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PTV Margin Reduction Study for 2-Phase Image-Guided VMAT Prostate Treatment

S Yaghoobpour Tari*, J Dubicki, A Hallock, Walker Family Cancer Center, St. Catharines, ON, CA

Presentations

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

Room: AAPM ePoster Library

Purpose: aim of this study is to determine if the reduction of the phase I prostate Planning Target Volume (PTV) margin for the CBCT-guided VMAT treatment can still ensure the Clinical Target Volume (CTV) coverage while further reducing the dose to the rectum.


Methods: patients already treated using 2-phase protocol for the intact prostate, seminal vesicles and pelvic nodes using daily CBCT-guided are chosen. For Phase I IGRT, the bony match is done along with the soft tissue match to ensure the prostate and seminal vesicles coverage. Two VMAT plans are created from the CTV and OARs on the planning CT for each patient. Each plan has a different PTV margin for the CTV of the prostate+seminal vesicles using current CTV+10mm, except 7mm posteriorly margin and proposed CTV+7mm, except 5mm posteriorly margin. The pre-treatment CBCTs from fractions 1, 5, 10, 15, and 20 with the translational shift recorded at the time of the treatment are imported to the Pinnacle treatment planning system. The CTVs and rectums are contoured on each CBCT image. Plans with different PTV margins were mapped from the planning CT to the daily CBCT data to assess the effect of daily changes in patient’s anatomy on DVH objectives and dependence on the PTV margin.


Results: V100% CTV dose for the CT planning data is 98.90±1.68% and 98.87±1.72% for current and reduced margins respectively and the V100% CTV dose for the cumulative daily CBCT data is 99.32±0.69% and 99.28±0.79% for current and reduced margins respectively. The mean dose to rectum is lower for the reduced margin plans by average of 2.43% for CT planning data and 2.26% for the cumulative daily CBCT data.


Conclusion: reduced margin of CTV is implemented in our GU protocol with the sufficient coverage for the prostate and reduced dose to the rectum.

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