Acute reaction parameters for human oropharyngeal mucosa

The purpose of this study was to determine the influence of changes in dose rate over the range 0.8-240 Gy/h on acute oropharyngeal mucosal reactions in human subjects, and to estimate the values of the important parameters that influence these reactions. Sixty-one patients requiring radiotherapy to palliate incurable head and neck cancer were treated on a telecaesium unit, using opposing lateral portals to total midline doses, varying between 30 and 42 Gy in 10 daily fractions over 2 weeks, at dose rates of 0.8, 1.8, 3.0 and 240 Gy/h according to a central composite study design. The severity and time course of reactions were charted at least twice weekly for each patient, using the EORTC/RTOG acute mucosal reaction grading system. Duration of reaction at each grade was observed to provide a more sensitive reflection of effect than the proportion of patients reaching any particular reaction grade. Analysis of duration by direct and indirect methods suggest alpha/beta ratios in the range 7-10 Gy and half-time (t1/2) values in the range 0.27-0.5 h, if mono-exponential repair kinetics are assumed. The t1/2 values are short and raise the question as to whether the repair kinetics of this tissue are well described by a mono-exponential function. Further prospective studies involving multiple daily fraction treatment regimes delivered at high dose rate, in which interfraction interval is deliberately varied, are needed to find out whether the parameters derived from this project are applicable to fractionated treatment courses at high dose rate.