Calculation of high-LET radiotherapy dose required for compensation of overall treatment time extensions

A method is presented that allows biological effective dose (BED) equations to be used to calculate compensatory doses for treatment time extensions when high-LET (linear energy transfer) radiotherapy schedules are used. The principles involved are the same as those for low-LET radiations, but incorporate two relative biological effectiveness (RBE) factors, RBE(max) and RBE(min), which represent the RBE at very low and very high fraction doses, respectively, with the actual RBE changing between these extremes. The method has the advantage that low-LET alpha/beta ratios and low-LET daily dose-equivalent repopulation factors are used in the calculations. The daily dose repopulation equivalents and increments in dose per fraction in the case of high LET radiotherapy are smaller than those for low LET.