Non-nuchal-type fibroma associated with Gardner's syndrome. A hitherto-unreported mesenchymal tumor different from fibromatosis and nuchal-type fibroma.

We describe a unique benign mesenchymal tumor in paraspinal location in a 13-year-old patient with Gardner's syndrome. The Gardner's syndrome in this patient consisted of multiple (more than 100) polyps throughout the entire colon with most in the cecum and rectum, three osteomas in the frontal area of the skull and one in the third right rib, and multiple superficial skin tumors. One of these cutaneous tumors was excised and histologically diagnosed as an epidermal cyst. Both father and uncle of this patient suffered from Gardner's syndrome as well. Microscopically the mesenchymal tumor was histologically different from nuchal type fibroma and fibromatosis. It consisted of a diffusely-growing fibrous mass composed of dense collagenous fibers and relatively numerous, bland-looking, spindle-shaped cells. The collagen fibers had haphazard spacing with no lobular arrangement. The collagen fibers were of a very coarse quality. No entrapment of adipose tissue, skeletal muscle or peripheral nerves was seen in the lesion. Immunohistochemically the tumor was vimentin positive and smooth muscle actin, muscle-specific actin, S-100 protein, cytokeratin and desmin negative.