Treatment planning considerations for permanent breast seed implant

Purpose: To determine an optimal planning strategy for permanent breast seed implant that minimizes dose heterogeneity without degrading coverage and conformity. Methods and Materials: A simple model was developed to investigate planning strategies incorporating a range of ¹⁰³Pd seed activities, needle and seed spacings, and implants in which seed positions are either restricted to or permitted outside of spherical planning target volumes (PTVs). To address more realistic target geometries, model parameters were used to retrospectively replan a 10-patient cohort in MIM Symphony. Results: We confirm that the current clinical modified uniform implantation pattern provides the most favorable dose distributions, given the resolution of the template grid and spacer length. We show that needle and seed counts for replans with seed placement permitted 0.3 cm outside of the PTV are most comparable to clinical preplans, but offer a 13 ± 11% average reduction in the V_PTV150%. Replans produced with seed placement 0.5 cm outside of the PTV provide the largest improvement in dose homogeneity, at the cost of a slight increase in irradiated volume and an increase in the number of needles and seeds. Conclusions: Implanting seeds beyond the PTV within a 0.3–0.5 cm margin, and optimizing seed activity on a per patient basis, allows for improvement in dose homogeneity. However, these plans require higher needle and seed counts and result in a small increase in irradiated volume. Before planning recommendations can be made, the implications of these changes must be investigated in the context of clinical outcome for permanent breast seed implant.