Comparative dosimetric study of single- vs. multi-channel cylindrical applicators in vaginal brachytherapy: Implications for organs at risk and vaginal surface doses

Purpose
This study aimed to compare the dosimetric effects of multi-channel (MC) and single-channel (SC) cylindrical applicators of various diameters on target volume, organs at risk (OARs), and vaginal mucosal doses in vaginal brachytherapy (VBRT).

Material and methods
Endometrial cancer patients undergoing VBRT post-operatively were included in the study. Dose distributions in reference parameters were calculated in patients treated with a MC applicator of 25, 30, or 35 mm in diameter, and plans similar to that used in a SC applicator were re-calculated in the same patient cohort by activating only the central axis channel of MC applicator. Comparisons were made between clinical target volume, dose parameters for OARs, such as bladder, rectum, and sigmoid colon, and dose parameters for vaginal mucosa. EQD2 (α/β = 3) values were also calculated for 0.1 cc doses to vaginal mucosa.

Results
The use of MC applicator, together with adequate target volume coverage, resulted in lower doses to OARs, such as the bladder, rectum, and sigmoid colon. In D0.1cc doses to the vaginal mucosa, no significant difference was observed between MC and SC plans with applicators of 25 and 30 mm in diameter. D0.1cc values obtained with a 35-mm MC applicator were lower compared with SC applicator, showing a statistically significant difference (p = 0.048). For both applicator types, the vaginal mucosal dose decreased with increasing applicator diameter.

Conclusions
In VBRT, MC applicators offer advantages in target volume and OARs dosimetry, and vaginal mucosal doses appear to be highly dependent on applicator diameter and geometry in addition to applicator type. In particular, MC applicators with larger diameter have the potential to reduce the need for small-volume, high-dose areas in vaginal mucosa. These findings highlight the importance of selecting the appropriate applicator diameter in VBRT planning.