Dosimetric study of Varian Universal Multi-Channel Cylinder System for High-Dose-Rate ¹⁹²Ir Brachytherapy

Recently, the Varian multichannel vaginal cylinder (MCVC) set for high-dose-rate ¹⁹²Ir brachytherapy was commercially released. This MCVC was distinct from our existing MCVC in its peripheral channel layout and tip design. This investigation sought to assess the dosimetric impact of these changes.

The dimensions of the virtual model for each applicator were compared against both physical and radiographic measurements. Volumetric dose distributions were generated in silico using a model-based dose calculation algorithm (MBDCA). To characterize the effects of the new peripheral channel layout on dose to adjacent areas ("dose-spill"), point doses were compared using two sets of applicator-based reference points: at surface or 5 mm radially from surface. To evaluate the dose-shaping capabilities, a dose distribution was generated for the new applicator and assessed against a representative dose distribution for a patient previously treated with existing equipment.

Based on both physical and radiographic measurements, virtual models were representative of each applicator within ±1 mm. Commissioning of the MBDCA was benchmarked based on AAPM Working Group on Dose Calculation Algorithms in Brachytherapy. The layout of the new applicator reduced dose-spill to other reference points significantly, as much as a factor of 16.3, compared with the existing equipment. The rounded tip shape and curve of the peripheral channels in the new applicator produced more conformity to its HR-CTV than existing equipment.

Compared with our existing equipment, the design changes in the new Varian MCVC set offered improved control of dose spill and better conformality to HR-CTV.