonal vertical planes, while values lower than 18 microGy were found in the horizontal plane. At 10 MeV, transmission behind 1 cm lead shield was found to be 42%. The use of internal shielding appeared to increase the photon scatter only slightly. Air kerma values outside the OR were generally lower than 1 mGy for an annual workload of 200 patients. Thus, the Liac can safely work in a conventional OR, while the need for additional shielding mainly depends on patient workload. Our data can be useful for centers planning to implement an IORT program using a mobile linear accelerator, permitting radiation safety personnel to estimate in advance the shielding required for a particular workload.

Figures

Figure 1
Figure 1
Side (a) and front views (b) of the Liac in parking position inside the operating room. Measurement planes (A, B, C), together with position n. 1 (0°) and direction from position n. 1 forward, are reported (not to scale). In the experimental set‐up, the Liac was positioned to simulate a standard treatment geometry, with the radiation head raised up with respect to the parking position shown. The beam stopper and an example of a movable vertical shield in front of the Liac, as well as an applicator docked to the head, are also shown.
Figure 2
Figure 2
Floor plan of the operating room and the adjacent areas. The typical working position of the Liac (outlined) as well as the points (A to F) where the air kerma values were calculated, are shown.

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Source: PubMed

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