Purpose: Cutaneous angiosarcoma is a rare cancer that forms in the blood and lymph vessels, occurring most often in the scalp and face -- regions involving complex topography. Surface applicator brachytherapy (SABT) may be effective in conformally treating such lesions while minimizing dose to organs at risk (OARs). TG-186 presents a model-based calculation method accounting for tissue heterogeneities. This study compares the TG-43 and TG-186 methodologies, assessing the dosimetric impact of different density materials on dose to angiosarcoma targets and OARs.
Methods: 12 previously treated angiosarcoma plans were evaluated. Radio-opaque wires marked the tumor volume as well as a 5cm clinical margin expansion at CT simulation. Treatment planning was performed using Oncentra Brachy 4.5.3. 100% isodose was prescribed to 3mm depth in the marked target, allowing up to 8mm depth locally where visible evidence of disease on the CT. Contoured OARs include brain as well as bilateral orbits, lenses, lacrimal glands, cochlea, and parotids. The 100% TG-43 isodose volume was designated as the planning target volume (PTV) for comparison. The plans were recalculated using Oncentra’s Advanced Collapsed-cone Engine (ACE) algorithm based on TG-186’s recommendations. HU-based densities were applied with unchanged dwell times. A paired Wilcoxon signed-rank test was utilized to compare dose differences between the two calculations.
Results: Using ACE compared to TG-43, the PTV median V100% decreased 19.8% (p=0.001) and the D90% decreased 7.1% (p=0.001). All OARs except for the parotids also showed decreases in median dose (p<0.05): Brain Dmax 5.8%, V80% 91.3%, V50% 22.3%; mean orbit 9.5%, lens 11.0%, lacrimal gland 9.4%, and cochlea 7.4%.
Conclusion: 12 scalp angiosarcoma SABT plans were recalculated using TG-186 formalism. Calculations using the TG-43 model are shown to underestimate doses for targets and OARs for superficial targets. Future studies will be required to evaluate any clinical impact of transitioning to TG-186 calculations.