Computed Tomography-Guided and Magnetic Resonance-Guided Adaptive Radiotherapy for Gynecological Stereotactic Body Radiotherapy Treatments: A Dosimetry and Workflow Analysis

Purpose: To evaluate the dosimetric benefit of online adaptive radiotherapy (oART) for adjuvant gynecological stereotactic body radiotherapy (SBRT) and to compare plan quality and workflow efficiency between computed tomography-guided adaptive radiotherapy (CTgART) and magnetic resonance-guided adaptive radiotherapy (MRgART) platforms. Materials and methods: Ten patients (50 fractions) treated between September 2024 and October 2025 were analyzed (CTgART: 35 fractions; MRgART: 15 fractions). Each adapted plan was compared with its corresponding non-adapted plan recalculated on the anatomy of the day. Wilcoxon signed-rank tests (W) were used to determine statistical significance (p < 0.05). Adapted plans between CTgART and MRgART were compared dosimetrically using Mann-Whitney U tests (U) for statistical significance (p < 0.05). For each comparison, the dose constraints defined by the trial protocol were used as a baseline. Timestamps from each adaptive system were extracted from the system logs and compared for each fraction. Results: Daily adaptation substantially improved paravaginal planning target volume (PTV) coverage (V28.5Gymean: 98.0% vs. 88.2%, W = -1205, p < 0.01) and clinical target volume (CTV) coverage (V30Gymean: 98.2% vs. 82.0%, W = -1189, p < 0.01), with 72% (36/50 fractions) of non-adaptive fractions failing the target goals. Adaptive planning also substantially improved bowel sparing (V25Gymean: 27.4cc vs. 56.1cc, W = 1207, p < 0.01). Between the CTgART and MRgART platforms, paravaginal target coverage was comparable. CTgART achieved higher nodal PTV coverage (V28.5Gymean: 95.4% vs. 90.7%, U = 89.5, p < 0.01), while MRgART demonstrated improved bladder sparing (V20Gymean: 23.4% vs. 43.4%, U = 15, p < 0.01). The CTgART workflow was more efficient (average completion time of 49.4 vs. 91.9 minutes) and had fewer interruptions (5.7% vs. 33.3%). Conclusions: Online adaptation is essential to consistently meet protocol targets in gynecological SBRT, enhancing both target coverage and organs at risk sparing. While CTgART and MRgART can both achieve clinically acceptable plans, the CTgART workflow currently demonstrates superior workflow efficiency.