18-Hour Planar Scintigraphy Versus SPECT/CT for Sentinel Lymph Node Detection in Early-Stage Endometrial Cancer

Background/Objectives: The preoperative detection of sentinel lymph nodes (SLN) using technetium-99m (Tc-99m) is crucial for surgical staging in early-stage endometrial cancer (EC). The optimal imaging timing and modality remain debated. This study compares early planar scintigraphy (30 min), SPECT/CT (1 h), and 18-h planar scintigraphy after a single Tc-99m injection. Methods: A total of 125 patients with early-stage EC underwent SLN mapping with Tc-99m (120 MBq). Imaging included 30-min planar scintigraphy, SPECT/CT (1 h), and 18-h planar scintigraphy on the day of surgery. Detection sensitivity, the bilateral mapping rate, and image quality (signal-to-noise ratio (SNR), contrast factor (C-factor)) were evaluated, with intraoperative gamma probe detection and histopathology as references. Results:The 18-h planar scintigraphy achieved the highest SLN detection sensitivity (94.4%, 118/125), compared with SPECT/CT (87.2%, OR = 2.48, 95% CI: 0.98-6.27, p = 0.051) and 30-min scintigraphy (72.0%). Only the 18-h protocol underwent intraoperative and histopathological verification; results for 30-min planar and 1-h SPECT/CT were based on imaging alone, which limits direct comparability. Bilateral detection was higher at 18 h (80.80%) than SPECT/CT (73.60%). All SLNs detected at 18 h were confirmed intraoperatively and histologically, yielding 100% PPV (95% CI: 96.9-100.0%) and NPV (95% CI: 59.0-100.0%). The 18-h protocol showed superior imaging contrast (C-factor: 10.30 ± 1.22) despite lower residual activity. The method remained effective in patients with BMI ≥ 30 (94.00%). Only 1.60% of patients required hysterectomy before mapping due to background interference. Conclusions: The 18-h planar scintigraphy is a highly effective, low-cost, and accessible method for SLN detection in early-stage EC, potentially reducing the need for SPECT/CT, radiation exposure, and costs.