Thursday, July 11, 20244:00 PM - 5:00 PM PP01 Presentation Time: 4:00 PM

Purpose

In Ir-192-based high dose rate (HDR) brachytherapy, the dose delivered at the central axis of single-channel vaginal cylinder (SCVC) is inherently lower than the surrounding areas due to the anisotropic dose distribution of the Ir-192 seed. In principle, this situation could be addressed by introducing custom shielding into the dome of SCVC to shape the dose distribution near the central axis. Aided by advanced dose calculation methods, including both Monte Carlo (MC) simulations and Model-Based Dose Calculation Algorithms (MBDCAs), the shape and density of the shielding materials could provide additional degrees of freedom for inverse dose optimization. In this work, we investigate the potential benefits of this approach by investigating the efficacy of an SCVC with custom shielding (SCVC+S) design to flatten the distal dose profile.

Materials and Methods

A standard SCVC with 140 mm cylinder height (h) and 30 mm outer diameter (OD) with an air channel of 1.6 mm along the central axis for the source transit was used in the initial investigation. The cylinder is capped by a half-spherical dome with a matching diameter (d) of 30 mm. While the existing SCVC is composed entirely of water-equivalent material, the SCVC+S design includes a cavity within the dome which will be injected with stainless steel (mass density of 8.0 g/cc, 6.49 electron density, 13415 HU) shielding material. Three distinct SCVC+S dome designs were explored: A) dome composed of entirely stainless steel except an air channel of d=5 mm along the central axis; B) dome containing one disk “washer” (OD=27 mm, inner diameter (ID)=20 mm, h=1 mm); C) dome containing one disk “washer” (OD=13 mm, ID=3 mm, h=2.5 mm) stacked on a short cylinder (d=13 mm, h=2.5 mm). The SCVC/SCVC+S geometries were independently modeled, with the GammaMed Plus 232 HDR ¹⁹²Ir (dosimetrically identical to Bravos 232A HDR192) radioactive source model for 1) AcurosBV v1.8.0.867816 (Varian Medical Systems, Palo Alto, CA) MBDCA, with dose reported to medium for a 1 × 1 × 1 mm³ grid; and 2) Geant4 MC with TOPAS v3.8 toolkit, with tracklength estimator (TLE) dose for 1 × 1 × 1 mm³ grid, with n=1 × 10⁸ histories.

Results

After modeling the 4 (one SCVC and three SCVC+S) applicator designs in EclipseBV and TOPAS, the dose grids were obtained. PORTEC A3 (3.5 mm lateral, 5 mm superior) reference point and lateral dose profiles up to lateral radius (r) of 15 mm were extracted from a plane 5 mm superior to the SCVC tip (normalized to 100% dose at the central axis). The traditional SCVC has region of lowest dose at central axis, with large shoulders, 109% dose at A3, 116% dose at r=10 mm. Design A of SCVC+S has its highest relative dose at central axis, with no shoulders, 92% dose at A3, 61% dose at r=10 mm. Design B of SCVC+S has a wide flat region with shoulders, 100% dose at A3, 104% dose at r=10 mm. Design C of SCVC+S has a high central region with large shoulders, 94% dose at A3, 82% dose at r=10 mm.

Conclusions

Traditional SCVC inevitably create regions near the central axis with lower relative dose than the surrounding region. Our initial MBDCA and MC simulations show promising results which demonstrate that a simple stainless steel “washer” embedded in the tip can provide a viable filter to produce a uniform dose profile at the 5 mm depth plane. In addition, a redesign of the shielding could turn the central axis into a region of highest dose, reversing the classic SCVC dose profile. Our ongoing work is investigating computational methods to optimize stainless steel disk shapes (namely h, ID, OD) to flatten the profile at arbitrary depths and different cylinder diameters. We are also working with our institution's 3D Collaborative for Medical Innovation (3DC) for rapid physical prototyping of our SCVC+S design.