Dosimetric Impact of Baseline Shift Corrections during Online Adaptive Prostate Mrgrt on Elekta Unity.
Abstract
Purpose
Quantify dosimetric differences between original online adaptive plans (adapt-to-position [ATP] or adapt-to-shape [ATS]) and baseline shift (BLS)–corrected plans for prostate MRgRT on 1.5 Tesla MR-Linac. BLS is a fast, during treatment, replan applying virtual couch shift (segment aperture morphing) to correct for motion/prostate drifts.
Methods
Fourteen prostate patients were selected (7 prostate-only: 40 Gy/5; 7 prostate plus DIL boost: 40/45 Gy/5). Rectal spacer utilization was 13/14. Preparation included enema and 1/2 cup water. Thirty-seven BLS events were identified; (34 analyzed). Each adapted plan was compared with the corresponding BLS plan,by shift magnitude: 3 mm (2.5-3.4mm) and 4 mm (≥3.5mm). Metrics included CTV Dmean, PTV_Prostate and PTV_DIL D0.035cc, and rectum/bladder/urethra D0.035cc. Plans used a 3 mm dose grid/1% statistical uncertainty per plan. Percent differences (adaptive vs BLS) were summarized as mean±SD.
Results
BLS increased dose hotspots and mean-dose metrics relative to adapted plans, with larger deviations for larger shifts. . Mean bladder volumes were prostate+DIL 126.7 cc and 219.4 cc; prostate-only 199.0 cc and 177.3 cc for 3mm and 4mm respectively. Prostate+DIL: urethra D0.035cc increased by 1.72%±1.53% (3mm) and 4.66%±2.74% (≥4mm); rectum D0.035cc increased by 0.78%±1.58% and 3.12%±2.10%; CTV Dmean increased by 1.08%±0.75% and 2.85%±1.56%. Prostate-only: CTV Dmean increased by 2.38%±1.81% and 1.83%±0.67%; PTV_Prostate D0.035cc by 4.22%±2.43% and 3.21%±0.50%; and urethra D0.035cc by 3.94%±3.94% and 3.01%±1.62.
Conclusion
Relative to adapted plans, BLS increased target mean/hotspot dose and OAR high-dose metrics, most notably urethral hotspot dose, particularly for larger shifts. Because BLS is typically applied mid-to-late fraction, the fraction-averaged impact may be smaller than the full-plan differences reported. BLS enables rapid geometric correction, but repeated use across/early-in fractions instead of re-adaptation should be approached cautiously due to potential dose increases.