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Do Organ-At-Risk Contours Need to Be Accurate in Prostate Radiotherapy?

H Liu*, B Sintay, Q Shang, D Wiant, Cone Health, Greensboro, NC

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: contouring is time- and labor-intensive and subject to inter- and intra-observer variabilities. Furthermore, contours drawn prior to treatment do not necessarily represent anatomy over the treatment course. This study aims to investigate the impact of OAR contour accuracy on the treatment outcome.


Methods: seminal vesicles (SV), bladder, and rectum were manually contoured on planning CT and daily CBCT for 20 prostate patients. OAR (bladder and rectum) contours were automatically generated with atlas-based segmentation algorithms on planning CT. Treatment plans were generated via a novel automated planning application that accessed a knowledge-based planning solution for both auto-generated OARs (auto-plan) and manual OARs (manual-plan). Both low-risk patients (LRP, CTV=Prostate) and high-risk patients (HRP, CTV=Prostate+SV) were included in this simulation. Doses were transferred from planning CT to CBCTs based on the clinical shifts, and daily dose comparisons were performed for CTV, bladder and rectum between auto- and manual-plans.


Results: mean expansion of the planning contours needed to cover 95% of the daily CBCT contours were: 2.6±2.0 mm for prostate, 4.4±3.8 mm for prostate+SV, 5.0±5.2 mm for bladder, and 5.1±3.8 mm for rectum. Overlap index/Dice similarity coefficient of 0.89±0.10/0.77±0.11 for bladder, and 0.76±0.15/0.70±0.07 for rectum were found between auto- and manual-contours. For both LRP and HRP, adequate daily CTV coverages were achieved for both auto- and manual-plans, and no significant daily dose differences were observed between auto- and manual-plans for OARs. For LRP: dDmax: -0.05±0.73Gy (bladder) and -0.41±0.80Gy (rectum); dD60: 0.4±0.9% (bladder) and 1.0±1.3% (rectum). For HRP: dDmax: -0.1±0.56Gy (bladder) and -0.26±0.76Gy (rectum); dD60: 0.2±0.6% (bladder) and 1.1±2.0% (rectum).


Conclusion: contours manually drawn are being used to represent anatomy that fluctuates by multiple millimeters on a daily basis thereby making the time and effort questionable. Automated contouring tools offer improvement in contour consistency, and provide acceptable delivered doses compared with precision manual contours.

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

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