Radiosensitive human tumour cell lines may not be recovery deficient

Split-dose studies have been performed on four human tumour cell lines of widely differing radiosensitivity in order to characterize the relationship between cellular recovery and radiation dose. Previous studies using the split-dose experiment have usually measured recovery at a single dose level and assumed an underlying multi-target model of radiation effect. This predicts that the recovery ratio should reach a plateau when the dose used per fraction is beyond the shoulder of the acute survival curve. In contrast, the linear-quadratic model predicts that the recovery ratio will increase steeply as a function of dose and will never reach a plateau. Our results show that recovery increases with increasing dose and therefore no single value of the recovery ratio can be used for comparative purposes. Using these data, we have derived a value for the beta-component of the linear-quadratic model that is independent of alpha. In addition we propose that the beta-parameter derived in this way provides the most satisfactory basis for intercomparison of cellular recovery between cell lines of differing radiosensitivity. Cellular recovery at any given dose was greatest in the most radiosensitive cell line, suggesting that increased radiosensitivity does not result from decreased recovery capacity. The results suggest that cells with steep acute radiation survival curves and which show little split-dose recovery may not be recovery deficient. Consequently, using such cells in attempts to correlate recovery with the underlying molecular processes of radiation damage repair could lead to misleading results.