Tumorigenic Potential of High-Z, High-LET Charged-Particle Radiations

The potential for radiogenic neoplasia from charged-particle irradiation has been estimated using the Harderian gland of the mouse as a test system. Particles ranging in Z from Z = 1 (proton) to Z = 41 (niobium), in energy from 228 to 670A MeV, and in LET from 0.4 to 464 keV/μm were produced at the Lawrence Berkeley Laboratory BEVALAC. Expression of the tumorigenic potential of the initiated cells was enhanced by hormones from isogeneic grafts of pituitaries. The goal of the studies was to estimate the initial slope of the relationship between increased tumor prevalence at 16 months after irradiation and the dose received. Initial slopes were measured with good precision for 60 Co γ rays and the Bragg plateau beams of 228A MeV 4 He ions, 600A MeV ^{56}{\rm Fe}$ ions, and 350A MeV ^{56}{\rm Fe}$ ions. The ratio of the initial slope for these ions to that of 60 Co γ rays give an estimate of the maximum RBE for radiogenic neoplasia. These values were 2.3 for the 4 He ions, 40 for 600A MeV ^{56}{\rm Fe}$, and 20 for 350A MeV ^{56}{\rm Fe}$. In the studies reported here the prevalence of tumors as the result of pituitary isografts was not enhanced after irradiation with ^{56}{\rm Fe}$ ions. It remains to be seen how effective pituitary isografts are for enhancement of radiogenic neoplasia from other ions at different LET values. A risk analysis was undertaken using particle fluence rather than dose as the independent variable. This analysis provides a value for a "cross section" expressed in μ m2. This parameter expresses as the increase in proportion of mice with one or more Harderian gland tumors per unit increase in particle fluence. The plot of the cross section (risk coefficient) as a function of LET is monotonic, with no clear evidence of a maximum value of the risk coefficient for even the highest LET particle used.