EVG-Based Periarterial/Perivenous Invasion (periA/V) as a High-Sensitivity Surrogate Marker for Lymph Node Metastasis in pT1 Invasive Breast Carcinoma of No Special Type

Background/Objectives: Conventional assessments of lymphatic invasion in the primary tumor may fail to identify lymph node metastasis (LNM) in breast cancer. We evaluated periarterial or perivenous invasion (periA/V), using Elastica–van Gieson (EVG)-stained sections, as a histological marker associated with LNM in invasive breast carcinoma of no special type (IBC-NST), focusing on the impact of invasive tumor size. Methods: We retrospectively analyzed 213 IBC-NST cases. PeriA/V was defined as tumor nests in direct contact with perivascular elastic fibers on EVG-stained sections. Diagnostic performance was compared with that of conventional LI markers (hematoxylin and eosin and D2-40), with stratified analyses by pathological T category (pT1 vs. pT2–4) and pT1 subcategories (pT1a, pT1b, and pT1c). Results: LNM was observed in 87 cases (40.8%). Overall, periA/V demonstrated high sensitivity (97.7%) and negative predictive value (NPV; 93.5%). In pT1 tumors (n = 130), periA/V achieved 100% sensitivity and 100% NPV (27/27), and was consistently present in all node-positive pT1b–c tumors. In multivariate analyses, periA/V remained independently associated with LNM in the pT1 group (odds ratio [OR]: 16.08, p = 0.003) and pT1c subgroup (OR: 14.7, p = 0.010). In pT2–4 tumors, periA/V became frequent regardless of nodal status, indicating reduced discriminatory value. Conclusions: In this exploratory single-center cohort, EVG-based periA/V demonstrated high sensitivity for LNM in pT1 IBC-NSTs, with periA/V negativity consistently observed among node-negative cases. These preliminary findings suggest that periA/V may potentially contribute to LNM risk assessment in early-stage breast cancer.