Apocrine Carcinoma of the Breast

Introduction

The word apocrine in medical literature is used in general to denote a gland or cell exhibiting a distinct type of glandular secretion in which the free end of the secreting cell is cast off along with the secretory products accumulated therein, resembling apocrine sweat glands. Apocrine morphology in breast pathology refers to distinct cytological features in mammary epithelial cells including eosinophilic and granular to foamy/vacuolated cytoplasm, centrally to eccentrically located nuclei, prominent nucleoli, and distinctive cell borders. It is most commonly seen in the setting of apocrine metaplasia in benign fibrocystic changes. Apocrine differentiation can also be seen in other benign breast lesions such as apocrine cysts, apocrine papillary lesions, and apocrine adenosis or adenoma ( Fig. 26.1 ). When apocrine cytoplasmic and nuclear changes are present in cytologically malignant and invasive lesions, a diagnosis of invasive carcinoma with apocrine differentiation can be rendered.

Invasive apocrine carcinoma was traditionally considered to be a morphological variant of invasive carcinoma of various histotypes. In the literature, the term invasive carcinoma with apocrine differentiation thus represents a heterogeneous group of tumors. Historically, due to the varied criteria used by different investigators to diagnose apocrine carcinomas or invasive carcinoma with apocrine differentiation, there has been obvious variability in reporting of the associated clinical and pathological features associated with these lesions. Recent advances in understanding the molecular apocrine signature and apocrine morphology in breast carcinoma have led to more refined characterization of this subgroup of tumors, which has been recognized by the World Health Organization (WHO) as a special subtype of carcinoma with a unique morphology and hormone receptor (HR) profile.

This chapter summarizes prior significant reports on apocrine tumors and the current general view on apocrine differentiation in breast cancer. Most remarks are regarding invasive tumors unless indicated.

Clinical Presentation

Pure apocrine carcinomas are rare and account for less than 1% of invasive breast tumors. While most of these tumors are believed to be sporadic, some of the carcinomas with apocrine morphology had been observed in patients with germline PTEN mutation or Cowden syndrome.

The clinical presentation of pure apocrine carcinoma is similar to that of invasive ductal carcinoma of no special type (IDC-NST). The tumor either is detected incidentally on mammographic examination (when small) or presents with a painless mass. The reported age range is quite wide (19–86 years), but some reports have suggested a higher incidence in older and postmenopausal women. There are no appreciable differences between apocrine and nonapocrine tumors with regard to bilaterality, location within the breast, or stage at diagnosis.

Clinical Imaging

Although the data on apocrine carcinomas are limited, the mammographic, ultrasound, and magnetic resonance imaging (MRI) findings do not appear to be distinctive and are similar to IDC-NST.