A comparison of the potential therapeutic gain of p(66)/Be neutrons and d(14)/Be neutrons

Purpose: To determine the relationship between photon sensitivity and neutr on sensitivity and between neutron RBE and photon resistance for two neutro n modalities (with mean energies of 6 and 29 MeV) using human tumor cell li nes spanning a wide range of radiosensitivities, the principal objective be ing whether or not a neutron advantage can be demonstrated. Methods and Materials: Eleven human tumor cell Lines with mean photon inact ivation doses of 1.65-4.35 Gy were irradiated with 0-5.0 Gy of p(66)/Be neu trons (mean energy of 29 MeV) at Faure, S.A. and the same plating was irrad iated on the same day with 0-10.0 Gy of Cobalt-gamma-rays, Twelve human tum or cell lines, many of which were identical with the above selection, and s panning mean photon inactivation doses of 1.75-4.08 Gy, were irradiated wit h 0-4 Gy of d(14)/Be neutrons (mean energy of 6 MeV) and with 0-10.0 Gy of 240 kVp X-rays at the Essen Klinikum, Cell survival was determined by the c lonogenic assay, and data mere fitted to the Linear quadratic equation, Results: 1, Using the mean inactivation dose, a significant correlation was found to exist between neutron sensitivity and photon sensitivity. However , this correlation was more pronounced in the Faure beam (r(2) = 0.89 p les s than or equal to 0.0001) than in the Essen beam (r(2) = 0.65, p = 0.0027) , 2, No significant relationship could be established between neutron RBE a nd photon resistance for both modalities (p = 0.69 and p = 0.07, respective ly), 3, Using alpha-coefficients as a criterion, the neutron sensitivity fo r the Faure beam correlated with photon sensitivity (p 0.001), but this did not apply to the Essen beam (p = 0.27). 4. The neutron RBE for the Essen b eam derived from alpha-coefficients showed a steep increase with photon res istance (p = 0.003). In the Faure beam there was no increase of RBE with ph oton resistance (p = 0.494). Conclusion: Radiobiological differences between high-energy and low-energy neutrons are particularly apparent in the dependence of the neutron RBE on photon sensitivity. The increase of RBE with photon resistance is more pron ounced in the low-energy Essen neutrons than in the high-energy Faure neutr ons. An RBE advantage is indicated for photon-resistant cell lines and this is particularly apparent in the low-dose range using alpha-coefficients as compared to the mean inactivation dose. The clinical application of low-en ergy neutrons may be more restricted because of poor penetration and lack o f skin sparing. However, these neutrons discriminate better between photon- sensitive and photon-resistant cells giving an RBE range of 2-6 and a mean RBE of 4,1, than high-energy neutrons where the RBE range is 1.6-3.5 and th e mean RBE is 2,4, From the radiobiological point of view it, therefore, ap pears that the clinical potential of low-energy neutrons is considerably un derrated. (C) 2000 Elsevier Science Inc.