Fast neutron effects on multicell spheroids.

Accumulation and repair of neutron damage was measured using multicell spheroids irradiated with neutrons at the Hammersmith Cyclotron. A relative biological effectiveness (RBE) of about 1.5 (ratio of Do values) was found for aerobic single cells or small spheroids. Small spheroids accumulated more sublethal neutron damage than single cells, as indicated by a wider shoulder on the survival curve. However, neither single cells nor spheriods repaired sublethal damage when two doses of neutrons were separated by six hours or less. A "therapeutic gain" was observed for larger spheroids containing hypoxic cells, due to an increased RBE of fast neutrons for hypoxic cells amounting to about a factor of two.