APPARENT RATES OF PROLIFERATION OF ACUTELY RESPONDING NORMAL-TISSUES DURING RADIOTHERAPY OF HEAD AND NECK-CANCER

Total doses, fraction sizes, and overall times reported to give "tolerance level" reactions in oral, pharyngeal, or laryngeal mucosa vary greatly in different countries. The variation in fraction size reviewed here is from 1.15 Gy (France) to 3.4 Gy (UK), and in overall time from 11 days (UK) through 3, 4, and 5 weeks (UK and Canada) to 7 weeks (USA and France). Using linear quadratic corrections for fraction size, and regression of the resulting "LQ doses" against overall time, it is found that the average rate of increase of dose for these normal tissue reactions is gamma/alpha = 63 to 69 cGy per day (along the initial slope of the underlying dose-response curve), which corresponds to 53 to 65 cGy per day when 2 Gy fractions are used. The range of uncertainty overlaps with that of estimated tumor proliferation rates in the head and neck by other authors, but there is no trend for these normal tissues to be proliferating consistently faster than tumors in the same sites. These conclusions must be interpreted with caution because of differences in acceptance of reaction level, treatment volume, and dose specification.