HER2-positive apocrine carcinoma of the breast: a population-based analysis of treatment and outcome

[Apocrine lesions of the breast]

Apocrine breast lesions encompass a spectrum of histopathological abnormalities, ranging from benign apocrine metaplasia to invasive apocrine carcinomas. Their defining feature lies in cells with abundant eosinophilic cytoplasm and round nuclei with prominent nucleoli. These cells strongly express the androgen receptor while lacking estrogen receptor-alpha and progesterone receptor expression. Benign lesions, frequently associated with mammary cysts or papillomas, lack nuclear and architectural atypia. In contrast, atypical apocrine lesions exhibit significant nuclear and structural abnormalities, posing diagnostic challenges when distinguishing them from apocrine ductal or lobular carcinoma in situ. Diagnosis relies on the extent of atypia and the presence of tumor necrosis. Invasive apocrine carcinomas are rare, accounting for less than 1% of all breast cancers, and predominantly occur in postmenopausal women. Histologically, they are often grade 1 or 2 tumors. Approximately 50% exhibit HER2 amplification and overexpression. Immunohistochemically, they are characterized by positivity for FOXA1 and GATA3, and negativity for FOXC1 and SOX10, and variable expression of TRPS1. These carcinomas belong to the molecular apocrine carcinoma family, which includes HER2-enriched tumors driven by HER2 addiction and androgen receptor-positive luminal tumors, a subtype of triple-negative breast cancers. The latter are defined by androgen receptor pathway activation and are frequently associated with PI3K pathway alterations and cell cycle dysregulation, suggesting potential therapeutic targets.

Rare Breast Cancers Review

BACKGROUND/OBJECTIVES

Breast cancer is one of the most common malignancies in women, with rare subtypes presenting unique clinical challenges. This review provides a comprehensive analysis of rare breast cancers, including both epithelial and non-epithelial subtypes, and explores their epidemiology, pathology, prognosis, and treatment approaches.

METHODS

A systematic review was conducted focusing on recent advancements in the treatment of rare breast cancer subtypes. Articles were selected based on criteria emphasizing studies from the past five years, with older foundational studies included where necessary. The analysis incorporated molecular profiling, clinical trials, and advancements in targeted and immunotherapies, where possible.

RESULTS

Rare epithelial subtypes, such as tubular, mucinous, and medullary carcinomas, demonstrate distinct clinical and pathological features, with generally favorable prognoses compared to invasive ductal carcinoma (IDC). Non-epithelial cancers, including sarcomas and primary breast lymphomas, require individualized treatment due to aggressive behavior and poor prognosis in certain cases. Recent advancements in targeted therapies (e.g., HER2 inhibitors, PI3K inhibitors, and PARP inhibitors) and immunotherapies (e.g., PD-1 inhibitors) have shown promise in improving outcomes for specific molecularly characterized subtypes.

CONCLUSIONS

While the management of common breast cancers has become increasingly sophisticated, rare subtypes continue to pose challenges due to limited research and small patient populations. Advances in molecular profiling and next-generation sequencing are pivotal in identifying actionable mutations and expanding personalized treatment options. Future research should focus on clinical trials and collaborative efforts to refine treatment strategies and improve outcomes for these rare subtypes.

The assessment of tumor-infiltrating lymphocytes in invasive apocrine carcinoma of the breast in relation to the HER2 status

In the current study, we assessed the prevalence and molecular features of HER2-low phenotype in the apocrine carcinomas of the breast (ApoCa) and its relationship with tumor-infiltrating lymphocytes (TILs). A cohort of 64 well-characterized therapy-naïve ApoCa was used. The TIL distribution was assessed using the hematoxylin and eosin whole slide/scanned images following the international TILs working group recommendations. Next-generation sequencing (NGS) was performed in a subset of HER2-low ApoCa. All patients were women, with a mean age of 62 years. Forty-three carcinomas were pure apocrine carcinoma (PAC; ER-/AR+), and the remaining 21 were classified as apocrine-like carcinomas (ALCs; ER+/-, AR+/-). HER2/neu was positive (score 3+ by IHC and/or amplified by FISH) in 20/43 (47%) PAC and 4/21 (19%) ALC. The prevalence of HER2-low expression (scores 1+ or 2+ without HER2 amplification) in ApoCa was 39% without significant differences between PAC and ALC (P = 0.14); however, the HER2-low phenotype was more prevalent in triple-negative PAC than in ALC (P < 0.001). Levels of TILs were low (≤10%) in 74% of ApoCa (median 5%, range 0%-50%). TIL levels were significantly higher in ALC than in PAC (P = 0.02). HER2 status had no impact on TIL distribution (P = 0.45). The genomic profile of HER2-low ApoCa was similar to other subtypes of ApoCa. ApoCa has predominantly low TIL, particularly PAC. The prevalence of the HER2-low phenotype in ApoCa is high, which should have therapeutic and clinical implications given the recently approved therapies with antibody-drug conjugates (ADCs) for HER2-low breast cancers.