Non‐compliant neutron dose associated with an internal lead slab in a radiotherapy treatment vault

BACKGROUND: In high energy radiotherapy treatment vaults, photoneutrons can be produced by direct photon interactions with barrier materials and have a mean energy of approximately 20% of the nominal photon energy. Excess photoneutrons outside a treatment vault are always a concern due to their significant radiobiological efficacy, or Q value. METHODS: A pre-clinical regulatory inspection of a newly installed linear accelerator detected legally non-compliant photoneutron dose levels in a controlled area outside the treatment vault. The non-compliant dose was measured for a 10 MV photon beam external to the primary barrier improperly containing an internal 2.0 cm lead slab. RESULTS: Based on measurements at a photon energy of 10 MV, using a workload W = 1500 Gy/week, a use factor of U = 1, and an occupancy factor of T = 0.5-which are NCRP-151 recommended values-an equivalent neutron weekly dose of 443 ± 38 µSv was calculated for a point in a controlled area external to the vault (point A in Figure 1). This results in a calculated annual neutron dose at this point of 22.1 ± 2.2 mSv, which exceeds the legal occupational annual limit of 20 mSv for a controlled area in our jurisdiction. The lead slab was removed. Neutron measurements taken subsequent to the removal of the slab detected values slightly above natural background. CONCLUSIONS: The probable source of the non-compliant neutron dose outside the treatment vault was the presence of the lead slab.