Utilizing target tracking and fast dose optimization strategies to drive efficient PTV margin reduction in MRIgART
Creators: name: Snyder, Jeffrey
Publisher: name: University of Iowa
Year: 2026
Subjects: Intra-fraction
MRI guided adaptive radiotherapy
real-time adaptive radiotherapy
SBRT
Target Tracking
VMAT
public abstract
Medical Imaging

description: Purpose: Long treatment session times in MRI guided adaptive radiotherapy (MRIgART) reduce patient throughput and increase the magnitude of intra-fraction motion, limiting the full potential of PTV margin reduction and organ at risk (OAR) sparing. This work evaluates a new leaf sequencer and dose optimizer combination called optimal fluence levels and pseudo gradient descent (OFL+PGD) and tests the algorithms ability to reduce treatment plan optimization times and temporal efficiency gains of incorporating VMAT into MRIgART. An intra-fraction dose accumulation method and a sub-fraction dose re-optimization technique are presented to characterize and compensate for the impact of intra-fraction target motion.
description: Materials and Methods: Optimization time, delivery time, and plan quality were retrospectively compared between IMRT plans generated with the OFL+PGD algorithm and the standard clinical Hyperion optimizer for a variety of anatomic sites including prostate, pelvic oligometastases, liver, and pancreatic cancer. Similarly, the time benefits of incorporating VMAT into the MRIgART workflow were evaluated by optimizing prostate SBRT plans with the OFL+PGD algorithm (OFL+PGD VMAT), the clinical Hyperion algorithm with VMAT (Hyp VMAT), and Hyperion based IMRT (Hyp IMRT). Optimization time, estimated delivery time, and plan quality as compared to Hyp IMRT were evaluated. The impact of intra-fraction motion on GTV coverage was assessed by tracking the GTV position on the cine MRI images acquired during treatment delivery and creating an intra-fraction dose distribution for each IMRT beam. Intra-fraction GTV coverage was assessed for 2 mm, 3 mm, and 5 mm PTV margins for pelvic oligometastases. For prostate cancer a novel sub-fraction workflow is presented which incorporates dose re-optimization to account for intra-fraction prostate drift as identified on cine MRI imaging. The efficacy of using 2 mm margins with the sub-fraction workflow was tested by reconstructing the intra-fraction prostate dose with no corrections applied (no-adapt), a single adaption prior to treatment (ATP), and with the full sub-fraction workflow (intra-adapt).
description: Results: For IMRT, the OFL+PGD algorithm reduced optimization times by 51.4 ± 5.0 % (p = 0.002) as compared to the Hyperion algorithm. When incorporating VMAT the total reduction in treatment session time was 33.1 ± 194.9 seconds (p = 0.138) and 402.7 ± 115.1 seconds (p &lt; 0.001) for Hyp VMAT and OFL+PGD VMAT, respectively as compared to Hyp IMRT. Equivalent plan quality in terms of OAR sparing and target conformity were found between both algorithms. When considering the dosimetric impact of intra-fraction motion for pelvic nodes our work found that the 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. The bowel D0.5cc was reduced by 27.4 ± 12.3% (4.0 ± 2.2 Gy) when using 2 mm margins as compared to 5 mm margins. For prostate SBRT the minimum dose received by the prostate was less than 95 % of the prescription dose in 84 %, 36 %, and 10 % of fractions, for the non-adapt, ATP, and full sub-fraction intra-adapt groups, respectively.
description: Conclusion: OFL+PGD significantly improves MRIgART efficiency by reducing optimization times. To date, the delivery time benefits of VMAT in MRIgART have not been realized due to the increased optimization times associated with VMAT plans. Our work overcomes this challenge, demonstrating that OFL+PGD VMAT reduces treatment session times by greater than 6 minutes. A novel intra-fraction dose accumulation methodology has been developed which found that PTV margin reduction below the currently used standard is feasible for pelvic oligometastases. For other anatomic sites, such as prostate, the fast intra-fraction dose re-optimization strategy presented was effective in mitigating intra-fraction target motion and enabled 2 mm PTV margins. These PTV margin reductions led to reduced OAR doses.