ESTRO 36 Abstract Book

S453 ESTRO 36 2017 _______________________________________________________________________________________________

Intestinal toxicity (IT) may affect the quality of life of patients (pts) treated with whole-pelvis intensity- modulated radiotherapy (WPIMRT) for prostate cancer. The aim of this investigation is to identify quantitative bowel dose-volume relationships for acute patient- reported IT. Material and Methods A cohort of pts was enrolled in 6 Institutions within a registered prospective trial. Pts were treated with conventional or moderate hypo-fractionation to prostate/prostatic bed and WPIMRT delivering 51.8 Gy (median dose, range: 50.4-56.1 Gy) to pelvic nodes while sparing the bowel outside PTV as much as possible. Acute IT was evaluated by the Inflammatory Bowel Disease Questionnaire pertaining to the Bowel Domain (IBDQ-B) filled in by pts at baseline and at mid-point/end of RT. IBDQ-B includes 10 items (#item) scored on a 7-point scale (worst symptoms=lower scores). The 25 th percentiles of the most severe worsening (Δ) between baseline and half/end RT were set as end-points. The correlation between end-points and bowel loops cc/% DVH/DSH (from V5Gy to V60Gy) as well as selected clinical parameters was investigated through multi-variable logistic regression. Goodness of fit was estimated by the Hosmer Lemeshow test (HL) and the Brier score (BS); performances of the model were assessed by the calibration plot. Internal validation was performed by 1000 bootstrap resampling. Results Data of 206 pts (80 radical, 79 adjuvant, 47 salvage RT) were available: 25/109/72 pts were treated with fixed- fields, rotational and Tomotherapy technique respectively. A relatively small but significant Δ (p<0.05) was found for all questions: the median Δ was 2 points for #1 (bowel movements) and 1 point for #5 (loose stools), #17 (gas passage) and #24 (urgency to defecate with empty intestine); Δ was 0 for the remaining 6 items that were then disregarded in current analysis. No DVH/DSH parameters were correlated with Δ, except for ΔIBDQ5≤-3 (25 th percentile, 43/191). The resulting model after backward selection of variables (R 2 =0.89, slope:1.037, optimism corrected BS=0.17, Figure 1) included absolute V42Gy and age (protective). Due to the correlation between DVH variables, three values representing ‘low’ (V20), ‘intermediate’ (V30) and ‘high’ (V42) dose levels were also considered to define an overall 'DVH-shape” predictor. When grouping pts according to best cut-off values assessed by ROC curves (high risk: V20>470cc, V30>245cc, V42>110cc; low risk: the other pts, figure 2), an alternative model including high-risk DVH-shape (OR:9.3) and age (protective, OR:0.94) may be suggested. The model showed very good calibration (slope:1.003, R 2 =0.92) and accurate prediction even after bootstrap-based internal validation (corrected BS=0.16).

Conclusion Auto-Plan in Pinnacle allowed fast planning of FFF VMAT plans for partial breast cancer radiotherapy. Compared to forward planned tangential radiotherapy the WMAT plans were better at both sparring of the ipsilateral lung and in covering the PTV. The VMAT plans could be delivered quickly, and as a result patients treated in breath hold could be treated with half the number of breath holds.

Poster: Physics track: (Radio)biological modelling

PO-0846 Bowel dose-volume relationship for patient- reported acute intestinal toxicity from pelvic IMRT C. Sini 1 , B. Noris Chiorda 2 , P. Gabriele 3 , G. Sanguineti 4 , S. Morlino 5 , F. Badenchini 6 , D. Cante 7 , V. Carillo 8 , M. Gaetano 8 , T. Giandini 9 , V. Landoni 10 , A. Maggio 11 , L. Perna 1 , E. Petrucci 7 , V. Sacco 2 , R. Valdagni 12 , T. Rancati 6 , C. Fiorino 1 , C. Cozzarini 2 1 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 2 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy 3 IRCC-Candiolo, Radiotherapy, Torino, Italy 4 Regina Elena Institute – IFO, Radiotherapy, Roma, Italy 5 Fondazione IRCCS Istituto Nazionale Tumori, Radiation Oncology, Milano, Italy 6 Fondazione IRCCS Istituto Nazionale Tumori, Prostate Cancer Program, Milano, Italy 7 ASL TO4 Ospedale di Ivrea, Radiotherapy, Ivrea, Italy 8 Centro AKTIS Diagnostica e terapia, Radiotherapy, Napoli, Italy 9 Fondazione IRCCS Istituto Nazionale Tumori, Medical Physics, Milano, Italy 10 Regina Elena Institute – IFO, Medical Physics, Roma, Italy 11 IRCC-Candiolo, Medical Physics, Torino, Italy 12 Fondazione IRCCS Istituto Nazionale Tumori, Radiation Oncology- Prostate Cancer Program- UNIV Hematology and Hemato-Oncology- Università degli Studi di Milano, Milano, Italy

Purpose or Objective