Lobulated red nodule on right lower back.

Abstract

A 60-year-old man with a history of basal cell carcinoma (BCC) on his forehead excised 10 years ago presented to the dermatology clinic with an enlarging red bump on his back for the past few months. He had no other personal or family history of malignancy and no known radiation exposure (Fig. 1). Physical examination revealed a 2 cm lobulated red nodule with multiple serpiginous vessels coursing through it located on the right lower back. There was no regional lymphadenopathy clinically. Histopathology showed lobules of basaloid tumor cells displaying vesicular, overlapping nuclei, and prominent nucleoli. Admixed aggregates of mature sebocytes, scattered foci of necrosis, and easily identifiable mitotic figures were seen (Fig. 2). Sebaceous carcinoma. This is sebaceous carcinoma, with an estimated incidence of 1–2 cases per 1,000,000 person-years.1, 2 Most cases of sebaceous carcinoma are ocular, and extraocular sites are usually in the head and neck area as a yellowish or red nodule with ulceration.1, 3 Histopathology shows sheets of basaloid and sebaceous cells sheets or asymmetric sebaceous lobules and other signs of malignancy like necrosis, pleomorphism, hyperchromatism, and numerous mitotic figures.1 Mohs surgery is considered first-line for removal of nodules suspicious for sebaceous carcinoma, as is wide-local excision with frozen sections.1 Further studies should include margin assessment of both peripheral and deep tissue edges, with regional lymph node biopsy if there is clinical suspicion of lymphatic involvement.1 As this patient had no clinical lymphadenopathy, no lymph node biopsy was indicated. However, the further pathological analysis showed negative nuclear staining for MSH2 and MSH6, concerning for Muir–Torre syndrome. Muir–Torre syndrome is an autosomal dominant phenotypic variant of Lynch syndrome, and typical findings include a constellation of sebaceous cutaneous neoplasms and visceral malignancies, most commonly colon cancer.4 Immunohistochemistry can be conducted on sebaceous neoplasms to identify mismatch repair gene mutations, and a thorough review of family and personal history of neoplasms as well as personal history of radiation exposure and immunosuppression is indicated. With this patient's multiple sebaceous nodules and DNA mismatch mutation on histology, extensive malignancy workup was started. To date, computed tomography (CT) revealed a multicystic pancreatic tail lesion and multiple metastatic lesions to the liver, with plans to conduct further colonoscopy and upper gastrointestinal (GI) imaging. This case is an excellent example of how dermatologic evaluation can help identify unsuspected internal disease. Sebaceous hyperplasia is incorrect. This is common in older adults and can often be confused with nodular BCC.3 Clinically, these appear as smooth yellow soft umbilicated papules and may elicit sebum with lateral compression, and histopathology would show enlarged sebaceous glands with normal morphology. Typical MCC includes a cutaneous or subcutaneous pink to violet dome-shaped rapidly growing papule or nodule with intact overlying skin in an immunosuppressed patient.3 Histopathology reveals aggregated, deeply blue staining small basaloid or lymphoma-like cells in sheets with CK20 positivity. SCC is incorrect, as these lesions are often plaques or nodules and can be differentiated (hard, keratinized) or undifferentiated (soft, no keratinization).3 These are tumors of pleomorphic keratinocytes that extend into the dermis, and histopathology yields keratinocyte anaplasia, multiple mitoses, and varying degrees of keratinization. FeP is considered a rare variant of BCC characterized by a skin-colored, firm, sessile, fleshy papule, or plaque which often develops in patients with a history of BCC.5 Histopathology is definitive with thin anastomosing basaloid or squamous strands of keratinocytes in a fibrous stroma projecting down from the epidermis, resembling a honeycomb.