Brachytherapy in endometrial cancer: Quantification of air gaps around a vaginal cylinder

Purpose Vaginal vault brachytherapy, prescribed to 0.5 cm, is used in endometrial cancer postoperatively to reduce the risk of vault recurrence. However, it is unknown if the vaginal mucosa is closely opposed to the vaginal cylinder, or if air gaps lift the mucosa from the cylinder. The aim of this study was to quantify the extent, magnitude, and effect on dose of any air gaps around the vaginal cylinder using CT-scan images. Methods and Materials Twenty-five patients treated between January and December 2005 were included in the study. The brachytherapy was prescribed to 0.5 cm with a reference volume length (RVL) of 4 cm. Pelvic CT scans taken before the first treatment with cylinder in situ were examined for presence of air gaps around the cylinder within the RVL. Results The median number of air gaps was 1 (range, 0–5) with a mean average area of 0.20 cm2 (range, 0.02–1.65). This resulted in an average of 0.86% (range, 0–6.3) of the vaginal surface within the RVL being raised from the surface of the cylinder. Over the air gaps, the dose the mucosa received at 0.5 cm was on average 86.7% (range, 54.7–97.3) of that which it would have received if there was no air gap. Overall, the dose at 0.5 cm of the whole vaginal mucosa within the RVL was 99.6% (range, 100–96.0) of that prescribed. Conclusions Most postoperative patients with endometrial cancer do not have any clinically significant air gaps around the vaginal cylinder used to administer brachytherapy to the vaginal vault.