Prospective Paired Comparison of MRI- and CT-Based Planning for Adjuvant Vaginal Cuff Brachytherapy in Endometrial Cancer: An Exploratory Study

Objectives: To assess the impact of magnetic resonance imaging (MRI) versus computed tomography (CT) on target delineation and dosimetry in adjuvant vaginal cuff brachytherapy (VBT) for endometrial cancer. Methods: Twenty-five consecutive patients receiving adjuvant high-dose-rate VBT after hysterectomy were prospectively included. All underwent standardized treatment of 4 × 5 Gy in 5 mm tissue depth using a vaginal cylinder with a fixed loading pattern; individualized optimization was permitted for suboptimal coverage or proximity of organs at risk (OARs). For each patient, clinical target volume (CTV) and OARs (bladder, rectum, sigmoid, bowel) were contoured on both CT and MRI of the first fraction. Dosimetric parameters (CTV D90, D98, D50; OAR D2cc) were compared. Treatment characteristics and outcomes were analyzed retrospectively. Results: Median CTV was smaller on MRI (9.8 cm3) than on CT (13.6 cm3). CTV coverage was comparable between modalities, with mean EQD2 D90 of 5.32 Gy per fraction on MRI versus 5.10 Gy per fraction on CT (p = 0.433) and mean EQD2 D98 of 3.95 Gy per fraction on MRI versus 3.80 Gy per fraction on CT (p = 0.491). In contrast, EQD2 D50 was significantly higher on MRI-based plans than on CT-based plans (9.12 vs. 8.62 Gy per fraction, p = 0.012). OAR D2cc values showed no significant differences between modalities. Most patients (80%) were treated with the institutional standard plan, and 20% required individualized optimization. After a median follow-up of 25 months (range 0–72), five patients (20%) recurred (one local, one regional, three distant), and one (4%) died. No grade ≥ 3 gastrointestinal or genitourinary toxicities occurred among patients with available data. Conclusions: In this small exploratory cohort, MRI-based planning delineated smaller CTVs, whereas the evaluated target-coverage and OAR-dose parameters were broadly similar between modalities. These findings suggest that CT-based planning may be adequate for many routine cases, with MRI potentially useful for selected, anatomically complex patients. Confirmation in larger multicenter studies is required.