The Dosimetric Impact of Air in Vaginal Vault Brachytherapy

Abstract

Vaginal vault brachytherapy is an adjuvant treatment option to reduce risk of local recurrence for intermediate and high risk endometrial cancer. The aim of this planning study is to examine the dosimetric effect of air gaps between the surface of a cylindrical vaginal stump applicator and mucosal wall with dose prescribed to a depth of 5mm from the mucosal surface. The applicability of the TG 43 formalism in the presence of air was also assessed. The planning CT scans for a retrospective sample of 82 patients treated between January 2014 and October 2015 were examined for the presence of air gaps. These were quantified in terms of the radial and longitudinal aspect with reference to the applicator, frequency per patient and location along the applicator. The effect of the geometric displacement of the target tissue was calculated on the planning system. Ionisation chamber measurements were also made in a phantom to estimate the uncertainty of the TG43 algorithm resulting from the impact of air on the dosimetry. 75 of the 82 patients that were analysed had at least one air gap. The overall incidence of air gaps was 91.4%. Of the total gaps observed, 23.7% were located above the tip of the applicator. The most common radial size was between 2 and 3mm (43.0%) which resulted in an average dose reduction using the TG43 formalism of 14.8%. Only 4.3% of the gaps were above 5mm in radial size. There is a strong correlation between radial dimension and TG43 calculated dose reduction, showing a Pearson’s correlation coefficient of 0.92. Phantom measurements indicated that the inhomogeneity introduced by air resulted in a maximum of 2.3% deviation from doses calculated using TG43 (Figure 1). It has been established that the TG43 formalism is not significantly compromised by the presence of air. The error in dose calculation using TG43 will shift the prescription isodose by at most 0.5 mm towards the mucosal surface with an air gap diameter of 10mm. However the presence of the air does produce a displacement of the mucosal surface away from the applicator and this causes a clinically significant dose reduction of 33.7% for 5mm of air. An approximately linear relationship is seen between increasing radial diameter of air and percentage dose reduction caused by the displacement by air of the tissue at the prescription point.