Decrease in mouse testes DNA 28 days after exposure has been used to biologically characterize the University of Washington 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 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 carcinogenesis risk to a patient population receiving neutron therapy is 2 % over a 15-yr period and 18 % over a 30-yr period post-therapy.
Use of neutron radiation as a potential improvement in the treatment of human cancer currently is being investigated at several radiation biology laboratories and therapy centers. Review of the relevant neutron radiobiological data for various tumors and late normal tissue effects in rodents indicates no definitive biological advantages of neutrons over conventional photon irradiation. The radiobiological data are corroborated by results from Phase I and I1 clinical studies at the University of Washington involving 74 neutron and 53 mixed beam (neutron/photon) treated patients with glioblastoma multiforme or advanced squamous cell carcinoma of the head and neck region. An analysis of the clinical data indicates no definitive improvement with regard to 2-yr survival rate and mean survival time for neutrontreated patients as compared to 218 photon-treated patients having similar severity of disease. Normal tissue complication rates following neutron or mixed (neutron/photon) therapy appear to be equal to or greater than those following conventional photon therapy.
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