Dosimetric Impact of Residual Intrafraction Motion in Gated MR-Guided Prostate SBRT With Focal Dose Intensification
- 1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA
Abstract
PurposeTo 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 MethodsThirty 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.
ResultsMedian 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 %.
ConclusionResidual 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.
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BibTeX
@misc{tam2026dosimetric,
title = {{Dosimetric Impact of Residual Intrafraction Motion in Gated MR-Guided Prostate SBRT With Focal Dose Intensification}},
author = {K. H. Tam and S. Jogi and A. Sobremonte and J. D. Ohrt and D. J. Rhee and J. Yang and Y. Ding and K. K. Brock and P. A. Balter and C. J. Hassanzadeh and M. K. Rooney and S. Choi and R. J.-h. Park and C. Tang and K. R. Patel and S. J. Frank and P. T. Tran and D. E. Hyer and N. Tyagi and E. D. Subashi},
howpublished = {medRxiv},
year = {2026},
month = sep,
doi = {10.64898/2026.09.11.26362864},
url = {https://www.medrxiv.org/content/10.64898/2026.09.11.26362864v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Dosimetric Impact of Residual Intrafraction Motion in Gated MR-Guided Prostate SBRT With Focal Dose Intensification
AU - K. H. Tam
AU - S. Jogi
AU - A. Sobremonte
AU - J. D. Ohrt
AU - D. J. Rhee
AU - J. Yang
AU - Y. Ding
AU - K. K. Brock
AU - P. A. Balter
AU - C. J. Hassanzadeh
AU - M. K. Rooney
AU - S. Choi
AU - R. J.-h. Park
AU - C. Tang
AU - K. R. Patel
AU - S. J. Frank
AU - P. T. Tran
AU - D. E. Hyer
AU - N. Tyagi
AU - E. D. Subashi
PY - 2026
DA - 2026/09/11
PB - medRxiv
DO - 10.64898/2026.09.11.26362864
UR - https://www.medrxiv.org/content/10.64898/2026.09.11.26362864v1
AB - PurposeTo 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 MethodsThirty 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. ResultsMedian 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 %. ConclusionResidual 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.
ER -