Accelerated partial breast irradiation dosimetric criteria for the strut-adjusted volume implant

Purpose Current guidelines for high-dose-rate accelerated partial breast irradiation using single-entry implants are based on the National Surgical Adjuvant Breast and Bowel Project B-39/Radiation Therapy Oncology Group 0413 protocol, which assumed a balloon implant geometry. We have developed robust plan evaluation criteria specifically for the strut-adjusted volume implant (SAVI). Methods and Materials Plan evaluation criteria were established using a “training data set” of 62 SAVI treatment plans and included the percentage volume of target receiving 90%, 95%, and 100% of the prescription dose (V₉₀, V₉₅, and V₁₀₀), the absolute volume of target receiving 150% and 200% of prescription (V₁₅₀ and V₂₀₀), and the maximum doses to skin (D_{skin max}) and ribs (D_{rib max}). “Ideal” and “expected” (routinely achievable) thresholds were determined for each criterion and compared to B-39 guidelines. A “test data set” collected from the next 25 patients was analyzed using the developed plan evaluation criteria. Results Ideal (expected) dosimetric thresholds established from the training data set were V₉₀ ≥ 98% (95%), V₉₅ ≥ 95% (92%), V₁₀₀ ≥ 91% (88%), D_{skin max} < 90% (100%), and D_{rib max} < 100%. Thresholds for V₁₅₀ and V₂₀₀ were stratified by SAVI size: V₁₅₀ ≤ 30 cc (50 cc) and V₂₀₀ ≤ 15 cc (20 cc) for 6–1 and Mini; V₁₅₀ ≤ 50 cc (50 cc) and V₂₀₀ ≤ 20 cc (20 cc) for 8–1 and 10–1. There was no significant difference between the training and test data sets in the fractions of patients meeting the criteria. Overall, 58% and 95% of 87 plans met all ideal and all expected criteria, respectively. Conclusions The plan evaluation criteria developed specifically for the SAVI device are robust, providing increased target coverage and increased skin sparing compared to B-39 guidelines.