DERIVATIONS OF RELATIVE BIOLOGICAL EFFECTIVENESS FOR THE HIGH-LET RADIATIONS PRODUCED DURING BORON NEUTRON-CAPTURE IRRADIATIONS OF THE 9L RAT GLIOSARCOMA IN-VITRO AND IN-VIVO

Purpose: Relative biological effectiveness (RBE) values for the high l inear-energy-transfer particles produced during boron neutron capture therapy have generally been based on theoretical considerations or on in vitro experiments. The purpose of this study was to independently d etermine RBE values for all of the boron neutron capture therapy dose components. Methods and Materials: Clonogenic cell survival data were obtained for 9L rat gliosarcoma cells irradiated in the Brookhaven Med ical Research Reactor thermal neutron beam both in vitro and as an int racerebral tumor. These data were analyzed using the linear quadratic model for cell survival to derive measured RBE values for all beam com ponents and for a number of different boron compounds. Results: In the absence of boron, the combined effects of the protons from the nitrog en capture, N-14(n,p)C-14, and the fast neutron scatter, H-1(n,n')p, r eactions generated RBEs of 3.7 in vitro and 3.2 in an in vivo/in vitro excision assay, compared to 250 kVp X rays using an end point of 1% c ell survival. Apparent RBEs for the B-10(n,alpha)Li-7 reaction product s were calculated from cell survival data following reactor irradiatio ns in the presence of the amino acid p-boronophenylalanine, the sulfhy dryl dodecaborate monomer or dimer, or boric acid. Apparent RBEs for t he B-10(n,alpha)Li-7 reaction ranged from 1.2 to 9.8 depending on whic h boron compound was used. RBEs from the in vitro studies were consist ently higher than from the in vivo/in vitro studies. Under any conditi ons, the apparent RBE for the B-10(n,alpha)Li-7 reaction with p-borono phenylalanine was higher than that with any other boron compound teste d. Conclusions: Generally accepted RBE values for the fast neutron and N-14(n,p)C-14 reaction components of the total dose are too low. The apparent RBEs calculated for the B-10(n,alpha)Li-7 reaction were compo und-dependent and consistent with differences in the distribution of B -10 relative to glioma cell nuclei.