Feasibility of PTV Margin Reduction for Prostate SBRT Treated Using MRI-Guided Radiotherapy
J Snyder1*, J St. Aubin1, D Dunkerley1, D Hyer1, (1) University of Iowa Hospitals and Clinics, Iowa City, IA
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Purpose: The ability to correct for inter-fraction motion using MRI-guided adaptive radiotherapy should enable reduction of the PTV margins for prostate SBRT treatments. The purpose of this study is to examine prostate intra-fraction motion to determine feasible margin reductions and the potential impact on organ-at-risk (OAR) sparing.
Methods: One patient with prostate cancer previously treated on the MRI-linac was retrospectively evaluated. In this study, five plans using five different PTV margins were created. The margins were created by uniformly expanding the prostate by 5mm, 4mm, 3mm, 2mm, and 1mm. The prescription dose for all plans was 36.25 Gy in five fractions and each plan had 95% PTV coverage. OAR sparing for each different margin was evaluated relative to the 5mm margin plan used clinically. Sagittal and coronal cine MR images were collected during each clinical treatment delivery and used to retrospectively monitor the position of the prostate throughout treatment. This positional data was used to evaluate the various PTV margins.
Results: Compared to the 5mm margin plan, reductions in rectum D1cc doses of 5.8%, 11.7%, 22.7%, and 25.6% were measured for the 4mm, 3mm, 2mm, and 1mm margin plans, respectively. Similarly, dose reductions of 8.1%, 26.9%, 50.3%, and 55.8% were found for the bladder D5cc dose for the 4mm, 3mm, 2mm, and 1mm margin plans, respectively. Amongst all five fractions the average time that the prostate remained within the PTV margin during treatment would have been 100.0%, 99.1%, 86.9%, 47.3%, and 22.7% for the 5mm, 4mm, 3mm, 2mm, and 1mm margin plans, respectively.
Conclusion: This work shows that uniform margin reductions as low as 4mm may be clinically feasible in prostate SBRT and that such reductions can reduce OAR doses. Further margin reduction may be possible if non-isotropic margins are used.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported through an industrial grant from Elekta (Stockholm, Sweden)
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