Super-Resolution Ultrasound Microvascular Features in Breast Invasive Ductal Carcinoma: Associations with Ki-67 Status and Stromal Tumor-Infiltrating Lymphocyte Categories

Background/Objectives: Super-resolution ultrasound (SRUS) depicts tumor microvascular structure and flow. We examined associations with Ki-67 status, stromal tumor-infiltrating lymphocyte (sTIL) categories, and immunohistochemistry-based surrogate subtypes in untreated breast invasive ductal carcinoma (IDC). Methods: This single-center retrospective secondary analysis included 72 surgically confirmed IDCs. Ki-67 was categorized as low (<20%; n = 31) or high (≥20%; n = 41) and sTILs as three ordered categories. Eight SRUS fields underwent endpoint-specific false-discovery-rate correction. Revision-stage logistic and ordinal logistic models examined four selected features separately, adjusting for age and log-transformed lesion volume. Results: Complexity levels and vessel ratios were higher in high-Ki-67 tumors (both q < 0.001). Maximum curvature based on the sum-of-angles metric (MaxCurvatureSOAM) and density entropy (DenEntropy) were inversely associated with the sTIL category (Spearman ρ = −0.475 and −0.411; both q < 0.001). These four association directions persisted after age/volume adjustment (all q < 0.001), although Ki-67 effect estimates were imprecise and sensitive to individual observations. Subtype differences were exploratory, with the two smallest groups containing nine and six tumors. A supplementary two-feature analysis did not establish incremental clinical value over the vessel ratio alone. Conclusions: SRUS features showed exploratory imaging–pathology associations that persisted after limited clinical adjustment. Residual confounding, small subtype groups, platform-specific measurements, and the absence of external validation preclude claims of independent biological effects or clinical decision utility.