Linac Leakage Dose Received by Patients Treated Using Non-coplanar Radiotherapy Beams

As non-coplanar beams are increasingly used to deliver cranial and extra-cranial stereotactic radiotherapy treatments, radiation leakage from the accelerating waveguide, bending magnet and other components of the medical linear accelerator (linac) that are not conventionally brought into proximity to the patient becomes increasingly concerning. In this study, the leakage dose in the patient plane was measured using optically stimulated luminescence dosimeters placed along the treatment couch, at 10 cm intervals. A Varian iX linac was operated in 6 and 10 MV photon mode, with all jaws and multi-leaf collimators closed. Dose measurements were made (a) using a “standard” setup, with couch and gantry at zero degrees and (b) using a worst-case non-coplanar setup, with the couch at 90° and the gantry at 30° (rotated over the couch). Results indicated that the leakage dose in the patient plane was uniformly low (less than 2 cGy/10,000MU) at all measurement positions for both energies, using the standard setup. However, when the gantry and couch were rotated, there was a systematic increase in dose to 4.2 cGy/10,000MU at a point 80 cm from isocentre (below the bending magnet). While these doses are within recommended out-of-field dose limits for static beam treatments, if IMRT/VMAT factors are applied then the leakage dose to the patient from non-coplanar treatments may become unacceptable. Specific checks of out-of-field dose from non-coplanar beam directions are advisable prior to acceptance of new or modified linacs.