HER2-Positive Classical Invasive Lobular Carcinoma of the Breast in a Young Female: A Rare Clinicopathological Entity Challenging the Conventional Lobular Phenotype

Abstract

Background: Classical invasive lobular carcinoma (cILC) is usually characterized by hormone receptor positivity, loss of E-cadherin expression, and absence of HER2 amplification. HER2-positive cILC is exceptionally rare and represents a distinct molecular subset of breast carcinoma.

Case Presentation: A 41-year-old female presented with a palpable left breast lump. Ultrasonography revealed a suspicious irregular hypoechoic lesion. Core needle biopsy demonstrated a malignant epithelial neoplasm composed of discohesive cells infiltrating the stroma in characteristic single-file arrangements. Immunohistochemistry showed complete loss of E-cadherin expression, diffuse HER2 overexpression (3+), GATA3 positivity, and androgen receptor expression, confirming HER2-positive classical invasive lobular carcinoma.

Conclusion: HER2-positive cILC is a rare but clinically significant variant of breast carcinoma. Recognition of this entity is important because HER2-directed therapies may substantially influence prognosis and treatment outcomes. Continued reporting of such cases is essential to improve understanding of their biological behavior and therapeutic implications.