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Purpose or Objective:
Unflattened surface HDR Brachytherapy applicators commonly suffer from dose fall off on the side of the dose distribution. Indeed treatments of larger than 1 cm lesions are reportedly more likely to fail. Recent research documented that in addition to missing dose at the side of the applicator vertical type HDR Brachytherapy surface applicators are subject to assymmetric underdose in the middle of the treatment region. 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. In order to overcome the limitation of increased treatment time of applicator flattening for horizontal type applicators we utilize the possibility of using a source position nearer to the surface to generate a flattened dose distribution together with reduced treatment time.
Material and Methods:
A 40 mm Varian VariSource GM11010080 applicator was used for the modification (Varian Medical Systems, Inc., Palo Alto, CA, USA). The source position is 1.5 cm from applicator tip. The depth of evaluation is 0.5 cm solid water material. 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 flattening elements. All dose measurements and flatness evaluations were performed with Gafchromic EBT3 film Lot #: 12021402 and the FilmQA software, flatness and symmetry toolbox (both Ashland Speciality Ingredients, Bridgewate, NJ, USA).
Results:
The generated compensation element is of toroidal shape, for the standard source position 1.5 cm from applicator tip, has a maximum thickness of 1.5 mm in surface direction. The output of the applicator with flattening element occurred to be 75% of the unflattened one. The diameter of 80% nominal dose area increased from 35.2 mm with the unflattended applicator to 50.2 mm with the flattening element in place. The asymmetric central low-dose artefact can be compensated to a clinical acceptable minimum dose. When utilizing the source position 1 cm from tip a prototype filter could bring the width of the 80% dose area to 45.0 mm, above the nominal applicator size, and output to 112 % of an unflattened applicator. The position 0.5 cm from tip is still considered flattable with increased low dose area in out of field tissue due to applicator geometry when quick treatment is of clinical interest.
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 and on the applicator side. With the appropriate flattening body for a vertical source type applicator it is possible to utilize a source position nearer to surface and compensate for dose output loss when using a dose flattening element.