RBE for lung and cord

Abstract

The sparing effect of fractionation of neutron dose is very small for late damage in tissues. This is seen in the almost flat isoeffect curve for damage to skin, CNS and to lung. This means that differences in the RBE curves for these tissues are determined by differences in the slopes of the photon isoeffect curves. The relevant slopes of the photon isoeffect curves giving the exponent of N in the Ellis formula are 0.24 for subcutaneous tissue, 0.27 for lung damage and 0.38–0.45 for damage to spinal cord, while the exponent for N for neutrons for these tissues is 0.04 for subcutaneous tissues and zero for lung and spinal cord. The slopes of the RBE curves for lung and cord or for skin and cord when RBE is plotted against dose/fraction of photons are significantly different, and the RBE at a γ ray dose/fraction normally used in therapy of about 2 Gy, is significantly higher for spinal cord than for lung or skin. The sparing of damage by extending overall treatment time for both lung and CNS is small for X or γ-irradiation. For neutron irradiation the sparing is similar to that with photons for the CNS but is much less with neutrons than with photons for the lung. This is because different mechanisms are responsible for this type of sparing of damage in the two tissues. In lung slow, repair is involved while in the spinal cord, the sparing is related to the slow cell proliferation.