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Metastatic tumors and simulators
It is important to distinguish cutaneous metastases, particularly metastatic adenocarcinoma, from primary adnexal tumors of the skin. Adenocarcinoma metastatic to the skin is commonly of breast or lung origin. Focal areas of glandular differentiation may be highlighted with a mucicarmine stain. Most metastatic tumors are situated in the dermis, although occasionally epidermotropic metastases form intraepidermal nests.
Pearl
Positivity with both cytokeratin 5/6 and p63 or p40 is suggestive of a primary cutaneous adnexal tumor over adenocarcinoma metastatic to the skin.
Breast carcinoma
Key Features
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Poorly differentiated adenocarcinoma
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Various patterns: single cells infiltrating through collagen, cords and tubules of atypical cells, collections of cells with glandular formation, clusters of cells in pools of mucin, and dense sheets of atypical cells
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Occasionally, there is epidermotropism
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Gross cystic disease fluid protein (GCDFP)-15+, estrogen receptor+, and cytokeratin (CK) 7+
Breast carcinoma is the most common cause of cutaneous metastatic disease in women. In general, metastases are seen on the chest wall, sometimes as a result of direct extension of the tumor. Various clinical and histologic presentations are possible. Distinct subtypes are discussed next.
Carcinoma en cuirasse
Key Features
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Rectangular punch
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Busy dermis
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Dense collagen
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Single files of hyperchromatic cells with nuclear molding (black box cars)
A cuirasse is a suit of armor made of leather. Carcinoma en cuirasse presents with woody induration of the skin. The skin is infiltrated by single files of hyperchromatic nuclei with prominent nuclear molding. Dense collagen is laid down between the tumor cells. Because the dermis is sclerotic, the punch is rectangular rather than tapered.
Inflammatory carcinoma (carcinoma erysipeloides)
Key Features
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Tumor cells within dilated lymphatic vessels
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Congested capillaries
Clinically, the lesions present with skin erythema that ranges from faint macular erythema to an erysipelas-like presentation. Inflammation is usually absent histologically, and the erythema is likely secondary to blood vessel congestion.
Alopecia neoplastica
Key Features
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Sclerotic dermis
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Infiltrative cords of atypical cells
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Loss of hair follicles
Occasionally, metastatic breast carcinoma presents as skin-colored to slightly erythematous patches of alopecia on the scalp. Clinically, it is often mistaken for alopecia areata. A biopsy is performed when hair fails to regrow in response to intralesional injection of corticosteroid.
Lung carcinoma
Key Features
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Metastases from the lung may be of the small cell type, adenocarcinoma, squamous cell carcinoma, or undifferentiated
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Most are thyroid transcription factor (TTF)-1+
Lung carcinoma is the most common cause of cutaneous metastases in men. Generally, the metastases present on the trunk as a single nodule or cluster of papules.
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