Answer: Fibromatosis

Histology: The tumor is composed of bland fibroblastic cells with minimal mitotic activity separated by thin strands of collagen. Thin-walled vessels appear to stand out from the lesion, in that there is a space between them and the fibroblastic proliferation. The tumor has an infiltrative border. The tumor cells are negative for muscle markers such as desmin or neural markers like S100 protein, but instead show nuclear labeling for beta-catenin protein.

Discussion: Schwannomas and neurofibromas would be expected to show S100 protein labeling, and not nuclear beta-catenin reactivity. These nerve-sheath tumors would feature nuclei with more angulated profiles, and a schwannoma would not be expected to be infiltrative like the current lesion. A Solitary Fibrous Tumor would label for CD34, and feature alternating cellularity in the form of highly collagenized zones alternating with more cellular foci.

Fibromatosis may be sub-categorized into superficial lesions, which arise in the skin and subcutaneous tissue, and deep lesions. Superficial lesions include Palmar Fibromatoses (Dupuytren’s contracture), Plantar Fibromatoses and Penile Fibromatoses (Peyronie’s Disease). Deep Fibromatoses include those of the abdominal wall (typically referred to as desmoids), those of the abdominal cavity (including pelvic, retroperitoneal, and mesenteric lesions), and those of the deep soft tissue. The current lesion is an example of the latter. Unlike superficial fibromatoses, deep fibromatoses are associated with alterations in the APC/beta-catenin pathway. Hence, only deep fibromatoses demonstrate nuclear labeling for beta-catenin.