ESTRO 2021 Abstract Book

S77

ESTRO 2021

Elena, Radiotherapy, Rome, Italy; 3 Istituto di Candiolo - FPO-IRCCS, Radiotherapy, Candiolo, Italy; 4 Ospedale Regionale Parini-AUSL Valle d’Aosta, Radiotherapy, Aosta, Italy; 5 Ospedale di Ivrea, Radiotherapy, Ivrea, Italy; 6 Cliniche Gavazzeni-Humanitas, Radiotherapy, Bergamo, Italy; 7 Azienda Sanitaria dell’Alto Adige, Radiotherapy, Bolzano, Italy; 8 Ospedale degli Infermi, Radiotherapy, Biella, Italy; 9 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiotherapy, Milan, Italy; 10 Azienda Ospedaliero Universitaria S. Maria della Misericordia, Radiotherapy, Udine, Italy; 11 Istituto di Candiolo - FPO-IRCCS, Medical Physics, Candiolo, Italy; 12 IRCCS Istituto Nazionale dei Tumori Regina Elena, Medical Physics, Rome, Italy; 13 Ospedale Regionale Parini- AUSL Valle d’Aosta, Medical Physics, Aosta, Italy; 14 Fondazione IRCCS Istituto Nazionale dei Tumori - Prostate Cancer Program, Radiotherapy, Milan, Italy; 15 San Raffaele Scientific Institute, Radiotherapy, Milan, Italy Purpose or Objective Bowel toxicity from whole pelvis radiotherapy (WPRT) is still an important issue also in the IMRT era due to the impact of intestinal symptoms on patients’ daily quality of life. The current study focused on acute patient-reported intestinal symptoms after WPRT for prostate cancer: the main goal was to assess the bowel dose-volume relationships. Materials and Methods A population of 415 patients enrolled in a registered multi institute trial underwent radiotherapy with radical (31%), adjuvant (33%) or salvage (36%) intent at a median dose to pelvic lymph nodes of 53Gy. The maximum variation between baseline and radiotherapy mid-point/end toxicity in terms of bowel symptoms was assessed by ten items of Inflammatory Bowel Disease Questionnaire (∆IBDQ) filled in by patients. The 25 th percentile values of ∆IBDQ were considered as clinically significant endpoints. Patients with severe starting symptoms (baseline IBDQ score <5) were excluded from analyses (rate: 2-7% according to the endpoint). Small bowel loops were contoured for all patients, according to the study guidelines. Mean absolute DVHs of bowel loops for patients with/without IBDQ-based toxicity were compared (two sided t-test). Based on the most significant differences, selected DVH based parameters were combined with prospectively collected clinical information through multivariate logistic regression analysis (MVA) in order to derive predictive models for specific patient-reported symptoms. Results IBDQ ranged between 0.2-1.5 points considering separately each IBDQ item. Four out of ten symptoms (IBDQ1=frequency, IBDQ5=diarrhea, IBDQ17=gas passage, IBDQ24=urgency) exhibited a median worsening ≥1 and analyses focused on them. A dose-volume effect was observed from DVH analyses for IBDQ5 (see Figure 1), IBDQ24 and overall Bowel Domain, showing V40-V50 to be the most predictive DVH region. At MVA, DVH (best cut-off: V46Gy≥80cc) and baseline scores (Odd-Ratio: 0.35-0.65) resulted the most predictive independent variables for the three endpoints (see Figure 2 for diarrhea symptom). The final models were reliable (Hosmer and Lemeshow test: 0.453-0.956), well calibrated (calibration plot: slope=0.922-1.069, R 2 =0.725-0.875) and moderately discriminative (Area Under the Curve: 0.628-0.669). An internal bootstrap-based validation confirmed their robustness.

Conclusion Bowel loops V40-V50 are associated with the risk of patient-reported moderate worsening in diarrhea and urgency symptoms, in particular for those patients with lower IBDQ baseline scores.