A benign tumor that is usually solitary, painless, palpable mass that is firm in consistency and slightly movable and often fluctuant. It can occur in any location, but tends to involve the muscles of the thighs, buttocks, and shoulders. On microscopic examination, there is abundant mucoid material and relative hypo cellularity and loose reticulin fibers. Vascular structures are sparse. The cells have a stellate shape with small hyper chromatic pyknotic nuclei and scanty cytoplasm. Some myxomas may show focal areas of hyper cellularity. However absence of nuclear atypia, mitotic figures or necrosis helps to rule out malignancy.

A benign, painless soft tissue tumor that arises in a muscle, usually in a large muscle of the thigh, buttocks, shoulder, or upper arm. It is associated with point mutations of the GNAS gene. Grossly it is characterized by a gelatinous and lobulated cut surface. Morphologically it is composed of uniform spindled and stellate-shaped cells without atypia. The cells are separated by myxoid stroma. No recurrences have been reported.