Meningothelial hamartoma of the scalp. A case report with immunohistochemical studies.

An unusual and, apparently, hitherto undescribed congenital lesion of the scalp which proved to be heterotopic meningothelial tissue is reported, and its clinical, morphological, and theoretical implications are reviewed. A 14-month-old male infant exhibited a soft tissue lesion on the midline of the parietal scalp since birth. The lesion had grown in size since birth. Histological examination showed an admixture of mature adipose tissue, bands consisting of bundles and small nodules of dense collagen, both enclosed and bordered by rests and strands of meningothelial cells. A network of vessel-like channels lined by plump hyperchromatic cells with spindle-shaped nuclei and occasionally multinucleated giant cells was one of the prominent features. Immunohistochemically, these cells were positive for vimentin, but staining was negative for EMA and all other antibodies tested. A fibrocollagenous stalk via bony defect showed no arachnoid cell rests. The authors believe that the herein described lesion and the hamartoma of the scalp described by Suster and Rosai (1990) may represent varying morphological expressions of a pathogenetically related process. Precautions appropriate to the possibility of intracranial extension must be taken at the time of surgery.