Answer: Fibromatosis

Histology: The lesion consists of an infiltrative proliferation of fascicles of spindled cells involving skeletal muscle and fibroadipose tissue. The individual cells are bland with bipolar nuclei, open chromatin and small punctate nucleoli. No atypia or mitotic activity is appreciated. The cells are embedded in a collagenous stroma. The individual capillaries stand out at low power.

Discussion: Deep fibromatoses, or desmoid tumors, are neoplastic proliferations of fibroblasts. In young adults, the most common site of desmoid tumors is in abdominal wall of women, where they may occur after previous surgery. Some desmoid tumors are hormonally responsive. Desmoid tumors also occur in extra-abdominal sites, where they arise from fascia and connective tissues. The shoulder girdle is the most common location of extra-abdominal desmoid tumor, but they also occur in the head and neck. Desmoid tumors do not metastasize but can be locally aggressive and difficult to completely excise due to their poorly defined, infiltrative edges.

Desmoid tumors may also be associated with genetic syndromes such as Gardner’s syndrome , which is an autosomal dominant disorder in APC on chromosome 5 with familial adenomatous polyposis and desmoid tumors. Even non-syndromic desmoid tumors have abnormalities in the APC/Wnt/beta-catenin pathway, and as expected, desmoid tumors display nuclear immunoreactivity for beta-catenin.

The histologic features characteristic of desmoid tumors are bland, spindled fibroblasts with minimal mitotic activity, prominent pink collagen and thin delicate vessels. The nuclei of the endothelial cells are darker than the nuclei of the bland fibroblasts, so that the blood vessels tend to stand out or “pop out” at low power. This is a useful feature of desmoid tumors.

The differential diagnosis of desmoid tumors includes other spindle cell lesions. Nodular fasciitis consists of spindled myofibroblasts with a “tissue culture-like” appearance. Leiomyomas have blunt-ended nuclei, perinuclear vacuoles, and bright pink cytoplasm; they are positive for actin and desmin. Neurofibromas display mast cells, “shredded-carrot” collagen, and patchy immunoreactivity for the S100 protein. Solitary fibrous tumors are comprised of spindled to epithelioid cells with a “patternless pattern” and immunoreactivity for CD34, CD99 and bcl-2. Fibrosarcomas display atypia and mitotic activity.

Reference(s):
– Wu JM, Montgomery E. Soft tissue tumors: Classification and pathology. Surg Clin North Am. 2008 Jun;88(3):483-520, v-vi.