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To estimate the α/β ratio of late rectal toxicity in case of prostate cancer radiotherapy; and to investigate whether correcting the rectal dose-volume histogram(DVH)for differences in dose per fraction benefits the prediction of normal tissue complication probability(NTCP)model.Methods and materials:A total of 213 patients received a conformal radiotherapy for localized prostate cancer to a total dose of 70 Gy in 7 weeks,either at 2 Gy/fraction(fr),5 fr/week(94 patients)or at 2.5 Gy/fr,4 fr/week(119 patients).The endpoint was the 3 year ≥Grade 2 rectal toxicity rate(according to the LENT/SOMA criteria).The Lyman-Kutcher-Burman(LKB)NTCP model was fitted(by optimizing the model parameters)to the endpoints in two different ways:firstly by using rectal DVHs representing the physical dose,and secondly by using a L-Q model correction with an estimated α/β additional parameter.Profile likelihood method was used to calculate 95%confidence interval(CI)for the estimated parameters.Akaike information criterion(AIC)and likelihood ratio test were used to evaluate the goodness of fit.Univariate analysis and receiver operating characteristic(ROC)curves were used to assess whether L-Q corrected NTCP model was better at toxicity prediction.Results:The median follow-up was 78 months(range,8 to 155 months).The 3 year rates of rectal toxicity for the 2 Gy/fr and 2.5 Gy/fr were:9.7%(95%CI:3.5-15.5%))and 21.0%(95%CI:13.1-28.1%)(2.5 Gy/fr)respectively.Using the L-Q corrected LKB model,the estimate of α/β was 7.17Gy(95%CI:5.21,9.13)and the corresponding NTCP parameters were TD50=71Gy(95%CI:69,73),n=0.08(95%CI:0.06,0.10)and m=0.09(95%CI:0.07,0.11).The AIC for the corrected and standard NTCP models were,84.58 and 85.31,respectively,and the corresponding likelihood ratio test shows that the corrected model better fits the data(p=0.02).The toxicity prediction of corrected NTCP gave smaller p value(0.003 vs.0.026)and bigger area under curve(AUC)value(0.68 vs.0.61)of ROC curve.Conclusions:The estimated alpha/beta ratio,which is in agreement with previous studies,allows for a better NTCP prediction and suggested that the late rectal damage could be correlated with the acute reactions.