ESTRO 2020 Abstract book

S50 ESTRO 2020

classified as procedural only (n=63 (5%)). For the bladder, prospectively scored Grade 3+ GU morbidity was used as endpoint (n=51 (4%)). In the multivariate analysis we accounted for collinearity of the DVH parameters. The discriminating ability of the model was described by the area under the curve (AUC), and model calibration was evaluated by the Hosmer-Lemeshow (HL) test. Results In the final multivariate models, both GR2A2B and GR2B (Fig. 1) LRB was associated with age, anti-coagulant use, rectum volume, rectum D5%, and V75 of the rectum wall. The Grade 3+ GU endpoint was associated with anti- coagulant use, prostate volume, bladder D5%, equivalent uniform dose (EUD) (n=0.18) of the bladder, and V75 of the bladder wall. The AUC and HL test values were 0.66 and p = 0.49 for the GR2A2B LRB model, 0.73 and p = 0.35 for the GR2B LRB model, and 0.64 and p = 0.09 for the Grade 3+ GU morbidity model (Fig. 2), indicating good model fits.

Conclusion Multivariate NTCP models were developed for both LRB and GU morbidity following passive scattered PT for prostate cancer. All endpoints were associated with many of the same predictors. In particular, anti-coagulant use, V75 of either the full structure or wall, as well as D5%, were important prognostic factors in this cohort. Consequently, these predictors may be used to optimise proton treatment planning.