Quality assurance in breast cancer brachytherapy: Geographic miss in the interstitial boost treatment of the tumor bed

Purpose: To assess the role of geographic misses in the interstitial boost treatment of breast cancer patients and to evaluate methods of optimizing breast implants in design, performance, and dosimetry. Methods and Materials: During lumpectomy, the tumor excision sites of 89 patients were marked by five hemoclips. Postoperative radiographs demonstrated the clips' positions with respect to the extension of the surgical cavity, which was demarcated by air and hematoseroma. Twenty-seven selected patients received interstitial boosts to the tumor bed. The implant was first designed according to the clinical assumptions of the tumor bed's topography and then compared with the radiological findings. Prior to brachytherapy, the planning of the implant's dimension and the needle guidance was performed under simulator control. Dose distributions were first calculated following the Paris System and then electively optimized for the target volume by changing source positions and dwell times. Results: Compared to clinical estimations, the radiological determination of the tumor bed's location revealed an overall potential of topographic errors of 51.8% (14 out of 27 patients), rising up to 78.5% in patients with large adipose breasts (11 out of 13 patients). This observation was due to a high mobility of the tissue, leading to varying turner site projections at the time of mammography, surgery, and brachytherapy. In all patients, the presimulation of the implant resulted in an adequate coverage of the target volume. In 17 of the 27 treated patients, dose distributions were modified to achieve a higher dose delivery in zones where a higher residual tumor load was expected (boost-in-boost). Conclusion: Breast implants have a high potential of geographic misses that can be avoided by intraoperative clip demarcation. The delineation of the tumor bed allows for dose reports actually referring to the target volume and not to the implant system to be obtained. In addition, modern afterloading techniques offer possibilities of individual dose planning with regard to high risk subvolumes within the implanted tissue.