An estimate of the patient risk in cyclotron neutron radiotherapy using mouse testes as a biological test system

Decrease in mouse testes DNA 28 days after exposure has been used to biologically characterize the University of Washinton fast neutron radiotherapy facility. The cyclotron fast neutron RBE for testes damage was found to be independent of depth in a tissue equivalent absorber. However, the RBE increased with decreasing field size and was greater in the penumbra as compared to the the primary beam. The rad dose in the shielded region of the facility was 2.8% of the primary beam rad dose with a patient phantom in the primary beam. Under these conditions, the average RBE for testes damage was 4.9 in the shielded region as compared to 3.0 in the primary beam. Measurement of Na activation at depths in an absorber indicated that neutrons in the shielded region are fairly penetrating. Based on these and other data, an estimate of the lethal carcinogenisis risk to a patient population receiving neutron therapy is 2% over a 15-yr period and 18% over a 30-yr period post-therapy. (author)