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

Abstract

Background:The recent trend toward 10 MV 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 10 MV to negligible at 6 MV has scarcely been characterized quantitatively, yet can elucidate important practical insights. Purpose: To measure photoneutron yields in a medical linac at 8 MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6 MV while reducing photoneutron production which is present at 10 MV. Methods: A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15 MV 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. Results:The photoneutron production for energies below 10 MV 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 8 MV was about 1/10 of the value at 10 MV, and about 10 times higher than detector background at 6 MV. Conclusions: Photoneutron production drops off below 10 MV, but is still present at 8 MV. An 8 MV beam is more penetrating than a 6 MV 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 10 MV beams considering the benefits to facility requirements and non-therapeutic patient dose.

In reply to Masjedi et al

“…Finally, we note that for other reports of photoneutron yield, significant variation in results exist depending on the type of medical linear accelerator used in the study (see for instance, Table 2 of Naseri and Mesbahi, 2 and Figure 4 of Carlone and colleagues 1 ). This is presumably due to the inherent difficulties of conducting this type of experiment in a hospital environment, and the variation in composition and construction of different medical linear accelerators.…”

“…Medical linear accelerators typically use a 5.5 MeV electron beam to produce a 6 MV beam. Since this is below the activation threshold for most common elements (see Figure 5 in our paper 1 ), and since medical linear accelerator OEMs do not include other energies below 10 MV in commercially available systems, there was not any data available in the literature at any energy below 10 MV for a commercially available medical linear accelerator. We believe this to be an important area where more data will benefit the radiotherapy community since it may enable and help to justify technique development for volume‐based radiotherapy treatments at intermediate energies.…”

“…We thank Dr. Masjedi for taking the time to review our paper, Measurement of neutron yield for a medical linear accelerator below 10 MV, 1 and to provide thoughtful commentary on our experimental methods. The motivation for this work was to address the lack of experimental data in the literature regarding neutron doses from medical linear accelerators at intermediate energies, between 6 and 10 MV.…”