Author: Sam Beddar, Brett Bocian, David B. Flint, Benjamin Abraham Insley, Patrick James Jensen, Rachael M. Martin Paulpeter, Joshua S. Niedzielski, Luis Augusto Perles, Reza Reiazi, Gabriel O. Sawakuchi π¨βπ¬
Affiliation: University of Miami, MD Anderson Cancer Center, The University of Texas MD Anderson Cancer Center, Empyrean Medical Systems, UT MD Anderson Cancer Center, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center π
Purpose: The Harrison-Anderson-Mick (HAM) applicator is an Ir-192 High-Dose-Rate (HDR) intraoperative radiotherapy device. It features a silicone body with embedded catheters that guide the Ir-192 source to planned dwell positions. Treatment plans assume a flat applicator using the TG-43 formalism, though in practice the applicators often need to be bent to conform to the topology of the target. We used a Monte Carlo (MC) model to assess the dosimetric accuracy when the applicator is deformed as well as the impact of scatter and backscatter opposite the target by using infinite and semi-infinite phantoms.
Methods: A 6-catheter HAM applicator was built in TOPAS (v.3.9). The center of the catheters is fixed 1-cm apart in a silicone flap of 22-cm length and 8-mm thickness, with catheters fixed 5-mm away from the target surface. A validated Ir-192 source of Elektaβs microSelectron remote afterloader and its photon energy spectrum were also included in the simulation. Dose was tallied in 1-mm cubical voxels using track length estimator normalized to 1-cm away from catheters (reference depth). Relative dwell times were optimized by the Oncentra Brachy (v4.6.0) treatment planning system (TPS). Flat and curved applicators were simulated in a 100x100x100 cm3 phantom down to a maximum uncertainty of 0.7% (1Ο) within the dose grid. The two-dimensional dose distribution maps of curved applicators were deformed to match the flat applicator for direct comparison. The setups above were simulated in infinite and semi-infinite water phantoms.
Results: The flat applicator simulation agreed with TPS calculation within 1.7%. We found the doses for the curved applicators in infinite phantom were 5-15% lower compared to flat ones. The doses in the semi-infinite phantom were further lower than the ones in infinite phantom by 5%.
Conclusion: The dose reduction observed in the bent applicator setup indicates the reduced contribution from the adjacent catheters.