Feasibility of Adapt-To-Position Treatment of Prostate Patients Undergoing Ultra-Hypofractionated MR-Guided Adaptive Radiotherapy
S Burleson*, V Brennan, M Zelefsky, N Tyagi, Memorial Sloan-Kettering Cancer Center, New York, NY
Presentations
PO-GePV-M-177 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To investigate the feasibility and dosimetric outcome of performing adapt-to-position (ATP) workflow on prostate cases undergoing ultra-hypofractionated radiotherapy with boost to dominant intraprostatic lesion (DIL).
Methods: 10 prostate patients were treated on Unity MR-Linac using a five-fraction regimen with 40Gy to prostate and 45Gy simultaneous-integrated-boost to DIL. All patients were treated clinically using adapt-to-shape (ATS) workflow with full re-plan and contour adjustments. Retrospectively, an ATP plan was generated on the original planning CT (pCT) to account for daily prostate shifts based on soft tissue match. The ATP plan was also re-calculated on the daily MR (dMR) with the adjusted contours-of-the-day to see the realistic impact of performing ATP. Dosimetric data to OAR’s (bladder, rectum, bowel) and targets (PTV_Prost, PTV_DIL) were compared between the treated ATS (ATS_txt), ATP (ATP), and the ATP plan recalculated on dMR (ATP_recalc). Volume and positional/center of mass (COM) changes for targets were compared between pCT and dMR’s.
Results: PTV_Prost and PTV_DIL coverage decreased more than 10% on ATP_recalc compared to the ATS_txt in 20% and 28% of fractions respectively. This coverage drop was not seen on any of the ATP plans. The average volume change of the PTV_DIL contour on dMR compared to pCT for the course of treatment ranged from 0.5-10 cc indicating during treatment changes. The difference in relative distance between the PTV_Prost COM and the PTV_DIL COM between the pCT and dMR’s ranged from 0.3-1.42 cm. Certain OAR constraints were violated on ATP_recalc but were met on ATP.
Conclusion: The decrease in target coverage and change in relative COM highlights that ATP planning is not an optimal approach for MR-guided adaptive prostate radiotherapy when boosting the DIL since ATP does not reflect daily variation in position and deformation of soft tissue anatomy and may result in OAR dosimetric parameters being exceeded.
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