A 20-year-old male presents with a large mass deep in his right thigh. At surgical resection, the mass is 7 cm in diameter, with infiltrative borders. Two images of the mass are shown here.

A 20-year-old male

presents with a large mass deep in his right thigh. At surgical

resection, the mass is 7 cm in diameter, with infiltrative borders. Two

images of the mass are shown here.

The diagnosis in this case is desmoid tumor.  Also called deep-seated fibromatoses, desmoid tumors may be extra-abdominal, abdominal, or intra-abdominal. They usually present as large, infiltrative masses which may be painful. Histologically,
they are composed of plump fibroblasts arranged in fascicles that infiltrate adjacent tissues. In these image, the tumor cells are infiltrating between skeletal muscle cells. Despite their nasty histologic appearance, and their tendency to recur (sometimes persistently) after resection, desmoid tumors are benign and (by definition) do not metastasize.