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Planning Strategies to Improve Dose Homogeneity for Permanent Breast Seed Implant

A Frederick

A Frederick*1,2, T Meyer1,2, M Roumeliotis1,2, (1) University of Calgary, Calgary, AB, (2) Tom Baker Cancer Centre, Calgary, AB


SU-I-GPD-T-42 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To re-evaluate the implantation pattern currently used in permanent breast seed implant (PBSI) to determine if an improvement in dose homogeneity is feasible without compromising target coverage.

Methods: A retrospective cohort of 5 patients, with PTVs well-separated from the skin, was selected. The clinical pre-plans (CPs) for this cohort utilized the recommended modified uniform implantation pattern: 1.0cm needle-needle spacing, 1.0cm seed-seed spacing in the PTV periphery and 1.5cm seed-seed spacing in the PTV interior. A minimum peripheral dose of 90Gy, using ¹⁰³Pd seeds with 2.5U activity, was prescribed to the PTV. These patients were re-planned in MIM Symphony™ for two scenarios: seed placement restricted to (1) the PTV or (2) the PTV plus a 5mm margin. The seed activity was modified until the PTV coverage criteria of V90%≥98% and V100%≥95% were just met.

Results: The average PTV size was 68.3cm³ (range, 44.8–106.1cm³). The mean pre-implant PTV coverage for the CPs in terms of the V90%, V100%, V150%, and V200% was 97.2% (range, 96.4–98.0%), 94.9% (range, 93.7–95.6%), 63.4% (range, 59.4–68.9%), and 20.7% (range, 19.2–22.3%). A higher seed activity was necessary to maintain target coverage when seed placement was restricted to the PTV. Compared to the CPs, this resulted in an average increase of 5% and 8% in the V150% and V200%. Implanting seeds outside of the PTV allows for a reduction in seed activity and a drastic improvement in dose uniformity; an average reduction of 28% and 6% in V150% and V200% with equivalent conformity index was observed in comparison to the CPs.

Conclusion: This work suggests that there is a potential for significant improvement in dose uniformity for PBSI, which may reduce the risk of adverse side effects. This methodology will be used with a more comprehensive patient population to develop planning guidelines.

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