Purpose: MRI-guided adaptive radiotherapy should enable PTV margin reduction in pelvic oligometastatic disease due to the fact that inter-fraction variations can be accounted for with adaptive planning prior to treatment. The aim of this study is to determine the feasible extent of margin reduction and the impact on OAR sparing.
Methods: One patient with a single pelvic oligometastasis previously treated on the MRI-linac was retrospectively evaluated. The clinical treatment delivered 40Gy in five fractions using a uniform 5mm expansion of the GTV. In this study, additional plans were created using uniform 4mm, 3mm, 2mm, and 1mm margins. All plans were normalized for equivalent PTV coverage and OAR sparing was compared to the clinical plan. During each fraction, cine MR images were acquired at 5 frames per second. A research tracking algorithm was used to monitor the position of the GTV throughout each fraction. Target drift values were used to assess the feasibility of each PTV expansion.
Results: Reduction in bowel D0.03cc doses of 10.6%, 14.5%, 29.4%, and 31.7% were measured for the 4mm, 3mm, 2mm, and 1mm margin plans, respectively, as compared to the clinical 5mm PTV margin. Similarly, dose reductions of 7.5%, 15.1%, 21.6%, and 24.2% were found for the rectum D0.03cc dose for the 4mm, 3mm, 2mm, and 1mm margin plans, respectively. The GTV remained within the 5mm, 4mm, and 3mm PTV expansions greater than 99.9% of the time for all 5 fractions. The GTV remained within the PTV margin on average 99.1% and 42.4% of the time for the 2mm and 1mm margin expansion, respectively.
Conclusion: This work shows that margins as small as 2mm may be clinically feasible for pelvic oligometastatic disease and that such reductions can substantially reduce OAR doses. A study incorporating a larger cohort of patients will follow.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by an industrial grant from Elekta (Stockholm, Sweden)
Image-guided Therapy, MRI, Treatment Planning
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking