Impact of dose-distribution uncertainties on rectal ntcp modeling II: Uncertainty implications

A trial of nonescalated conformal versus conventional radiotherapy treatmen t of prostate cancer has been carried out at the Royal Marsden NHS Trust (R MH) and Institute of Cancer Research (ICR), demonstrating a significant red uction in the rate of rectal bleeding reported for patients treated using t he conformal technique. The rate of bleeding has been shown to fall signifi cantly as the extent of rectal wall receiving a planned dose-level in exces s of 57 Gy is reduced. Dose-distributions delivered to the rectal wall over the course of radiotherapy treatment inevitably differ from planned distri butions. In a previous paper estimates were obtained of the uncertainties i n some planned rectal dose-distribution parameters generated by patient set up error, rectal wall movement and the variable degree of rectal wall diste nsion. Here these uncertainties are combined to obtain estimates of the tot al planning uncertainties in rectal dose-distribution parameters thought li kely, on the basis of mechanistic biological modeling, to correlate strongl y with the complication rate. Working from these totaled uncertainty values , together with values of patient-to-patient and technique-to-technique dif ferences in planned dose-distribution parameters, it can be inferred that t he rectal dose-distribution uncertainties: (i) Have only a marginal impact on fits of a normal tissue complication probability (ntcp) model to RMH/ICR dose-distribution and grade 1, 2, 3 bleeding data (slightly flattening obs erved volume-response curves); (ii) only slightly reduce the power of a 2 x 100 patient trial of conformal versus conventional prostate radiotherapy t o detect a significantly lower rate of grade 1,2,3 rectal bleeding amongst conformally treated patients; (iii) do not diminish the information content of individual planned patient dose-distribution data to the point where th e fitting of technique-averaged data would provide as sensitive a test of t he existence of a volume effect as the fitting of individual patient data. (C) 2001 American Association of Physicists in Medicine.