Neutrons and carcinogenesis: a cautionary tale.
The best estimates for radiation induced cancer and leukemia are based on the Japanese survivors of Hiroshima and Nagasaki. With the earlier dosimetry systems of the 1960s, it was possible to drive an RBE (relative biological effectiveness) for neutrons from the Japanese data, because it was thought that there was a significant neutron dose at Hiroshima compared with Nagasaki. The estimated RBE of about 20 was consistent with laboratory estimates for oncogenic transformation in vitro and tumors in animals. The revised dosimetry of the 1980s [DS 86] essentially eliminated the neutron component at Hiroshima, and consequently removed the only neutron RBE estimate based on human data. However, recent neutron activation measurements indicate that there may indeed have been thermal neutrons at Hiroshima, and measurements of the ratio of inter- to intra-chromosomal aberrations in peripheral lymphocytes of survivors also tend to indicate that the biologically effective dose was dominated by neutrons. Another area in which the large biological effectiveness of neutrons assumes importance is the production of photoneutrons in high energy medical linear accelerators (Linacs). An increasing number of accelerators operating in the 18 to 20 MV range are coming into routine clinical use and at this energy, photoneutrons generated largely in the collimators result in a total body dose to the patient. The increased risk of second malignancies must be balanced against the slight improvement in percentage depth doses compared with more conventional machines operating at 6 to 10 MV, below the threshold for photoneutron production.
Hall
E J
EJ
Center for Radiological Research, Columbia University, New York, NY 10032, USA.
D002471:Cell Transformation, Neoplastic; D002869:Chromosome Aberrations; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D018499:Linear Energy Transfer; D008297:Male; D009381:Neoplasms, Radiation-Induced / Q000209:etiology*; D009502:Neutrons; D010315:Particle Accelerators; D011829:Radiation Dosage; D012062:Relative Biological Effectiveness; D018570:Risk Assessment; D014916:Whole-Body Irradiation