Scientists investigate rare breast cancer subtype to improve future care

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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