Clinical features

•Slight predilection for males

•1.9 per 100,000 person years

•Wide age range (8-91, median 73)

•May arise in an eccrine poroma (11%)

•Exceptionally complicates nevus sebaceus

•Lower limb (44%)> trunk (24%)> head (18%) and upper limb (11%)

•Sometimes ulcerated verrucous plaque

•Recurrences (17%), nodal metastases (19%), systemic spread (11%), mortality (7-11%)

Histological features

•Association with UVB and immunosuppression

•In situ (malignant hidroacanthoma simplex)

•Continuity/origin from epidermis (arises from the acrosyringium, distinguishes this tumor from hidradenocarcinoma)

•Pushing or infiltrating deep margin

•Broad anastomosing epithelial trabeculae 

•Variable nuclear pleomorphism

•Some tumors are deceptively bland & diagnosis depends upon growth pattern or is made in retrospect after metastasis

•Mitoses often conspicuous and sometimes abnormal

•Variable necrosis (may be comedo-like)

•Variable lymphovascular invasion

•Variable perineural infiltration

•Ducal differentiation invariably present

•Intracytoplasmic lumina

•Squamous differentiation

•Apocrine, clear cell, Bowenoid, pigmented & sarcomatoid (metaplastic) variants

•Exceptionally shows eccrine syringofibroadenoma-like foci

•Metastases are sometimes epidermotropic

Immunohistochemistry

•CK5, CK7, CK19, Cam 5.2, CD117, BerEP4, CEA & EMA +ve (Images from Dr. Conti’s case)

•Merkel cell polyomavirus is probably a passenger virus rather than representing an oncogenic driver

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