Low-dose RBE and Q for X-ray Compared to Gamma-ray Radiations

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Abstract
The standard comparison radiation for designation of radiation protection values of Q for high-LET [linear energy transfer] radiation has been low-LET radiations having LET of 3.5 keV/.mu.m or less. Radiobiological studies at high doses and dose rates have indicated that the effectiveness per rad of 250 kVp [kilovolt peak] X-rays is within 10 or 20% of that of 60Co .gamma.-rays. From microdosimetric considerations, however, at very low doses, 250 kVp X-rays may be substantially greater in effect per rad than are .gamma. rays. The theory of dual radiation action leads to the formulation .epsilon. = .kappa.(Sz D + D2) for dose effect relationships for low-LET radiation, in which zeta is a physically determined microdosimetric quantity equal to the dose average of the specific energy Z and K is a constant related to target sensitivity. The theory thus predicts that at very low doses at which the linear term dominates, the RBE [relative biological effectiveness] should be the ratio of the values of zeta, provided that the LET dependence of K is minimal in the LET range used for comparison. The ratio of the physically determined zeta values for 250 kVp X rays vs. 60Co .gamma.-rays is about 2-1, leading to the prediction that the X-rays at low doses should be about twice as effective per rad as are .gamma.-rays. Quantitative biological studies extending into the low dose linear portion of the dose effect curve are in accord with predicitons and the RBE of 250 kVp X-rays compared to 60Co .gamma.-rays for low doses and dose rates, appears to be of the order of 2. The physically determined zeta values for several low-LET radiations indicate that the effectiveness per rad may differ by as much as 4. The implication of these findings is that the reference low-LET radiation must be defined more narrowly than it presently is in order to avoid ambiguity.