SU-FF-T-272: Measurement of Leakage Radiation From Fixed Electron Applicators On Siemens Primus Accelerators

Purpose: To characterize leakage radiation from fixed electron applicators on Siemens Primus accelerators and to ascertain whether placement of bolus and/or leadrubber outside the field on a patient's skin reduces dose to the skin from leakage radiation. Method and Materials: First, measurement of leakage from electron applicators (10×10–25×25 cm2) on a Primus unit was performed as a function of vertical position along and lateral distance from applicator body using XV films and cylindrical ion chamber with 1 cm build‐up cap. Second, using a parallel‐plate chamber inserted at the surface of a “solid water” phantom, the leakage at 4 cm from applicator body was measured after “solid water” slabs of thicknesses 0–3 cm were placed between detector surface and applicator sidewall. Finally, to determine the thickness of leadrubber needed for shielding leakage radiation on a patient's skin, transmission of leakage from 20×20 cm2 applicator through various thicknesses (0–6 mm) of leadrubber was measured.Results: Maximum leakage observed at 2 cm from applicator body at the front and right sides of applicator were 16% and 14%, respectively; these maxima were recorded for 18 MeV electron beams and applicator sizes of ⩾20×20 cm2. At lateral distances of ⩾4cm and ⩾10cm from applicator body, the maximum leakage observed was <10% and ⩽5%, respectively, for each energy and applicator size combination. In addition, introduction of water‐equivalent material of thicknesses 1–3 cm or sheets of leadrubber of thicknesses 1.5–6 mm between the detector‐phantom surface and applicator sidewall for 6–18 MeV electron beams, respectively, attenuated the leakage radiation by ⩾75%. Conclusions: The highest average leakage observed at a distance of 2 cm from applicator body was 15%; this value exceeds the IEC 60601‐2‐1 recommended limit of 10%. Dose to skin from leakage can be minimized by shielding with bolus or leadrubber.