Negative/Low Hormone Receptor and APOcrine Lobular Invasive Breast Carcinoma - A Real-World International Cohort Study

Inclusion Criteria: * Female or male patients aged ≥18 years; * Histologically confirmed ILC, confirmed by E-cadherin loss/aberrant expression and/or p120 cytoplasmic relocalization and/or CDH1 alteration; * HR-negative (ER \<1% and PR \<1%) or HR-low (ER and/or PR 1-10%) disease, as defined by ASCO/CAP guidelines, is eligible regardless of HER2 status (assessed according to 2025 ASCO/CAP criteria, with HER2-low and HER2-ultralow status recorded where assessable); * HR-positive (ER\>10% according to the ASCO/CAP guidelines) ILC is eligible only if HER2 status is positive; * Mixed ductal-lobular carcinomas are eligible provided that a clearly identified invasive lobular component is present and predominant (\>50% lobular) and HR-negative/low criteria are met; * Stage I-III disease at diagnosis; Patients with de novo stage IV disease who underwent surgery of the primary tumor will not be included in the main study cohort but may be captured in a separate exploratory cohort for dedicated analysis; * Patients who underwent surgery of the primary tumor at the participating institution, either upfront or after neoadjuvant systemic treatment; * Diagnosis occurred between 1 January 2000 and 31 December 2025; * Minimum follow-up of 12 months for patients without an event, unless recurrence or death occurred earlier; * Local review by a dedicated breast pathologist to confirm the diagnosis and the related molecular features, with particular attention to the confirmation of apocrine morphology. Exclusion Criteria: * Pure IC NST without any lobular invasive component; * ER or PR expression \>10% in the invasive component, in cases with negative HER2 status; * In situ lobular neoplasia (lobular carcinoma in situ) without an invasive component; * De novo stage IV disease (such patients, if they underwent surgery of the primary tumor, will be captured in a separate exploratory cohort for a dedicated analysis) * Synchronous invasive BC of another dominant histology requiring systemic treatment that precludes attribution of outcomes to HR-negative/low ILC; * Prior invasive BC under active systemic treatment at the time of diagnosis, unless clearly documented as unrelated and not expected to confound outcomes; * Insufficient data or follow up