ESTRO 2023 - Abstract Book

S732

Monday 15 May 2023

ESTRO 2023

Figure 1: CTV overlap within PTV in percentage per fraction (in blue) and averaged per patient (in yellow). Fractions with an additional ATP are marked with dotted bars. The applied 3 mm PTV margin for the prostate gland had at least a 97.5% averaged CTV overlap with the PTV in all patients. An additional ATP adaptation was applied in 32% of all fractions. By changing the margin the PTV volume was on average reduced by 25% (20 cc). Conclusion Based on intrafraction motion as observed in this patient cohort the 3 mm PTV margin expansion remained large enough to compensate for the observed intrafraction variations if combined with an additional ATP correction if a shift of the prostate exceeding 2 mm is observed in the PV scan. This indicates that the PTV margin reduction from 5 to 3 mm with online adaptive MR-guided radiotherapy on a MR-linac is safe to implement in the clinic. MO-0884 knowledge-based adaptive QA using dosimetric indicators for stereotactic adaptive RT for pancreas A. Price 1 , F. Forghani-Arani 1 , B. Maraghechi 1 , L. Marut 1 , J. Ginn 2 , P. Samson 1 , C. Robinson 1 , E. Laugeman 1 , H. Kim 1 , L. Henke 1 1 Washington University in St. Louis School of Medicine, Radiation Oncology, St. Louis, USA; 2 Duke University, Radiation Oncology, Durham, USA Purpose or Objective In this study we aim at developing knowledge-based tools for robust ART planning to detect variations or errors in plan quality for stereotactic pancreatic ART. We developed volume-based dosimetric identifiers which can be used to identify dosimetric deviations of ART plans from the simulation plan. Materials and Methods 57 patients treated using 0.35T MRgRT for pancreas cancer were included in this retrospective study. All patients received 50 Gy in 5 fractions. A total of 278 treatment fractions were adapted. The critical gastrointestinal luminal organs, namely stomach, duodenum, small bowel, and large bowel (SDSL) were prioritized over target coverage with the dose constraint of V36 < 0.5 cc. PTV-OPT was generated by subtracting the SDSL plus a 5mm margin from the PTV to allow for appropriate dose gradient from the prescription dose to the SDSL dose-limiting objective. Dosimetric information were retrieved from the treatment reports for both the initial and ART plans using an in-house Matlab program. Several metrics that potentially can identify poor quality plans were calculated. These metrics include V95% for target coverage for PTV and PTV-OPT, the ratio of D95% to D5% for PTV and PTV-OPT, the volume ratio between PTV-OPT and PTV, and MU per fraction. We calculated the difference of these values between each adaptive plan and the simulation plan. Simulation plans were used as the gold standard in this study based on the assumption that the simulation plans are of standard quality due to extended time for planning. We then calculated 95% confidence interval (CI) for the patient population. We evaluated whether variations in metrics for each adapted fraction exceeded the 95% CI, which were flagged for retrospective investigation to identify the cause of high variation in their dosimetric indices such as contouring or planning mistakes and determine sensitivity (Fig. 1). The same number of non-flagged ART plans were also evaluated for comparison and specificity of the tool. Results During the retrospective investigation of flagged adaptive plans, a list of causes for deviations were determined. These causes include excessive normalization, inappropriate alignment (Fig. 2), large anatomical changes, etc. For those plans not flagged by our tool, adaptive plans that were found to be low quality were the plans that started with the low quality simulation plan or were under-contoured. We estimated the sensitivity (true positive) and specificity (true negative) of our method to be 84% and 76%, respectively.