Low-Grade Myofibroblastic Sarcoma

Not clinically relevant

Unknown

Grossly, the tumor is usually a firm mass with pale, fibrous cut surfaces and ill-defined margins; a minority are well circumscribed with pushing margins.

Histologically, low-grade myofibroblastic sarcomas are characterized by a diffusely infiltrative growth pattern, and (in deeply located neoplasms) tumor cells often grow between individual skeletal muscle fibers. Most cases are composed of spindle-shaped tumor cells arranged in cellular fascicles or show a storiform growth pattern. Neoplastic cells have ill-defined palely eosinophilic cytoplasm and fusiform nuclei that are either elongated and wavy with evenly distributed chromatin or plumper, more rounded, and vesicular with small nucleoli. Rarely, hypocellular neoplasms with a more prominent collagenous (sometimes hyalinized) stroma have been described. Importantly, neoplastic cells show, at least focally, moderate nuclear atypia with enlarged, hyperchromatic, and irregular nuclei and slightly increased proliferative activity. These neoplasms may contain numerous thin-walled capillaries. The tumors may progress to morphologically higher-grade myofibroblastic sarcomas. By immunohistochemistry, neoplastic cells in low-grade myofibroblastic sarcoma show variable positivity for SMA and/or desmin. A subset show nuclear β-catenin staining.

Not clinically relevant

Low-grade myofibroblastic sarcoma often recurs locally, but metastases are rare, most often occurring after a prolonged time interval.

Not clinically relevant

Essential: diffusely infiltrative growth, often between skeletal muscle fibers; cellular fascicles of spindle cells with pale eosinophilic cytoplasm; at least focally moderate nuclear atypia; variable expression of SMA and/or desmin.