3148 - Dosimetric Impact of Intra-Delivery Motion In Stereotactic Body Radiotherapy to the Prostate and Dominant Lesion
Presenter(s)
K. H. Tam1, S. Jogi1, A. Sobremonte1, J. D. Ohrt1, D. J. Rhee1, J. Yang1, Y. Ding1, K. K. Brock2, P. Balter1, C. J. Hassanzadeh3, M. K. Rooney3, S. Choi3, R. J. H. Park4, C. Tang4, S. J. Frank4, D. Hyer5, N. Tyagi6, and E. D. Subashi1; 1Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 2Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 3Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 4Department of Genitourinary Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 5University of Iowa Hospitals and Clinics, Department of Radiation Oncology, Iowa City, IA, 6Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY
Purpose/Objective(s):
To quantify the dosimetric impact of residual intra-fraction motion during MRI-guided SBRT for localized prostate adenocarcinoma with simultaneous integrated boost to the dominant intraprostatic lesion.Materials/Methods:
A cohort of 30 patients treated with 5-fraction SBRT (36.25/40/45 Gy to PTV/CTV/GTV) was analyzed retrospectively (150 fractions). CTV-PTV margins were 3–4 mm isotropically, 2–3 mm posteriorly. Real-time MRI recorded target positions at 5 frames/second in sagittal/coronal planes and was synchronized with machine log files. The beam was automatically paused if the CTV extended beyond a user-defined out-of-PTV volume limit (0–5%). Motion range was defined as the 95th-percentile (p95) spread of target displacement. Motion vectors were also decomposed into drift and oscillatory components via low- and high-pass filtering. For gated and non-gated events, duty cycle was calculated as the fraction of time beam delivery remained uninterrupted. Delivered dose was reconstructed by integrating segment-level dose matrices weighted by monitor units at each motion state and compared to planned DVH metrics for GTV, CTV, and urethra. The effect of intra-delivery corrections by a baseline shift (BLS) plan was analyzed in all clinical use cases (32 fractions).Results:
Median (IQR) treatment time was 13.2 min (12.0–15.1 min) and duty cycle due to gating events was 93.2% (80.6–97.8%). Median p95 SI/AP/LR motion was 2.9/1.4/0.5 mm and it exceeded 3 mm in 47.7/13.2/0.7% of fractions; the dominant motion mode was SI drift (median p95 2.0 mm). Dose differences are summarized in Table 1. DVH deviations correlated strongly with displacement perpendicular to the radiation beam (r=-0.52, p<0.001). BLS corrections resulted in significant improvements in coverage for GTV V45Gy (median 81.2 vs 66.6%, p=0.002), GTV D95% (8.9 vs 8.6 Gy/fx, p<0.001), CTV V40Gy (90.8% vs 89.4%, p=0.03), CTV D95% (7.9 vs 7.8 Gy/fx, p=0.005), and reduced urethra D0.1cc (8.3 vs 8.7 Gy/fx, p=0.038). Acute GU grade =1 events were reported in 7 patients; no acute GI toxicity; maximum IPSS increase was 9.Conclusion:
Intra-delivery motion contributes to measurable differences between planned and delivered dose distributions during gated MRI-guided prostate SBRT. We present a dose reconstruction framework to systematically evaluate the dosimetric impact of residual motion. This approach enables patient-specific dose assessment and may inform the selection of individualized margins and adaptive strategies. Table 1. DVH Metric Deviations due to Residual Intra-Fraction Motion: (Reconstructed-Plan)/Plan| Per-fraction | Cumulative (5 Fx) | ||
| GTV | ?V45Gy | -5.6% (-15.3 to -0.2%) | -5.6% (-14.1 to 0.5%) |
| ?D95% | -1.2% (-2.4 to -0.1%) | -1.2% (-1.8 to -0.3%) | |
| CTV | ?V40Gy | -2.1% (-4.1 to -0.4%) | -0.9% (-2.8 to -0.6%) |
| ?D95% | -0.5% (-1.1 to -0.1%) | -0.3% (-0.6 to -0.1%) | |
| Urethra | ?D0.1cc | 0.2% (-0.1 to 0.6%) | 0.2% (0.1 to 0.8%) |