Context— Metaplastic carcinoma is a rare, triple-negative carcinoma of the breast that exhibits transformation of part or all of its glandular carcinomatous component into a nonglandular, or metaplastic, component. The World Health Organization currently recognizes 5 variants of metaplastic carcinoma based on their histologic appearance. Objective— To review the histologic classifications, differential diagnosis, prognosis, and recent laboratory studies of metaplastic breast carcinoma. Data Sources.— We reviewed recently published studies that collectively examine metaplastic carcinomas, including results from our own research. Conclusions.— Metaplastic breast carcinoma has a broad spectrum of histologic patterns, often leading to a broad differential diagnosis. Diagnosis can typically be rendered by a combination of morphology and immunohistochemical staining for high-molecular-weight cytokeratins and p63. Recent studies elucidate new genes and pathways involved in the pathogenesis of metaplastic carcinoma, including the downregulation of CCN6 and WNT pathway gene mutations, and provide a novel MMTV-Cre; Ccn6fl/fl knockout disease-relevant mouse model to test new therapies.

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Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine carcinoma with increased prevalence in patients with immunosuppression or B‐cell neoplasms. To the best of our knowledge, an association with cutaneous T‐cell lymphoma (CTCL) has not been previously described. In this report, we present two cases of MCC arising in the setting of CTCL. The first case was a female during her 70s with previously diagnosed stage IVA1 Sezary syndrome. Biopsy of a scaly patch showed two distinct abnormal cell populations. The first population consisted of hyperchromatic dermal and epidermotropic lymphocytes, expressing CD3 and CD4 with diminished CD7. The second population consisted of intraepidermal clusters of larger atypical cells that expressed synaptophysin, neurofilament, CK20, and Merkel cell polyomavirus transcript. The combination of findings was consistent with intraepidermal MCC in a background of CTCL. Excision showed residual intraepidermal MCC without dermal involvement. The second case was a male during his 50s with a longstanding history of mycosis fungoides, who presented with a new lesion on his right thigh. Biopsy and excision showed dermal MCC without secondary involvement by CTCL. Our cases show that MCC may rarely occur in the setting of T‐cell lymphoma, and that intraepidermal MCC may mimic epidermotropic T‐cells.

Annular erythema of infancy is a rare, benign disease characterized by enlarging annular patches and plaques that resolve spontaneously. Histopathology typically demonstrates a perivascular mixed lymphohistiocytic infiltrate with increased eosinophils. We present two cases of annular erythema of infancy, at ages 2-4 weeks, and review the literature on annular erythema of infancy. It is important to differentiate this distinct, benign disease from serious autoimmune or infectious processes, such as neonatal lupus erythematosus and syphilis, which may present with similar annular lesions in infancy.