Apocrine carcinoma: a rare form of breast cancer

This is article no. 1 in a series on Rare Diseases. Next article: Pseudo-Angelman Syndrome.

Apocrine carcinoma (AC) is a rare form of breast cancer, accounting for approximately 1-4% of all breast cancer cases worldwide. It affects a wide range of patients from 19 to 92 years of age, with the reported mean age varying from 53 to 62 years.

AC of the skin - primary cutaneous apocrine carcinoma - is the only other known cancer that arises from apocrine cells. This is a very rare cancer with limited research.

AC is commonly classified into two subtypes: triple-negative AC (TNAC) and HER2+ AC.

Another receptor not included in the ‘triple negative’ name is the androgen receptor (AR). A ‘pure’ apocrine carcinoma is ER-negative, PR-negative, but AR-positive. Among triple

negative ACs, ones that are AR-positive have a better prognosis.

AC is often associated with triple-negative breast cancers (TNBC), meaning that it does not express oestrogen receptors (ER) and progesterone receptors (PR), and produces very little to no HER2– all of which play key roles in the reproductive system. AC arises from apocrine metaplastic cells that are commonly located in the lobules of the breast. This disease can be aggressive and can metastasise to the lymph nodes and distant organs (eg. lungs, liver, and bone).

What makes AC different is the appearance of cells which have abundant granular eosinophilic or cytoplasm with fine empty vacuoles. Despite its rarity, focal apocrine differentiation is relatively common (reported in approximately 60% of not otherwise

specified [NOS] invasive ductal carcinoma) and shows clinical presentation and radiographic findings similar to that of invasive ductal carcinoma NOS.

TNBCs are generally aggressive and present a poor prognosis. However, studies show

apocrine breast cancer to have a better prognosis and low proliferative nature, despite its

poor response to neoadjuvant chemotherapy. Treatment of AC may include surgery,

radiation therapy, chemotherapy, hormone therapy, or targeted therapy.

The problem with TNACs is that therapies targeting the hormone receptors are ineffective.

Conversely, targeted therapy is seen to work relatively well with HER2-positive ACs despite them being more aggressive than TNACs.

ACs can be diagnosed through a series of tests—usually a mammogram, ultrasound, biopsy, and finally immunohistochemistry. The latter makes it possible to know the status of the ERs and PRs. As with most breast cancers the earlier the detection and treatment

implementation, the better the prognosis for the patient.

ACs can be hard to diagnose due to its rarity and non-specific presentation. AC has a low

proliferative nature, which is shown in its low Ki-67 index. Ki-67 has a higher presentation in

cells that have a high division rate. Slower division rates result in slower growth rates of the

tumour, and may imply that there is a better prognosis. This could be one of the reasons why apocrine triple-negative breast cancers have a better prognosis than other types of TNBCs.

There is promise in the future for AC, however this is not without its challenges. Due to its

rarity there are limited patients to participate in clinical trials which are essential in new

treatment development.

Written by Henrietta Owen & Sherine A Latheef

Related article: Epitheliod hemangioendothelioma

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