The design of a novel direction modulated brachytherapy vaginal cylinder applicator for optimizing coverage of the apex

Abstract

Purpose: High-dose-rate (HDR) vaginal cuff brachytherapy is an effective adjuvant therapy for women with stage I endometrial cancer. Although infrequent, failures do occur, most frequently at the vaginal vault. A potential cause of failure is insufficient dosimetric coverage at the vaginal apex due to cold spots from the anisotropic dose distribution of the source. Here, we propose a novel direction modulated brachytherapy (DMBT)-concept vaginal cylinder (VC) applicator that resolves this dosimetric issue.

Methods and materials: The novel DMBT-VC applicator was designed and simulated with the GEANT4 Monte Carlo code. The outer cylinder material chosen was polyphenylsulfone (PPSU) plastic, and the central part was a detachable rod, housing a single lumen made of either polyether ether ketone (PEEK) plastic or an MR-compatible tungsten alloy. The PPSU-based outer cylinder, together with the inner PEEK rod provides the dose distribution of a conventional VC applicator. The PEEK rod is then replaced with an MR-compatible tungsten alloy rod of the same dimensions to generate directional "pencil-like" beams to compensate for the anisotropic cold spots. Two widely used ¹⁹² Ir HDR sources, VS2000 and GammaMedPlus, were also simulated.

Results: The novel DMBT-VC applicator was able to remove the underdosage at the apex due to the anisotropy effect regardless of the HDR sources without unnecessarily increasing the dose to the periphery of the applicator. Also, further directional modulation to reach deeper in the apex by up to 14 mm beyond the VC surface was achievable, again without increasing the peripheral doses. Total treatment dwell times increased only by 7-13%.

Conclusions: The novel DMBT-VC applicator provides improved dose coverage at the vaginal apex by overcoming the classical anisotropy issue ubiquitous to all HDR brachytherapy sources. The next step in development of the device is manufacturing a prototype for clinical testing.

Keywords: Monte Carlo simulations; apex optimization; direction modulated brachytherapy vaginal cylinder applicator.