RESPONSE OF THE CENTRAL-NERVOUS-SYSTEM TO FRACTIONATED BORON NEUTRON-CAPTURE IRRADIATION - STUDIES WITH BOROCAPTATE SODIUM

The response of the central nervous system (CNS) to fractionated doses of boron neutron capture (BNC) irradiation was assessed using a rat s pinal cord model. The thermal neutron beam at the Brookhaven Medical R esearch Reactor (BMRR) was used for the spinal cord irradiations, with borocaptate sodium (BSH) as the neutron capture agent. Irradiations w ere given as a single dose or as two or four equal fractions. The ED(5 0) for radiation-induced myeloparesis, as indicated by limb paralysis within 7 months, after a single exposure to thermal neutrons in the pr esence of BSH (blood boron-10 content similar to 70 mu g/g) was 27.2 /- 0.9 Gy. This was expressed as the total physical dose to the blood. Dividing the radiation dose into two consecutive daily fractions or f our fractions given over 1 week, resulted in ED(50)=32.0 +/- 1.4 and 3 1.5 +/- 0.4 Gy respectively. Although there was no significant dose sp aring in moving from two to four fractions, there was a dose increment of similar to 17% as compared with single-dose irradiation. The varia tion in the relative biological effectiveness of the thermal neutron b eam, with dose per fraction, was established using data from a previou s study with single and fractionated doses of thermal neutrons in the absence of a neutron capture agent. This varied from 1.40 to 3.74 for thermal neutron dose per fraction in the range 13.6-1.5 Gy. Previously published CBE factors for both BSH and BPA have been recalculated in the present report to take into account the change in the RBE of the t hermal neutron beam with dose. In all cases the recalculated CBE facto rs were lower than those obtained previously. Values for this paramete r increased with fraction number. In the case of BSH, the CBE factor i ncreased from 0.36 +/- 0.03 after a single-dose to 0.51 +/- 0.06 after four fractions.