3376 - A Comprehensive Population Study of Prostate MR-Linac SBRT: Quantifying Motion-Induced Duty Cycle Variations and Simulated Non-Gated Dosimetry

Purpose/Objective(s):

Intrafraction prostate motion during SBRT can cause clinically meaningful target excursion and dose degradation. Population-level beam-on intrafraction motion is poorly quantified because acquisition of continuous, high-resolution intrafraction prostate imaging is challenging with conventional image-guidance approaches. Using a large cohort of continuous cine MRI acquired during prostate MR-guided SBRT, we quantified beam-on prostate displacement and duty-cycle distributions and evaluated the resulting changes in target coverage and OAR dosimetry using motion-perturbed dose simulations.

Materials/Methods:

Intrafraction cine MRI images were analyzed for 154 patients (770 fractions) with prostate carcinoma treated with MRI guided daily adaptive SBRT. Patients received 36.25-45 Gy in 5 fractions on a 0.35T MRI linear accelerator using gated delivery with continuous target tracking at 4-8 frames/second. A GTV that included the whole prostate and a 3-mm gating region of interest (ROI) were contoured on sagittal and coronal cine MRI. Beam delivery was interrupted when more than 5% of the GTV exited the ROI.

An automated computer-vision–based tracking pipeline quantified frame-by-frame GTV position. Beam-interruption events (BIEs) were summarized by gating duty cycle and displacement magnitude. The dosimetric impact of intrafraction motion was retrospectively evaluated using five simulated test plans, including three MR-guided SBRT reference plans and two X-ray–guided SBRT plans. Weighted dose accumulations were performed to simulate motion-perturbed scenarios. The final dose was modeled by compounding the static plan with a motion-affected volume using the lowest-decile duty cycle. Four motion perturbation scenarios were simulated based on the mean superior/inferior and anterior/posterior displacements observed using cine-MRI.

Results:

Duty cycle was high overall (94% ± 6%); however, the lowest decile of fractions exhibited duty cycles = 87%. Across all BIEs, 49% included an anterior component (mean at BIE: 5 ± 6 mm), and 60% included an inferior component (mean at BIE: 7 ± 6 mm) (Table 1). Motion perturbed plans showed a reduction in target coverage with the most pronounced effect observed for inferior displacements, with a mean ?PTV D95% of -4.7 ± 2.6%.

Conclusion:

Continuous cine-MRI shows that while prostate MR-guided SBRT achieves high duty cycles overall, patients in the lowest duty cycle decile (= 87%) experience inferior-dominant prostate displacement that, if untreated with gating, could reduce target coverage. Future work will involve using this for application of treatment margins for prostate SBRT with and without MRI guidance.

Table 1: Summary results of the intrafraction target displacements and the target and OAR dosimetric endpoints for motion simulation assuming mean displacements in each direction.

	Anterior	Posterior	Superior	Inferior
Displacement	5 ± 6 mm	1.3 ± 1.3 mm	1.5 ± 1.2 mm	7 ± 6 mm
PTV ?D95 (%)	-2.7 ± 1.3	-4.1 ± 5	-1.2 ± 1.7	-4.7 ± 2.6