TREATMENT TIME AND SUBLETHAL REPAIR IN RADIOTHERAPY

The repair of sub-lethal damage is now universally accepted by radiotherapists and its role in fractionated radiotherapy fully appreciated. However, it may also influence the survival of cells during a single daily treatment - a phenomenon not always considered. Chinese hamster ovary cells have been irradiated with total doses of 750 rad or 500 rad under optimum growth conditions and the effect of repair of sub-lethal damage on survival examined for a split dose given with 30 minutes separation. Our results show that repair of sub-lethal damage is a very rapid process and the recovery ratio, i.e. the increase in survival when the radiation dose is delivered in two or more parts, is approximately one per cent per minute. The results show quite clearly that both dose-rate and the total time for a daily treatment may significantly change the number of survivors following radiation. Using a carefully designed irradiation set up to achieve accurate dosimetry, and many replicate cell samples, it has been demonstrated that the recovery ratio is greater than 1.2 for time intervals of 30 minutes whether the radiation is given in two equal parts or five portions. The statistical significance of the data was extremely good with the 'p' value never worse than 0.01. Using these results we have examined the effect that changing a patient's treatment from a high dose-rate linear accelerator to a low dose-rate telecobalt unit might have on cure rates. This analysis suggests that doses should be increased for long daily treatment times and that care must be exercised when high-energy machines with different dose rates are used in multi-centre trials.