Purpose or Objective To retrospectively evaluate the incidence, magnitude, and dosimetric impact of any air pockets between the vaginal cylinders and the vaginal mucosa using CT-scan images. Material and Methods 120 postoperative vaginal cuff brachytherapy cases were analyzed for receiving the prescribed dose to 5 mm depth from the cylinder wall. CT-Based treatment planning was performed in each fraction. The incidence, vaginal mucosa displacement and air volume were assessed in each treatment. A Monte Carlo study has also been done to evaluate the dosimetric effect of air pockets around the vaginal cylinder. Results In 50 patients, a total of 90 air pockets were observed in 150 procedures. Four patients had pocket free insertion during the whole treatment sessions. The volume of air pockets ranged between 0.01 cm3 and 4.5 cm3 with average value of 2.5cm3, and the maximum displacement of vaginal mucosa from cylinder surface was between 0.2 and 2 cm with average value of 0.8cm. Thirty patients had no air pockets on their first fraction but in subsequent fractions. Twenty patients had incorrect applicator insertion as they have an air gap between applicator tip and cylinder dome ranged between 0.3 cm and 1.1 cm with average value of 0.8 cm. The Monte Carlo dosimetry also shows that the average dose reduction to the vaginal surface and 5mm-depth, at the air pocket, is respectively about 9.2% and 7.3% per 1 mm of the air thickness. Conclusion Air pockets between vaginal cylinder and the vaginal mucosa are observed in the majority of treatment fractions, and the probability of occurrence varies from patient to patient and procedure to procedure. The dose reduction effect of air pockets needs to be considered especially around the vaginal cuff using imaged based treatment planning in each fraction and the effect on the clinical outcome should be put under more investigation.

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