The RBE for renal damage after irradiation with 3 MeV neutrons

Abstract

Mouse kidneys were locally irradiated with single doses or up to 8 fractions of 240 kV X rays or 3 MeV neutrons. Damage was assessed from measurements of urine output, isotope clearance or hematocrit levels. All 3 assays gave steep dose-response curves by 4-5 mo. after irradiation. The repair capacity of the kidney was considerable after X-irradiation but was very small after irradiation with neutrons. Thus, the RBE increased sharply with increasing fractionation. After large doses, an RBE of 2.3-2.5 was measured, rising to 4.5-5.1 after 8 fractions of 4-5 Gy [Gray] X rays. Linear-quadratic analysis of these data has allowed RBE to be calculated outside the measured dose range. The limiting RBE predicted at very low doses per fraction is 20-26, whereas at extremely high doses it would be as low as 1.2-1.4. This indicates that high RBE values may be seen in a slow turnover tissue after low doses per fraction (within the clinically relevant range) although this may not be evident after larger doses. Such high RBE arise because of the shape of the underlying X-ray dose-response curve rather than the shape of the neutron curve.