H Liu*, C Amaloo, B Sintay, D Wiant, Cone Health, Greensboro, NC

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  H Liu

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PO-GePV-T-256 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Radiotherapy is a widely accepted standard of care for early stage prostate cancer, and it is believed that the plan quality and treatment outcome are associated with contour accuracy of both the target and organs-at-risk (OAR). The purposes of this study are to: (1) assess geometric and dosimetric uncertainties due to inter-observer contour variabilities and (2) evaluate the effectiveness of geometric indicators to predict target dosimetry in prostate radiotherapy.

Methods: Twenty prostate patients were selected for this retrospective study. Five experienced clinicians created unique structure sets containing prostate, seminal vesicles, bladder, and rectum for each patient. A fully automated script and knowledge-based planning routine was utilized to create standardized and unbiased plans that could be used to evaluate changes in isodose distributions due to inter-observer variability in structure segmentation. Plans were created on a “gold-standard” structure set, as well as on each of the user defined structure sets. The correlation between geometric and dosimetric indices was evaluated using R2 statistics for plans generated with different combination of structure sets.

Results: Inter-observer variability of contours during structure segmentation was very low for clearly defined organs such as the bladder but increased for organs without well-defined borders (prostate, seminal vesicles and rectum). For plans generated with the user defined structure sets, strong / moderate correlations (correlation coefficient 0.4 < |r| < 1 ) were observed between the geometric indicators for target structure agreement and target coverage for both low-risk and intermediate risk patient groups, while OAR indicators showed no correlation (|r| < 0.2) to final dosimetry.

Conclusion: Level of target delineation accuracy is crucial in order to maintain adequate dosimetric coverage regardless of the associated inter-observer uncertainties in OAR contours that had limited impact upon final dosimetry.

Funding Support, Disclosures, and Conflict of Interest: This research was supported by a grant from Varian Medical Systems, Palo Alto, CA.

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