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Keywords:

  • CD99;
  • CK20;
  • immunohistochemistry;
  • Merkel cell carcinoma;
  • paranuclear dot-like staining

Background

Merkel cell carcinoma (MCC) is a rare neuroendocrine cancer of the skin. The utility of CD99 (MIC-2) in the diagnosis of MCC has been previously studied, with reported rates of expression ranging from 13 to 55%. When specified, a membranous or cytoplasmic staining pattern was considered significant. Recent studies of CD99 have identified a paranuclear dot-like expression pattern in certain non-neuroendocrine pancreatic and colonic lesions. We recently noted paranuclear dot-like staining in several cases of MCC, including cases lacking cytokeratin 20 (CK20) expression.

Methods

Fourteen cases of MCC were stained with CK20 and CD99 antibody, and the pattern and intensity of staining were recorded. Seven cases of pulmonary small cell carcinoma (PSCC) and one case of primitive neuroectodermal tumor (PNET) were used for comparison.

Results

All 14 cases of MCC showed at least focal CD99 staining, with both membranous and paranuclear dot-like staining patterns identified. CK20 staining was present in 12/14 cases, with the characteristic dot-like pattern identified. Four of seven cases of PSCC showed CD99 staining, with two showing a finely granular dot-like staining pattern.

Conclusions

We report an unusual pattern of paranuclear dot-like expression of CD99 in 14 cases of MCC, two of which did not express CK20. This previously unrecognized expression pattern may be of use in differentiating MCC from other cutaneous malignancies, especially when CK20 expression is limited or absent.