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

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.

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.

Median (range) PTV_CTV₁₀₀ 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_CTV₁₀₀ by a mean of 10 ± 8%, and increased PTV_USV₁₀₀ 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 V₂₀₀.

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.