Abstract
Purpose: The Xoft Axxent Electronic Brachytherapy Skin Applicator and miniature x-ray source combination can be used for superficial skin or surface treatments. The applicator is a 25 mm SSD cone (1.0–5.0 cm diameter) with an embedded flattening filter. The applicator and source are decoupled, which could lead to variations in dose profile. Several potential problems are investigated for this system. Method and Materials: Dose profiles of the 35 mm diameter cone were measured in solid water using GAFChromic EBT film and in water using a stereotactic diode. Two flattening filter designs were examined. The effect of rotating the source or applicator independently, tilting the applicator with respect to the surface, modifying the source-filter distance, and skin collimation from 10 to 30 mm by means of tungsten aperture cutouts were examined. Results: The stepped disk filter design had flatness and asymmetry of 8.3% and 3.1%. The chamfered filter was 14.8% and 0.7% respectively. Rotation of the source and applicator produced a minimal change in flatness and asymmetry, with less than 2% change in flatness or asymmetry. The stepped-disk filter flatness changed by up to 7% with filter source displacement. Tipping the applicator off of the perpendicular axis by 6 degrees changed the symmetry by 14%. Beam profiles for cutouts ranging from 1.0–3.0 showed little change in flatness and asymmetry, and the sharp edges of the PTW were retained. Conclusion: The stepped filter design is superior. The source-filter is sufficiently concentric to prevent profile shifts, but the dose is very sensitive to the filter-source distance, as predicted by the geometric model. Small tilts of up to 2 degrees, or a 1 mm gap on one side of the cone, can be tolerated. Skin collimation preserves both profile and penumbra. Research Sponsored by Xoft, Inc.
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