Appropriate timing for postimplant imaging in permanent breast seed implant: Results from a serial CT study

Purpose: Postimplant analysis in permanent breast seed implant (PBSI) is performed at inconsistent times subsequent to seed implantation across cancer centers, creating challenges in the interpretation of dosimetric data and ultimately the correlation with clinical outcomes. The purpose of this study is to determine the most appropriate time postimplant to perform this analysis.

Methods and materials: Nine patients treated at our institution with PBSI were included in this analysis. Each underwent 4 postimplant CT scans: 0, 15, 30, and 60 days postimplant. A model of the accumulated dose was created by deformably registering the Day 15, 30, and 60 postimplant CT scans and dose matrices to the Day 0 scan, scaling for seed decay. The results from this model were compared to each individual postplan by integral comparison of dose-volume histogram curves for a dose evaluation volume.

Results: The Day 30 postplan showed the best agreement with the accumulated dose model and the smallest interpatient variability across the patient cohort. The mean (±SD) for the dose evaluation volume V₉₀, V₁₀₀, V₁₅₀, and V₂₀₀ for the accumulated dose model was 90 ± 7%, 86 ± 8%, 66 ± 14%, and 41 ± 16%, respectively.

Conclusions: Based on the results of this patient cohort, we recommend that postimplant dosimetric analysis for PBSI be performed approximately 30 days following the implant.