ESTRO 36 Abstract Book

S482 ESTRO 36 _______________________________________________________________________________________________

Receiver-operator characteristic curves were plotted, linking the extracted dose parameters with six patient- reported clinical endpoints: rectal bleeding, proctitis, sphincter control, rectal pain, and “How big a problem are bowels?” (≥Grade 1, ≥Grade 2). Statistical correlations between planned and accumulated DSMs were compared using the calculated area under the curve (AUC) presented on High-Low plots. Results For rectal bleeding, the 30, 40, and 60 Gy accumulated DSM dose-widths were significant predictors (AUC 0.629, 0.621 and 0.643 respectively), where planned dose was not (Figure 2a). For DSM dose-widths up to 70 Gy, AUC was greater for accumulated dose than planned dose. EUD was the strongest predictor of rectal bleeding from both accumulated (AUC 0.682) and planned (AUC 0.673) DSMs. The only significant predictor of proctitis was EUD of the accumulated DSM (AUC 0.673) (Figure 2b). Neither planned nor accumulated doses were predictive of the other endpoints

Conclusion For the first time, it has been possible to quantitatively demonstrate that accumulated delivered dose to the rectal wall is more strongly correlated with rectal bleeding and proctitis in prostate radiotherapy than planned dose. The results support the hypothesis that incorporating delivered dose into multi-variable predictive models could improve toxicity outcomes. Poster: Physics track: CT Imaging for treatment preparation PO-0881 4DMRI for RT planning; novel precise amplitude binning in the presence of irregular breathing I. Bones 1 , O.J. Gurney-Champion 2 , A. Van der Horst 1 , A. Bel 1 , T. Alderliesten 1 , G. Van Tienhoven 1 , K. Ziemons 3 , Z. Van Kesteren 1 1 Academic Medical Centre, Radiotherapy, Amsterdam, The Netherlands 2 Academic Medical Centre, Radiotherapy and Radiology, Amsterdam, The Netherlands 3 FH Aachen University of Applied Sciences, Medical Physics, Jülich, Germany Purpose or Objective Irregular breathing, often the case in clinical practice, introduces the need for proper outlier handling for 4DMRI reconstruction. Discarding outliers may lead to underestimation of the respiratory-induced organ motion. Our study aimed to develop and evaluate an amplitude binning strategy that reduces reconstruction artefacts while improving precision in the presence of irregular breathing. Material and Methods Twelve volunteers and 2 abdominal cancer patients were scanned with our 4DMRI sequence. In this 6 minute scan, 11 2D coronal slices were acquired repetitively (60 times) during free breathing, using a T2W TSE sequence (resolution: 1.3x1.6x5.0 mm 3 ). Prior to each slice acquisition, the position of the diaphragm was assessed using a 1D acquisition.