COMPARATIVE-ASSESSMENT OF SINGLE-DOSE AND FRACTIONATED BORON NEUTRON-CAPTURE THERAPY

The effects of fractionating boron neutron capture therapy (BNCT) were evaluated in the intracerebral rat 9L gliosarcoma and rat spinal cord models using the Brookhaven Medical Research Reactor (BMRR) thermal n eutron beam. The amino acid analog p-boronophenylalanine (BPA) was adm inistered prior to each exposure to the thermal neutron beam, The tota l physical absorbed dose to the tumor during BNCT using BPA was 91% hi gh-linear energy transfer (LET) radiation, Two tumor doses of 5.2 Gy s paced 48 h apart (n = 14) or three tumor doses of 5.2 Gy, each separat ed by 48 h (n = 10), produced 50 and 60% long-term (>1 year) survivors , respectively. The outcome of neither the two nor the three fractions of radiation was statistically different from that of the correspondi ng single-fraction group. In the rat spinal cord,the ED(50) for radiat ion myelopathy (as indicated by limb paralysis within 7 months) after exposure to the thermal beam alone was 13.6 +/- 0.4 Gy. Dividing the b eam-only irradiation into two or four consecutive daily fractions incr eased the ED(50) to 14.7 +/- 0.2 Gy and 15.5 +/- 0.4 Gy, respectively, Thermal neutron irradiation in the presence of BPA resulted in an ED( 50) for myelopathy of 13.8 +/- 0.6 Gy after a single fraction and 14.9 +/- 0.9 Gy after two fractions. An increase in the number of fraction s to four resulted in an ED(50) of 14.3 +/- 0.6 Gy, The total physical absorbed dose to the blood in the vasculature of the spinal cord duri ng BNCT using BPA was 80% high-LET radiation. It was observed that fra ctionation was of minor significance in the amelioration of damage to the normal central nervous system in the rat after boron neutron captu re irradiation. (C) 1995 by Radiation Research Society