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Introduction
Unflattened surface HDR Brachytherapy applicators commonly suffer from dose fall off on the side of the dose distribution. Recent research documented that in addition to missing dose at the side of the applicator vertical type HDR Brachytherapy surface applicators are subject to underdose in the middle of the treatment region due to a possibly tilted source in addition to self absorption in the longitudinal direction of the source. This artifact is clinically relevant because tumor cells in the middle of the treated area can end up irradiated insufficiently. In this work we present a surface-dose compensation body generated with a 3D printer that specifically addresses the dose irregularities of a vertical type HDR Brachytherapy surface applicator.

Materials and Methods
Previously investigated surface dose irregularities were used as a starting point to define the thickness profile of the compensation element. A 40 mm applicator was used as prototype applicator for the modification. The source position is 1.5 cm from applicator tip. The depth of evaluation is 0.5 cm solid water material. The nominal diameter of the dose distribution therefore equals 53.3 mm when a 50% isodose level as size definition is considered. A consumer grade 3D printer “UP! 3D, Beijing TierTime Technology Co. Ltd.” was used to print out a negative form with ABS plastic. Lippowitz type low temperature melting metal was used to mold the positive form of the compensation body of a prototype flattening element for a 40 mm vertical type surface applicator.

Results
The generated compensation element is of toroidal shape with a maximum thickness of 1.5 mm in surface direction. The output reduction as consequence of the flattening element occurred to be 25%. The diameter of 80% nominal dose increased from 35.2 cm with the unflattended applicator to 48.9 mm with the flattening element in place. The central underdosed region is compensated with the flattening element.

Conclusion
The presented prototype of a dose compensation body can remove the dose artefacts of a vertical type HDR Brachytherapy surface applicator including the clinical relevant underdosed central region.