Dosimetric comparison of single-versus multi-fraction CT-guided HDR vaginal brachytherapy in early-stage endometrial cancer

Purpose The aim of this study was to evaluate the dosimetric importance of adaptive daily planning in endometrial cancer patients undergoing high-dose-rate (HDR) intracavitary vaginal cuff brachytherapy (VB). Changes in target volume and critical organ doses were examined when the plan obtained in the initial fraction was reused in subsequent fractions. Thus, the potential contribution of adaptive planning was evaluated. Methods Computed tomography (CT) images obtained from each fraction of 15 endometrial cancer patients undergoing five fractions of HDR-VB were evaluated. A total of 75 plans were generated on the same day to ensure constant source activity. Catheter points were determined for the initial fraction, and dwell positions were activated according to the prescription length for all patients. For subsequent fractions, retrospective planning was performed using the dwell times from the initial fraction. Clinical target volume (CTV) dose parameters including D98, D95, D90, and homogeneity index (HI) were evaluated. D0.1 cc, D1cc, and D2cc values for the bladder, rectum, and sigmoid colon were compared. Dosimetric parameters were analyzed in EQD2 using an α/β ratio of 10 Gy for CTV and 3 Gy for organs at risk (OARs). Results CTV volumes were similar between single-CT and daily-CT plans for all fractions; however, statistically significant decreases were observed in D98 and D95 values. No significant differences were observed for HI, D90, or most OAR dose parameters. For the bladder, rectum, and sigmoid colon, no significant differences were generally observed, except for rectum D0.1 cc, which increased significantly (p = 0.008). Conclusion Our study demonstrated that daily CT-based planning resulted in small but statistically significant differences in some CTV dose parameters, while critical organ doses remained largely unchanged. These findings suggest that the additional dosimetric benefit of routine daily CT-based replanning may be limited in patients with stable anatomy and applicator geometry.