Immunoreactivity of CD99 in invasive malignant melanoma.

BACKGROUND: CD99, also known as p30/32, is a glycoprotein product of the MIC2 gene. It was originally utilized in immunohistochemistry as a unique marker for Ewing sarcoma, other primitive neuroectodermal tumors, and subsequently in other tumors. Its expression in malignant melanoma (MM) has not been well documented, with just two isolated cases of MM recently reported. Recent studies have documented CD99 expression in a significant percentage of atypical fibroxanthomas (AFX), posing potential diagnostic problems in differentiating these two entities. As mistaking MM for AFX based on immunohistochemical staining pattern has significant consequences, we sought to determine the percentage of invasive MM in our archives that have this staining pattern. METHODS: Seventy-eight cases of invasive melanoma were retrieved from our files. Each case was stained with mouse anti-human CD99 and evaluated for membranous expression. RESULTS: Our evaluation revealed that 47 of 78 MM cases (60%) stain positive for CD99. CONCLUSION: This study is the first to demonstrate, in a large series, the prevalence of CD99 expression in primary cutaneous melanoma. Additionally, this introduces in the histologic differential diagnosis of CD99 expressing dermal spindle cell lesions.

Colchicine intoxication diagnosed in a skin biopsy: a case report.

Colchicine toxicity is a rare, but well-described clinical entity. The histopathologic findings in various organ systems have been delineated in colchicine intoxication; however, skin findings have only been described in rare cases. We present a case of colchicine toxicity diagnosed on skin biopsy in a patient presenting with mental status changes. Her cutaneous manifestation consisted of a diffuse, blanchable, violaceous, morbilliform rash involving the trunk and proximal extremities. The histopathologic findings included metaphase-arrested keratinocytes with underlying basal vacuolization. These features, considered in the setting of multiorgan dysfunction and a known exposure of colchicine, led to the diagnosis.