A report describes a case of a 75-year-old man who presented with a lesion on the knuckle of 5 months duration. He documented that the lesion initially grew very rapidly before shrinking down to its current size. He refused any bleeding or pain but was suspected of having a splinter in the area around the time of the lesions appearance. The patient also reported spending much time outdoors and noted several recent insect and tick bites. The patient owned a boat and frequently went fishing. 

The patient previously received treatment for actinic keratoses but had no history of skin cancer and no family history of melanoma. Physical examination demonstrated a 2-cm erythematous nodule with central hyperkeratosis overlying the metacarpophalangeal joint of the right index finger. He underwent a shave biopsy.

The history of rapid growth followed by shrinkage and the craterlike clinical appearance prompted the diagnosis of the keratoacanthoma variant of squamous cell carcinoma (SCC). Periodic acid-Schiff green staining came negative for fungal or bacterial organisms, and the biopsy showed keratinocyte atypia and irregular epidermal proliferation confirming well-differentiated SCC. 

The patient subsequently was scheduled for Mohs micrographic surgery. Additionally, a tissue sample sent for panculture returned positive for Mycobacterium marinum infection before the surgery. Thus, Mohs surgery was canceled, and the patient was started antibiotic treatment with azithromycin, ethambutol and rifabutin. After 1 month of therapy, the lesion substantially improved, further supporting the diagnosis of M marinum infection over SCC.

This case underlines M marinum infection as a potential mimicker of SCC, mainly if the biopsy is relatively superficial, as it often occurs when obtained via the common shave technique. 

Source: Stashower J, Lee J, Noland MB. Atypical keratotic nodule on the knuckle. Cutis. 2022;110(6):E25-E27.