Measurement of neutron yield for a medical linear accelerator below 10 MV.

Abstract

The recent trend toward 10MV for volumetric radiotherapy treatment such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), and stereotactic ablative body radiotherapy (SABR) introduces photoneutron production, with implications for non-therapeutic patient dose and additional shielding requirements for treatment room design. The sharply nonlinear drop-off in photoneutron production below 10MV to negligible at 6MV has scarcely been characterized quantitatively, yet can elucidate important practical insights. To measure photoneutron yields in a medical linac at 8MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6MV while reducing photoneutron production which is present at 10MV. A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15MV by calibrating the bending magnet and adjusting other beam generation parameters. Neutron dose within the treatment room was measured using an Anderson-Braun type detector over a continuum of intermediate energies. The photoneutron production for energies below 10MV was measured, adding to data that is otherwise scarce in the literature. Our results are consistent with previously published results for neutron yield. We found that the photoneutron production at 8MV was about 1/10 of the value at 10MV, and about 10 times higher than detector background at 6MV. Photoneutron production drops off below 10MV, but is still present at 8MV. An 8MV beam is more penetrating than a 6MV beam, and may offer a suitable tradeoff for modern radiotherapy techniques such as VMAT, SRS, and SABR. Further studies are needed to better understand the impact on treatment plan quality between 8 and 10MV beams considering the benefits to facility requirements and non-therapeutic patient dose.