Estimate of the risk in radiation therapy due to unwanted neutrons.

The integral dose of accelerator-produced leakage neutrons to patients undergoing high-energy photon therapy is estimated and compared to other sources of integral dose. The leakage neutron component contributes about 5 g rad (1 rad = 10(-2) Gy) for a typical treatment course of 5000 rad. When averaged over a 70-kg tissue volume, the corresponding dose amounts to only 0.36 rad. From this, the risk of inducing fatal malignancies by leakage neutrons is estimated to be about 50 x 10(-6) per year following treatment. This is compared to other risks to which the patient is unavoidably exposed, and it is argued that the unwanted neutrons pose such small additional risk that regulatory intervention is not warranted. This assessment is performed without reference to neutron RBE or quality factor.