Intra-fraction motion of pelvic oligometastases and feasibility of PTV margin reduction using MRI guided adaptive radiotherapy

Abstract

PurposeThis study assesses the impact of intra-fraction motion and PTV margin size on target coverage for patients undergoing radiation treatment of pelvic oligometastases. Dosimetric sparing of the bowel as a function of the PTV margin is also evaluated.Materials and methodsSeven patients with pelvic oligometastases previously treated on our MR-linac (35 Gy in 5 fractions) were included in this study. Retrospective adaptive plans were created for each fraction on the daily MRI datasets using PTV margins of 5 mm, 3 mm, and 2 mm. Dosimetric constraint violations and GTV coverage were measured as a function of PTV margin size. The impact of intra-fraction motion on GTV coverage was assessed by tracking the GTV position on the cine MR images acquired during treatment delivery and creating an intra-fraction dose distribution for each IMRT beam. The intra-fraction dose was accumulated for each fraction to determine the total dose delivered to the target for each PTV size.ResultsAll OAR constraints were achieved in 85.7%, 94.3%, and 100.0% of fractions when using 5 mm, 3 mm, and 2 mm PTV margins while scaling to 95% PTV coverage. Compared to plans with a 5 mm PTV margin, there was a 27.4 ± 12.3% (4.0 ± 2.2 Gy) and an 18.5 ± 7.3% (2.7 ± 1.4 Gy) reduction in the bowel D0.5cc dose for 2 mm and 3 mm PTV margins, respectively. The target dose (GTV V35 Gy) was on average 100.0 ± 0.1% (99.6 – 100%), 99.6 ± 1.0% (97.2 – 100%), and 99.0 ± 1.4% (95.0 – 100%), among all fractions for the 5 mm, 3 mm, and 2 mm PTV margins on the adaptive plans when accounting for intra-fraction motion, respectively.ConclusionA 2 mm PTV margin achieved a minimum of 95% GTV coverage while reducing the dose to the bowel for all patients.

Simulating an intra-fraction adaptive workflow to enable PTV margin reduction in MRIgART volumetric modulated arc therapy for prostate SBRT

“…A preclinical motion management research package (MMRP) was used to retrospectively analyze and track the position of the prostate in each frame of the cine MRI acquisitions. Details of the target tracking algorithm have been previously published ( 39 , 60 , 61 ). Briefly, the algorithm begins by using 2D-3D template matching.…”

“…The DICOM coordinates from each of the eight treatment arcs were shifted to account for the prostates average position during the delivery of that beam. In this scenario, the isocenter position of each beam was shifted to represent the effective motion of the beam with respect to the fixed reference of the prostate on the daily MRI image ( 39 ). The shifted beam doses were then summed and composite doses were viewed within Velocity (Varian Medical Systems Inc., Palo Alter, CA, version 3.2.1).…”

“…Sub-fraction doses were first rigidly registered onto the daily MRI dataset. Next, intra-fraction dose was accumulated following the methodology provided by Snyder et al (39). Briefly, the prostate position as a function of time defined from the cine MRI images and target tracking was synchronized with the radiation therapy treatment delivery.…”

“…MRI guided adaptive radiotherapy (MRIgART) has emerged as a promising technique for treating prostate cancer. MRIgART enables real time cine imaging and target tracking without additional ionizing radiation or fiducial markers ( 38 , 39 ). It is estimated that one third of prostate patients require adaptive radiotherapy which can be applied online with MRIgART ( 40 ).…”