ESTRO 36 Abstract Book

S461 ESTRO 36 _______________________________________________________________________________________________

the iMPM. We conclude that the strong correlation between iMPM and iMM is caused by close proximity of the two muscles. However, the different shapes of the dose- response curves of both muscles suggest that they should be regarded as separate OARs and at least the iMPM should be delineated to estimate trismus risks. Furthermore, baseline MMO is highly predictive and is important to take into account in trismus models. PO-0850 Predicting late fecal incontinence risk after RT for prostate cancer:external independent validation A. Cicchetti 1 , B. Avuzzi 2 , T. Rancati 1 , F. Palorini 1 , C. Stucchi 3 , G. Fellin 4 , P. Gabriele 5 , V. Vavassori 6 , C. Degli Esposti 7 , C. Cozzarini 8 , C. Fiorino 9 , R. Valdagni 10 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate cancer program, Milan, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milan, Italy 3 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy 4 Ospedale Santa Chiara, Radiotherapy, Trento, Italy 5 Istituto di Candiolo- Fondazione del Piemonte per l'Oncologia IRCCS, Radiotherapy, Torino, Italy 6 Cliniche Humanitas-Gavazzeni, Radiotherapy, Bergamo, Italy 7 Ospedale Bellaria, Radiotherapy, Bologna, Italy 8 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy 9 San Raffaele Scientific Institute, Medical Physics, Milan, Italy 10 Università degli Studi di Milano, Oncology and Hemato- oncology, Milan, Italy Purpose or Objective To validating a predictive model for late fecal incontinence (FI) on a recent population of prostate cancer patients (pts) treated with radical radiotherapy. NTCP model was derived from literature. Material and Methods Population included 267 pts treated with Intensity Modulate Radiation Therapy (IMRT) in 2010-2014. Prescribed dose was between 68 and 80 Gy with conventional and hypo-fractionated (HF, from 2.2 to 2.8 Gy) treatment. Rectal toxicity was scored using the LENT/SOMA questionnaire. Follow-up (FU) was considered up to 2 years. The study endpoint was late FI. We chose to validate a model for prediction of chronic fecal incontinence, as evaluated through multiple measures during follow-up. Mean FI was defined as the average score during the FU period after RT. Mean incontinence >1 was the considered endpoint. Pts with at least three out of four FU points in the first 2 years were included (the 2- year point was mandatory). Literature based multivariate model included: mean rectal dose (Dmean), previous diseases of colon and previous abdominal surgery (SURG). Dose distributions were corrected EQD in 2 Gy fractions (alpha/beta=5Gy). Results 256 pts were available. Mean grade>1 FI was scored in 28 patients (10.9%). Univariate logistic analysis confirmed the risk factors reported in literature, with similar Odds Ratios (OR) for Dmean (1.04±0.03 vs 1.05±0.04) and SURG (1.90±1.70 vs 1.50±0.50). As consequence, NTCP models including Dmean and Dmean+SURG were evaluated through calibration plot. The models showed a clear trend (increasing observed toxicity rates with predicted risk), but the observed toxicity rates were underestimated. We guessed this scenario could be due to a hidden effect of HF (OR=2.20, 8.6% vs 17.6%), beyond standard correction using LQ model for late effects. The first approach was to directly evaluate the impact of HF, by including it as a variable into model (keeping coefficients for Dmean and SURG fixed at previously published values). It clearly improved calibrations. A further step was to include the time recovery effect into EQD2 correction

(gamma=0.7Gy/day), thus taking count of a possible consequential effect between acute and late damage. The figure reports calibration plot for all cases.

Conclusion The study confirms formerly published results on effect of abdominal surgery and dose to large rectal volumes as potential risk factors for late FI. Calibrations highlight a possible role of HF beyond linear quadratic correction. Inclusion of time recovery correction improved calibrations, but a further separate effect of HF was still detectable. This might be related to the selected alpha/beta (5Gy), which is currently accepted for late rectal toxicity. A more suitable value could be found for the longitudinal definition used in these trials (i.e., toxicity starting in acute phase and persisting during follow-up), instead of using the assumption settled in studies focusing on incidence of late peak events (such as rectal bleeding). PO-0851 Artificial neural networks for toxicity prediction in RT: a method to validate their “intelligence” E. Massari 1 , T. Rancati 2 , T. Giandini 1 , A. Cicchetti 2 , V. Vavassori 3 , G. Fellin 4 , B. Avuzzi 5 , C. Cozzarini 6 , C. Fiorino 7 , R. Valdagni 2 , M. Carrara 1 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Diagnostic Imaging and Radiotherapy- Medical Physics Unit, Milan, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milan, Italy 3 Cliniche Humanitas-Gavazzeni, Radiotherapy, Bergamo, Italy 4 Ospedale Santa Chiara, Radiotherapy, Trento, Italy 5 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milan, Italy 6 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy 7 San Raffaele Scientific Institute, Medical Physics, Milan, Italy