Theory of RBE. Progress report, 1 January-31 December 1978

The shapes of the blackness-exposure curves of special nuclear emulsions after exposure to x-rays of 15-150 kvp have been found to remain constant, at all applied voltages. Since the shapes of these curves mimic those of biological cells (for cell killing vs. exposure), no change is expected in the RBE of low LET radiations at low dose levels. This finding disagrees with radiobiological findings in which the RBE of gamma-rays relative to x-rays increased at low dose, and was expected to approach 4. A minimal model of a heavy ion beam incorporating secondary and tertiary particle production has been constructed, in support of our investigations of relative effectiveness. The model gives good agreement with depth-dose curves from He to Ar for 8-30 cm of penetration in water. When applied to the survival of T-1 kidney cells using the parameters of the theory of RBE assigned from earlier work and slightly modified to accomodate recently attained levels of hypoxia, good agreement is found with measured values of the RBE and OER in Ar beams, for these cells. Secondary particle distribution alters the depth dose distribution dramatically, but has a much smaller effect on these biological parameters. To illustrate the potential application of the model to heavy particle therapy, we have designed a ridge filter, and find once again that it will be impossible to achieve isoeffect volumes for both oxic and hypoxic cells in a single port irradiation. Where possible, the preferred mode for heavy ion therapy must be symmetric cross-firing, where it is relatively simple to achieve isoeffect volumes for both oxic and hypoxic cells