Deformable image registration for defining the postimplant seroma in permanent breast seed implant brachytherapy.

Purpose: To demonstrate the benefit of deformable image registration (DIR) to define postimplant seromas in permanent breast seed implants (PBSIs) by adapting preimplant seromas to postimplant images.

Methods And Materials: Planning and Day 0 CT scans from 15 consecutive PBSI patients were included. Seromas were contoured on the preimplant CTs. After implant, DIR (MIM Maestro; MIM Software, Inc., Cleveland, OH) was used to adapt preimplant seromas to postimplant scans. Resulting seromas were evaluated for accuracy and adjusted if required, and pre- and postimplant seroma volumes were recorded and compared. DIR was validated by assessing the spatial correspondence of surgical clips pre- and postimplant in eight cases. Retrospectively, rigid registration was also investigated.

Results: Surgical clip location corresponded between pre- and postimplant images with a median agreement of <1.0 mm. The seroma defined using DIR was an acceptable rendering of the postimplant seroma in 9 of 15 cases, and in the remaining six cases, minor alterations (mean, 1.0 ± 0.2 mm) were made. Mean change in seroma volumes between preimplant and postimplant imaging was -6.1 ± 26.4%. Rigid registration exhibited unacceptable variability, with poor concordance in postimplant seroma between different observers (median conformity index, 26.5%; range, 0-52.7%).

Conclusions: Deformable image registration can be used to adapt the preimplant seroma to postimplant PBSI images and is recommended for implant quality assurance in PBSI. Using deformable registration avoids uncertainties associated with poor seroma visualization and interobserver variability when rigid registration is performed. The accurate deformation of implanted surgical clips demonstrates the reliability of deformable image registration for PBSI.