Background Paraneoplastic pemphigus (PNP) is a rare autoimmune bullous disease classically associated with an underlying neoplasm. The heterogeneous clinical and histopathologic features of the disease make diagnosis challenging for clinicians. There are no formally accepted diagnostic criteria, and newer techniques for identifying antibodies directed against plakin proteins have largely replaced immunoprecipitation, the historic gold standard. Methods An analysis of 265 published cases of PNP was performed. The clinical, histopathologic, and immunologic features of PNP were assessed. Results Based on this review, we modified previous diagnostic criteria to capture 89.4% of PNP cases compared to 71.2% of cases captured by the most commonly referenced criteria devised by Camisa and Helm (p‐value < 0.01, z‐test; 95% CI [10.2, 33.6]). Conclusion These revised diagnostic criteria address the variable clinical, histopathologic, and biochemical features of PNP, allowing physicians to have greater confidence in diagnosis of this rare and often fatal disease. The revised criteria include three major criteria and two minor criteria, whereby meeting either all three major criteria or two major and both minor criteria would fulfill a diagnosis of paraneoplastic pemphigus. The major criteria include (a) mucous membrane lesions with or without cutaneous involvement, (b) concomitant internal neoplasm, and (b) serologic evidence of anti‐plakin antibodies. The minor criteria include (a) acantholysis and/or lichenoid interface dermatitis on histopathology and (b) direct immunofluorescence staining showing intercellular and/or basement membrane staining.

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Squamoid eccrine ductal carcinoma (SEDC) is a rare and underrecognized primary cutaneous tumor with a high risk for local recurrence and metastasis. The tumor has a biphasic histologic appearance consisting of a superficial portion indistinguishable from squamous cell carcinoma (SCC) and a deeper component demonstrating eccrine ductal differentiation. Because of superficial sampling, SEDC often is misdiagnosed as SCC during the initial biopsy. The diagnosis usually is made during complete excision when deeper tissue is sampled. Confirmation of the diagnosis can be achieved by immunohistochemical positivity for carcinoembryonic antigen (CEA), epithelial membrane antigen (EMA), cytokeratin (CK) 5/6, and p63. In this article, we review the clinical and histologic details of 5 patients with SEDC who underwent successful treatment with Mohs micrographic surgery (MMS) at a single institution between November 2018 and May 2020. We also review the histologic patterns that helped distinguish SEDC from SCC upon complete excision. Our findings support the use of MMS as the treatment of choice for SEDC, given that all of the patients we reviewed required more than 1 Mohs stage for complete tumor clearance, and none demonstrated evidence of recurrence or metastasis after a mean follow-up period of 11 months.Cutis. 2021;107:E5-E9. Squamoid eccrine ductal carcinoma (SEDC) is an aggressive underrecognized cutaneous malignancy of unknown etiology. 1 It is most likely to occur in sun-exposed areas of the body, most commonly the head and neck. Risk factors include male sex, increased age, and chronic immunosuppression. [1][2][3][4] Current reports suggest that SEDC is likely a high-grade subtype of squamous cell carcinoma (SCC) with a high risk for local recurrence (25%) and metastasis (13%). 1,3,5,6 There are as few as 56 cases of SEDC reported in the literature; however, the number of cases may be closer to 100 due to SEDC being classified as either adenosquamous carcinoma of the skin or ductal eccrine carcinoma with squamous differentiation. 1 Clinically, SEDC mimics keratinocyte carcinomas. Histologically, SEDC is biphasic, with a superficial portion resembling well-differentiated SCC and a deeply invasive portion having infiltrative irregular cords with ductal differentiation. Perineural invasion (PNI) frequently is present. Multiple connections to the overlying epidermis also can be seen, serving as a subtle clue to the diagnosis on broad superficial specimens. [1][2][3] Due to superficial sampling, approximately 50% of reported cases are misdiagnosed as SCC during the initial biopsy. 4 The diagnosis of SEDC often is made during complete excision when deeper tissue is sampled. Establishing an accurate diagnosis is important given the more aggressive nature of SEDC compared with SCC and its proclivity for PNI. 1,3,6 The purpose of this review is to increase awareness of this underrecognized entity and describe the histologic findings that help distinguish SEDC from SCC.