Dosimetric Impact of Residual Intrafraction Motion in Gated MR-Guided Prostate SBRT With Focal Dose Intensification

Purpose: To quantify the dosimetric effect of residual intrafraction motion during gated MR-guided prostate stereotactic body radiotherapy with simultaneous integrated boost to the dominant intraprostatic lesion, and to derive and evaluate target-specific, population-based margins designed to mitigate motion-induced coverage loss. Materials and Methods: Thirty patients were treated on a 1.5-T MR-Linac with 36.25, 40, and 45 Gy prescribed to the planning target volume, clinical target volume (CTV), and gross tumor volume (GTV), in 5 fractions. Across 150 fractions, segment-level dose matrices synchronized with time-resolved target displacements were combined to reconstruct per-fraction and cumulative dose. Directional prescription-isodose deviations were used to derive internal target volume margins, which were evaluated through offline replanning, gating-envelope analysis, and dose reconstruction. Results: Median 95th-percentile motion ranges were 0.5, 1.4, and 2.9 mm in the left-right, anterior-posterior, and superior-inferior directions; these exceeded 3 mm in 0.7%, 13.2%, and 47.7% of fractions, respectively. The median treatment time was 13.2 min (IQR, 12.0-15.1 min) with a median gating duty cycle of 93.2% (80.6%-97.8%). Per-fraction median GTV deviations were -1.2% (-2.4% to -0.1%) for D95 and -5.6% (-15.3% to -0.2%) for V45Gy; corresponding CTV deviations were -0.5% (-1.1% to -0.1%) and -2.1% (-4.1% to -0.4%). Cohort-derived margins of 2/2/1/2/1/2 mm for the GTV and 1/1/1/2/1/2 mm for the CTV in the left/right/anterior/posterior/superior/inferior directions achieved sufficient coverage in approximately 94% of fractions in each direction. Gated delivery using the proposed margins improved coverage but may reduce duty cycle by an average of 6.9 +/- 4.7 %. Conclusion: Residual intrafraction motion produced directionally asymmetric coverage loss, with the largest effect on the boosted GTV. Motion-inclusive dose reconstruction enabled derivation of practical asymmetric margins that improved robustness while identifying the tradeoff between target coverage and delivery efficiency.