Tolerance of spinal cord, lung and rectum after fractionated pions and X-rays.

The chronic and late effects of fractionated negative pi-mesons (pions) and X-rays were investigated in rat spinal cord and rectum, and in mouse lung. Range modulated pions with a 6 cm Bragg peak were used at an average dose-rate of 0.2 Gy/min. Control studies were performed with 250 kV or 300 kV X-rays. The following functional and histological endpoints were used: paralysis and white matter necrosis after spinal cord irradiation, ulceration and fibrosis of the rectum, breathing frequency and pneumonitis after lung irradiation. The presence of a large low-LET component was reflected by a significant dose-rate effect for spinal cord and lung. This resulted in RBE values smaller than unity for single doses, since the reference X-irradiations were done at high dose-rate. For small fraction sizes and short exposure times the influence of the dose-rate is insignificant. The RBE values at pion doses of 1 to 2 Gy per fraction were found to be not higher than 1.5 for all tissues studied. These values are similar compared to published data for acute effects. This is in contrast to the effects of neutrons, which in general show higher RBE's for late effects. It can be concluded from our studies that the radiobiological characteristic of pions in a spread Bragg peak are influenced by the low-LET component of the beam. Unlike with neutrons, there are no significant differences between the RBE values for late and acute effects.