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

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.

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.

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.

The probable source of the non-compliant neutron dose outside the treatment vault was the presence of the lead slab.