Desmoid tumor, also called deep-seated fibromatosis, is a benign tumor that is nonetheless a nasty beast. It is composed of fibroblasts, and it often presents as a large, infiltrative masses. Desmoid tumors may occur in patients with familial adenomatous polyposis (also called Gardner syndrome), an autosomal dominant disorder characterized by innumerable colon polyps (hence the name), as well as other lesions such as osteomas, jaw osteomas, odontomas, thyroid carcinoma, epidermoid cysts, fibromas, and sebaceous cysts. If the colon is not removed in a patient with familial adenomatous polyposis, the chance of colon carcinoma is virtually 100%.

Back to desmoid tumors. These tumors usually present as firm, rubbery, ill-defined, infiltrative masses. They may occur in extra-abdominal regions (for example, in the shoulder or thigh), within the abdominal wall itself (often attached to the rectus abdominus), or in the intra-abdominal region. Histologically, desmoid tumors are composed of bland-appearing fibroblasts in broad fascicles that infiltrate surrounding tissue. Check out the photo above: you can see muscle fibers being splayed apart by benign-looking, round-to-spindly tumor cells.

Although benign, desmoid tumors can act nasty. They are locally aggressive and invasive, and if you don’t excise them completely, they are likely to recur. Treatment with things that you normally only use for malignancies, such as tamoxifen, chemotherapy and radiation, has been effective in many cases.