The Diagnosis: Primary Cutaneous Mucinous Carcinoma
Primary cutaneous mucinous carcinoma is a rare tumor of the sweat glands that was first reported in 1952 by Lennox et al.1 These tumors are slow growing and have a predilection for the head and neck, with the eyelid being the most commonly reported location.2 In general, they present as erythematous asymptomatic nodules measuring less than 7 cm in diameter.2-4 Primary cutaneous mucinous carcinoma tends to have a good prognosis with complete resection, but cases of metastasis and recurrence have been reported.2 Although there is no standard of care, treatment typically consists of surgical management, as the tumors are nonresponsive to chemotherapy or radiation.4 Kamalpour et al2 compared outcomes for Mohs micrographic surgery versus standard excision, the former showing a lower percentage of poor outcomes. Of note, there were fewer cases treated with Mohs surgery in this study; only more recently reported cases have been treated with Mohs surgery.
Histologically, primary cutaneous mucinous carcinoma is composed of cords, tubules, and lobules of epithelial cells floating in large pools of basophilic mucin, separated by thin fibrovascular septa.5 It can be difficult to distinguish a primary tumor from a mucinous carcinoma metastasis with histology alone, especially on the breasts and in the gastrointestinal tract. Immunohistochemistry can be helpful in determining the origin of the tumor. A homologue of p53, p63 expressed in basal and myoepithelial cells of the skin can aid in the confirmation of a primary tumor when present.6,7 Negative staining for cytokeratin 20 and positive staining for cytokeratin 7 also are helpful in distinguishing a primary cutaneous mucinous carcinoma from a gastrointestinal tract metastasis.4,8
In our patient, no other symptoms were present that raised concern for an internal malignancy. Findings that supported a primary versus metastatic tumor included the clinicopathologic findings (Figure) as well as positive p63, cytokeratin 7, and negative cytokeratin 20 staining. The initial standard excision had tumor cells within 1 mm of the specimen margin; thus, a subsequent wider reexcision was performed. Reexcision was negative for tumor cells. Close follow-up with a primary care physician was recommended, with emphasis on colon and breast cancer screening. A follow-up mammogram was negative for breast cancer.
Cystic and papillary components are present in the tumor sample (H&E, original magnification ×20).