Optimum overall times II: Extended modelling for head and neck radiotherapy.

ABSTRACT

AIMS: A previous paper in this journal (part I) concluded that there was no pronounced optimum overall time, at least up to 70 fractions of 1.15 Gy at two fractions/day in 50 days. The maximum tolerable tumour doses increased only 2% from the best short schedules of 21 or 23 days to those of 50 days. Only this range was modelled in part I because it covered the fewest and the most fractions, and the longest overall times that will probably be used in practice. Most UK schedules, typically using five fractions a week, yield tumour effective doses about 10% less than the best schedules in other developed countries. The present paper covers a much wider range of fraction numbers from one to 115, and from 1 to 80 days. Some numerical errors in the Tables in part I are also corrected in the present appendix. These made no difference to the main conclusions just described. MATERIALS AND METHODS: Standard linear quadratic modelling was used, assuming at first alpha/beta=10 Gy, alpha=0.35 ln/Gy, Tk=21 days, Tp=3 days for tumours, but with Tk=7 days, Tp=2.5 Gy for acute mucosal reactions, as before. A late complications constraint of 70 Gy was accepted, and an acute constraint of 51 Gy (both at 2 Gy fractions). Alternative values of more rapid or slower repopulation were also explored (Tp=2 days or Tp=5 days, respectively). RESULTS: Optimal values were shown at 22-32 days for one fraction/day five times a week, and at 42-49 days for two fractions/day at 10 fractions/week. Repopulation caused a rapid fall in tumour dose after 30 days with one fraction/day, but not until after 50 days with two fractions/day, and so was not seen in part I with its too-practical end time. CONCLUSIONS: Biological modelling can extrapolate calculations outside the borders of published treatment schedules to clarify borderline situations. Optimum schedules in radiotherapy can reliably give more tumour control if two fractions/day are used. The potential gains are equivalent to about two fractions of 2 Gy as given by this modelling. However, the late complications will be less with some nearly tumour equivalent shorter schedules if optimally designed.