ESTRO 2021 Abstract Book

S1544

ESTRO 2021

Results The value of AUC of the GLM is 0.831 (95% CI, 0.701‐0.961), and 0.828 (95% CI, 0.700 ‐0.956) in the original and new validation cohorts, respectively, showing a reproducibility in the applicability of the original model (Fig 1.a, 1.b). Eight radiomic features and one clinical parameter were found to be significant with LASSO model. Rad‐score between pCR and non pCR group occurred in both training (P < .001) and validation cohorts (P < .001). AUC of the new model of 0.926 (95% CI, 0.859-0.993) for the training and 0.926 (95% CI, 0.767-1.00) for the validation group (Fig 1.c, 1.d, respectively) show an improvement in performance compared to the original model.

Conclusion GLM model has shown a good reproducibility in predicting pCR in LARC; prediction accuracy is enhanced with LASSO model developed in this study.

PO-1815 Longitudinal radiomic features for prostate on anatomical T2w MRI on 1.5T MR-Linac E. van der Bijl 1 , L. Kermeijer 1 , R. Monshouwer 2 1 Radboud University Medical Center, Department of Radiation Oncology, Nijmegen, The Netherlands; 2 Radboud University Medical Center, Department of Radiation Oncology, Nijmegen, The Netherlands Purpose or Objective Magnetic resonance imaging guided radiotherapy relies on daily MRI scans to delineate structures to adapt treatment plans and additional images to verify positioning just before, during and after irradiation. This leads to a unique dataset of multiple MRI’s acquired each fraction over the course of a treatment on the same scanner with a single protocol. Opening up the opportunity to study radiomic features over the course of treatment, which in the future might lead to the identification of imaging biomarkers for treatment response. The objective of the current work is to perform a first analysis of longitudinally extracted radiomic features in the prostate target structure as well as the femoral heads which do not deform and are stable over the fraction. Materials and Methods Included are ten low and intermediate risk prostate cancer patients treated on a 1.5T MR-Linac with 5x7.25Gy to the whole prostate gland, delivered in two to three weeks in fractions of approximately 1h. We retrospectively analyzed the four 3D T2 weighted anatomical scans acquired each fraction using standard settings provided by the vendor. From these