Abstract

Desmoplastic melanoma (DM), a type of spindle-cell melanoma separated into pure (PDM) and mixed (MDM) subtypes, can be a diagnostic challenge and easily confused for dermal scar. We report a 65-year-old Caucasian man who received a left thumb amputation for malignant melanoma, unclassified-type. The amputation specimen consisted of residual malignant melanoma with nevoid morphology as well as a spindle-cell component, initially interpreted to be dermal scar on low power scanning magnification, which was negative for HMB45 and Melan-A on the initial immunohistochemical panel. One of five sentinel lymph nodes submitted with the amputation was significant for an expanded and “scarred” capsular area that was diffusely positive for SOX10 on routine sentinel lymph node protocol immunohistochemistry. On review of the area initially interpreted as “brisk” dermal scar due to observer’s confirmation bias, a subsequent SOX10 stain confirmed the diagnosis of MDM. We show that MDM can result in a pure spindle-cell metastasis, which based on our literature search has not been previously emphasized. There is conflicting evidence in the literature for MDM and sentinel lymph node biopsy positivity compared to PDM. However, these discrepant conclusions may be due to challenges in detecting metastasis without appropriate immunohistochemistry. By sharing our experience in inattentional blindness, we hope to encourage increased utilization of SOX10 to assess sentinel lymph nodes, especially in the context of a primary melanoma with a spindle-cell component, in melanomas with mixed morphology, and in biopsies for patients with epidemiological risk for developing DM.

Financial Disclosure:
No current or relevant financial relationships exist.