Incorporating three-dimensional ultrasound into permanent breast seed implant brachytherapy treatment planning

Abstract Purpose Planning permanent breast seed implant (PBSI) brachytherapy using CT alone may reduce treatment accuracy because of differences in seroma visualization compared with ultrasound (US). This study evaluates dosimetric effects of seroma delineation in PBSI and the potential impact of incorporating three-dimensional (3D) US into PBSI treatment planning. Methods and Materials Spatially coregistered CT and 3D US images from 10 patients were retrospectively analyzed to simulate the PBSI procedure. Seromas contoured on CT and US defined clinical target volumes, CTV CT and CTV US , which were expanded to create planning target volumes (PTVs). PBSI plans were generated using PTV CT alone, and the resulting coverage to PTV US was evaluated. To assess the potential impact of transferring to an US-guided procedure, the CT-based plans were centered on CTV US . The volume encompassed by both PTVs was used to evaluate how 3D US can affect the planning procedure. Results Median (range) PTV CT V 100 was 95.6% (93.3–97.3%), resulting in PTV US coverage of 91.5% (80.5–97.9%). Centering plans on CTV US decreased PTV CT V 100 by a mean of 10 ± 8%, and increased PTV US V 100 by 5 ± 4%. The combined PTVs were a mean 9±6% larger than PTV CT . Acceptable dosimetry to the combined PTVs resulted in sufficient coverage to individual PTVs but with a mean 11 ± 24% increase to skin dose and 6 ± 8% increase in breast V 200 . Conclusions Differences in seroma visualization have dosimetric effects in PBSI. CT-based plans can underdose US-defined volumes and may not adequately translate to an US-guided procedure. Implementing 3D US into planning can potentially compensate for differences in delineation.