Inclusion of clinical risk factors into NTCP modelling of late rectal toxicity after high dose radiotherapy for prostate cancer.

Abstract

To fit an NTCP model including clinical risk factors to late rectal toxicities after radiotherapy for prostate cancer. Data of 669 patients were considered. The probability of late toxicity within 36months (bleeding and incontinence) was fitted with the original and a modified Logit-EUD model, including clinical factors by fitting a subset specific TD(50)s: the ratio of TD(50)s with and without including the clinical variable was the dose-modifying factor (D(mod)). Abdominal surgery (surg) was a risk factor for G2-G3 bleeding, reflecting in a TD(50)=82.7Gy and 88.4Gy for patients with and without surg (D(mod)=0.94; 0.90 for G3 bleeding); acute toxicity was also an important risk factor for G2-G3 bleeding (D(mod)=0.93). Concerning incontinence, surg and previous diseases of the colon were the clinical co-factors. D(mod)(surg) and D(mod)(colon) were 0.50 and 0.42, respectively for chronic incontinence and 0.73 and 0.64, respectively for mean incontinence score ⩾1. Best-fit n values were 0.03-0.05 and 1 for bleeding and incontinence, respectively. The inclusion of clinical factors always improved the predictive value of the models. The inclusion of predisposing clinical factors improves NTCP estimation; the assessment of other clinical and genetic factors will be useful to reduce parameter uncertainties.