Answer
Digital mucous cysts (choice “c”), also known as myxoid cysts, are quite common and are found often on fingers and occasionally on toes. But they are almost always located on the distal dorsal portion of the digit, between the cuticle and distal interphalangeal joint—close enough, in many cases, to compress the nail matrix, which leads to a longitudinal trough in the nail plate. Surface erosion of these lesions is unusual but is occasionally seen.
However, given the history, appearance, and especially the location of this patient’s lesion, this is the one diagnosis our patient almost certainly did not have. The others, discussed below, were all real possibilities, and since “common things occur commonly,” squamous cell carcinoma was the most likely.
Discussion
With this clinical picture, cancer is assumed until proven otherwise. In that regard, the presence of a longstanding wart in this location is especially significant, since human papillomavirus (HPV) is known to be potentially oncogenic. The patient’s heavily sun-damaged skin adds another layer of risk for malignant transformation.
The only way to sort through this differential diagnosis was to perform a shave biopsy, which confirmed the diagnosis of squamous cell carcinoma (SCC)—one that showed evidence of arising from a long-standing wart. The patient was referred for Mohs micrographic surgery, for two reasons: (1) this location does not lend itself to simple excision and closure, both because of the paucity of adjacent skin and because of the potential for damage to the underlying tendons, nerves, and blood supply, and (2) SCCs of nonsolar causation (besides HPV, these include ionizing radiation, arsenic, and chronic ulcers) have more potential for metastasis than do the far more common sun-caused SCCs. This is all the more reason to obtain adequate margins. Often in such cases, irradiation of the site is also done, postoperatively.
Had the biopsy not shown clear evidence of cancer, the other items in the differential diagnosis would have come into play. This would have necessitated an additional biopsy, this time to obtain tissue for acid-fast bacilli and bacterial and fungal cultures.
As of this writing, the patient is awaiting Mohs surgery. He will have to be followed closely for at least a year to watch for any signs of metastasis. With an intact immune system, his prognosis is excellent.