Author: Hamid Aghdam, DJ Chen, Heather Chen, Sharad Goyal, Yonas Solomon Mekuria, Martin Ojong-Ntui, Yuan James Rao 👨🔬
Affiliation: The George Washington University, The George Washington University Cancer Center 🌍
Purpose: Elekta Leipzig and Valencia Skin applicators are designed to treat skin lesions. The applicators limit the irradiation to the required treatment area using a densimet shielding. Valencia uses densimet flattening filters to provide a relative flat and homogeneous isodose pattern, while Leipzig does not use flattening filters. Commisioning of these skin applicators come with some challeges due to limitations of: 1) The treatment planning system does not consider the effects of the flattening filter and the effect of geometry of the applicators; 2) accurately measue the pdd and isodose dictributions is difficult due to the low energies.
Methods: Monte Carlo simulation has been conducted by J. Pérez-Calatayud, et al. The surface applicators need to be commissioned before putting into clinical application. Percent depth dose, dose reference point depth, effective dwell position and output at the reference depth all need to be verified. Gad-chromic film and software Image-J were used for PDD, dose distribution and effective source position, Well-chamber was used for output verification. Elekta Oncentra software and Flexitron remote after loader were used for planning parameter generation and delivery.
Results: Dwell position was verified to be 1121 with film, same as specification. Output accuracy was less than 3% difference between published data and our measurement. Reference point depth was within 1mm difference between expected and measured.
Conclusion: CT planning is not appropriate for Elekta Surface applicator treatment as the planning software does not take into account the densimet attenuation and geometry construction of the application. With careful measurement and commissioning, clinical set up can deliver an acceptably accurate dosimetry result. A spreadsheet calculation can be used to calculate the dwell time and a single dwell position with the calculated dwell time can enter into TPS to facilitate the treatment.