Dosimetric Evaluation for Various Applicator Setups with In-house Designed PMMA-Agar Phantom in Vaginal Cuff Brachytherapy

Background: Vaginal cuff brachytherapy after hysterectomy targets radiation to potential residual cancer cells. Our previous study proposed a three-ovoid applicator to improve dose coverage and stability over conventional applicators but was limited to two-dimensional plan comparisons. This study introduces a polymethyl methacrylate (PMMA)-Agar phantom for threedimensional (3D) dosimetric comparison of these applicators and verifies its applicability. Materials and Methods: Cylinder, two-ovoid, and three-ovoid applicators were compared. A PMMA frame (30 cm×20 cm×10 cm) with a central 10 cm diameter cavity for the Agar phantom was constructed. Using treatment plans delivering 3 Gy to a 5 mm thick bolus target, dose distributions were verified using Gafchromic EBT3 film (Ashland Inc.) and an Exradin A1SL thimble ionization chamber (Standard Imaging Inc.). Results and Discussion: Plan evaluations showed the three-ovoid applicator demonstrated improved dose uniformity and target coverage over conventional applicators. It yielded higher mean target doses (1.97 Gy vs. cylinder; 0.78 Gy vs. two-ovoid normal) and notably enhanced dose homogeneity, reducing a key uniformity index (D5 cm³/D99 Vol%) from 2.08 (cylinder) and 1.68 (two-ovoid normal) down to 1.61. These planned advantages were qualitatively supported by A1SL chamber and EBT3 film measurements showing dose distributions consistent with plans, experimentally verifying the three-ovoid's dosimetric advantage using the PMMA-Agar phantom. Conclusion: In this study, we experimentally verified the three-ovoid applicator's dosimetric advantage compared to the conventional applicators by using an in-house designed PMMA-Agar phantom for vaginal cuff brachytherapy. We plan to improve the experimental 3D dose verification accuracy in the future, considering the more realistic patient's internal organ structure using a 3D printing technique.