Author ORCID Identifier
Master of Science
William Y. Song, Ph.D.
Purpose: Anisotropy of the 192Ir source causes a lack of dose coverage at the apex in HDR vaginal cylinder brachytherapy (VC BT). In this study, we took advantage of the GEANT4 Monte Carlo (MC) simulation code to design a new DMBT vaginal applicator so that it maintains the dose coverage of the current vaginal cylinder at periphery everywhere while improving the apex coverage. In addition, since current treatment planning systems (TPS) considers the whole environment as water according to TG43 formalism, based on the capability of the MC in the calculation of dose in the material, the effect of inhomogeneity of the vaginal cylinder in the dose coverage was investigated.
Methods: The new DMBT vaginal applicator was designed to be a 30 mm diameter, single lumen vaginal cylinder, made of polyphenylsulfone (PPSU) plastic. The central part of the applicator, which includes the lumen, was considered to be a detachable 8 mm diameter tandem rod. In order to provide directional modulation, a tungsten rod similar to the dimensions of the detachable tandem was added to the simulation. The applicator works in two steps. First, we get a typical dose distribution based on the planning system using the applicator with a detachable polyetheretherketone (PEEK) plastic tandem in place. Second, the detachable tandem is replaced with a tungsten tandem to compensate for the lack of coverage at the apex utilizing a directional radiation beam generated. The same source dwell positioning is used for both steps, while the dwell time for the second step is a small fraction of the first step. Furthermore, in order to assess the effect of VC inhomogeneity, a separate simulation with the same dwell time and position based on TG43 model was performed and the results were compared. The MATLAB software was used for data analysis.
Results: The analysis showed that the new applicator can address the lack of coverage at the apex due to anisotropy (~2 mm), while simultaneously preventing from overdosing the periphery. Also, the analysis of the data indicated that there is a significant increase of dose at the surface of the cylinder (~19% at the periphery and ~28% at the apex), in comparison to TG43 model.
Conclusion: This new DMBT concept design can be considered as a possible solution for the lack of apex coverage due to anisotropy as there is a subset of patients who experience recurrences after brachytherapy, frequently in the vaginal apex. Further, based on the VC heterogeneity analysis, the increase of the dose at the surface of the cylinder indicated that prescribing the dose to VC surface involves significant uncertainty. In addition, the magnitude of the increase at the boundary is higher when prescribing to 5 mm beyond the VC surface and could potentially risk vaginal stenosis.
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Available for download on Monday, April 19, 2021