ESTRO 2023 - Abstract Book

S605

Monday 15 May 2023

ESTRO 2023

Prior to model development, 4 patients were removed at random. Step 1: We analysed EVs individually for ability to predict coverage; the best performing in each category were combined in a logistic regression model, adjusted to achieve the highest mean accuracy in the validation set using 10-fold cross

validation (random shuffle of training (n=11) and validation (n=5) sets). Step 2: Model coefficients were determined on the 16-patient training set. Step 3: The final model was tested on 4 unseen patients.

Results Step 1: The chosen EVs (rectal size on CT-FB, bladder width on CT-FB, max surface-to-surface distance between CTV-LR contours on CT-FB and CT-EB, and menopausal status) had 78% average accuracy (s.d. 17%) and 86% average specificity in the cross-validation. Step 2: The coefficients were -0.68, 0.47, 0.90, 0.70, the intercept was -0.75. When applied to the training set (n=16) the model had 87.5% accuracy, 80% sensitivity, 91% specificity. Step 3: When applied to the unseen test set (n=4) the model had 75% accuracy, 67% sensitivity and 100% specificity (one patient with adequate coverage was mis-classified as an occult mover). Overall, 8/20 patients were covered (6 in the training set, 2 in the test set). Conclusion Using 4 EVs, our model predicted with 75% accuracy and 100% specificity whether a patient’s interfractional motion will fall within the range on CT-EB and CT-FB. Predicted occult movers may have their ITVs extended, or be selected for more resource-intensive adaptive strategies. PD-0741 Online adaptive radiotherapy for low volume metastasized prostate cancer on the CyberKnife system V. Brand 1 , M.T. Milder 1 , F.E. Froklage 1 , M.E. Christianen 1 , K.C. de Vries 1 , M.S. Hoogeman 1 , L. Incrocci 1 1 Erasmus MC Cancer Institute, University Medical Centre Rotterdam, Radiotherapy, Rotterdam, The Netherlands Purpose or Objective The low α / β ratio of prostate cancer (PCa) suggests benefits of ultra-hypofractionation (UHF). However, for high-risk PCa patients this is challenging as seminal vesicles (SV) are included in the target volume. Our recently started UPRATE trial aims to prove the feasibility of SV PTV-margin reduction in low volume metastasized PCa patients. To this end, pre- treatment in-room CT-on-rails and online replanning are combined with intra-fraction fiducial tracking of the prostate on the CyberKnife. Here we present the plan protocol creation and validation in 10 patients, workflow optimization, and results of the first-in-men treatments. Materials and Methods Patients are treated with 6 once-weekly fractions of 6 Gy according to the STAMPEDE trial on a CyberKnife with a CT-on- rails system. The organ-at-risk (OAR) constraints were derived from large randomized UHF trials, e.g. PACE and HYPO-RT- PC. To increase delineation and plan optimization speed during the online adaptation, volume constraints are turned into absolute constraints. Based on a planCT, a patient specific optimization script with reduced sample points (to reduce optimization time) was saved to run on the fraction CTs. This method was validated using 10 previously treated patients with 4 CTs each. For each fraction CT, the initial prostate contour is rigidly transferred on this new CT by aligning the implanted intraprostatic fiducials. The OARs and SV are segmented by artificial intelligence (AI). Rigid registration and AI was chosen after comparison with deformable image registration on adaptation time and inter-observer variation. A radiation oncologist adjusts these contours where needed. Treatment times of 2 end-to-end dummy runs were recorded. Finally, the first four fractions delivered to our first UPRATE patient were timed and adherence to the constraints was recorded. Results Table 1 shows the UPRATE constraints compared to the STAMPEDE-, PACE- and HYPO-RT-PC-trial. These UPRATE constraints were met for 35/40 fractions for the 10 test patients. Of the 5 fractions that did not, 3 had bladder volumes <150 ml and 2 required PTV underdosage due to bowel within the PTV. The plan with reduced sample points, was clinically acceptable