Evaluation of a new bi-valve vaginal speculum applicator design for gynecologic interstitial brachytherapy

Purpose
We designed a bi-valve vaginal speculum high-dose-rate (HDR) interstitial gynecologic brachytherapy applicator. This allows for both a direct view of the cervix and image-guided brachytherapy applicator placement. The purpose of this study was to assess the validity of the new applicator.

Material and methods
The applicator was designed to have a 25-mm arc, which can be spread transversely to 35-mm wide, with 10 insertion holes; it was produced using a stereolithographic printer with biocompatible Dental SG resin. For resin radiodensity was measured in Hounsfield units (HU) using computed tomography (CT). Comparing the new applicator with a conventional intracavitary applicator, we evaluated the treated volume (including dimensions of 100% isodose volume at the central axis), V100/D90/D98 for a hypothetical cervix (a 2-cm-long and 4-cm-diameter cylinder), and dose points of organs at risk (OARs) (at 25 and 30 mm from the tandem). Based on dose-volume histogram (DVH) analysis of the cervix and dose points of OARs, the range of tolerance for the percent dose difference in the prescription dose was set at 5%.

Results
The mean radiodensity of the Dental SG resin, which was magnetic resonance imaging compatible, was 118 HU. Dimensions of the 100% isodose volume measured at the central axis were 4.4 × 6.6 × 7.4 cm for the new applicator and 4.3 × 6.0 × 7.7 cm for the intracavitary applicator. The 100% prescription dose volumes were 110 cc and 113 cc for the new and conventional applicator, respectively. The percent difference in the hypothetical cervix V₁₀₀, D₉₀, and D₉₈ between the new and intracavitary applicator were within 5%. The percent differences in dose points of OARs at 25 and 30 mm between the new and conventional applicators were within 5%.

Conclusions
Our speculum applicator can reproduce a conventional pear-shaped dose distribution. Our current clinical practice will use this applicator, which can improve the patient’s treatment results.