Micronucleus response of human glioblastoma and neuroblastoma cells toward low-LET photon and high-LET p(66)/Be neutron irradiation.

The identification of photon resistant tumors that are sensitive to neutrons is still an unresolved problem, and no radiobiological criteria have been developed that could help the selection of patients for neutron therapy. The micronucleus (MN) assay has been evaluated for this purpose in a panel of human glioblastoma and neuroblastoma cell lines spanning a wide range of photon sensitivities defined by mean inactivation doses ([Latin capital letter D with macron above][gamma]) of 1.25-3.21 Gy. We show that the relative biologic effectiveness (RBE) of the p(66)/Be neutrons is significantly correlated with inherent photon sensitivity (r = 0.89, p < 0.01), indicating that the panel of cell lines used is suitable to study the differential biologic response to neutrons and photons. We find that p(66)/Be neutrons are 1.43 to 5.29 times more effective per unit dose in inducing micronuclei than 60Co [gamma]-rays. Surprisingly, cells that are inherently photon resistant tend to show a higher yield of micronuclei following exposure to either photons or neutrons, but no significant correlation could be demonstrated. However, RBE values based on micronucleus yield were found to strongly correlate with RBE values derived from cell survival data (r = 0.91, p < 0.01). It is concluded that although micronucleus yield does not reflect intrinsic sensitivity to either photons or neutrons, the strong correlation between RBE calculated from micronucleus formation and RBE derived from cell survival demonstrates that the micronucleus endpoint has a potential for detecting photon resistant cells that show increased sensitivity to neutrons.