ESTRO 36 Abstract Book

S458 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective 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 The DVH shape of bowel loops is highly correlated with the risk of acute patient-reported loose stools due to WPIMRT for prostate cancer. Older age was found to be a protective factor. PO-0847 The dose-response curve of post-treatment FDG-uptake in lung tissue of irradiated NSCLC patients M. La Fontaine 1 , W.V. Vogel 1 , G. Persson 2 , G. Westman 2 , B. Reymen 3 , D. De Ruysscher 3 , J.S. Belderbos 1 , J.J. Sonke 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Department of Radiation Oncology, Amsterdam, The Netherlands 2 Copenhagen University Hospital- Rigshospitalet, Department of Radiation Oncology, Copenhagen, Denmark 3 Maastricht University Medical Centre, Department of Radiation Oncology, Maastricht, The Netherlands Purpose or Objective In NSCLC patients undergoing chemoradiotherapy, pneumonitis is a common occurrence. While its exact relationship with radiation dose is unknown, there is evidence that increasing lung dose leads to increased levels of pneumonitis. Using inflammation on post- treatment FDG PET scans as a measure of damage to normal lung tissue, the aim of this study was to investigate the relationship between pneumonitis and planned dose in a radiation dose-escalation trial.