Neutrons and carcinogenesis: a cautionary tale

Area of Science:

• Radiation biology
• Medical physics
• Epidemiology

Background:

• Radiation-induced cancer and leukemia risk estimates primarily rely on Japanese atomic bomb survivor data.
• Early dosimetry suggested significant neutron doses at Hiroshima, allowing for a relative biological effectiveness (RBE) estimation for neutrons.
• Revised dosimetry (DS86) minimized the neutron component, removing human-based RBE estimates.

Purpose of the Study:

• To re-evaluate the neutron dose and its biological effectiveness in Hiroshima survivors.
• To assess the implications of neutron RBE for radiation risk assessment.
• To examine the significance of photoneutron production in medical linear accelerators (Linacs).

Main Methods:

• Analysis of neutron activation measurements.
• Examination of inter- to intra-chromosomal aberration ratios in survivor lymphocytes.
• Review of photoneutron production in high-energy medical linear accelerators.

Main Results:

• Recent measurements suggest thermal neutrons may have been present at Hiroshima.
• Lymphocyte aberration data indicate a biologically effective dose dominated by neutrons.
• High-energy medical Linacs (18-20 MV) produce photoneutrons, contributing to patient dose.

Conclusions:

• Neutron RBE estimates based on Hiroshima data may need revision.
• The biological effectiveness of neutrons is crucial for accurate radiation risk assessment.
• Balancing the risks of photoneutron-induced second malignancies against clinical benefits of high-energy Linacs is necessary.