Alveolar soft part sarcoma: a review and update.

Alveolar soft-part sarcoma (ASPS) is one of the most unusual of the soft tissue sarcomas. Although it most commonly arises in the fascial planes or skeletal muscles of the lower extremities in adults, and in the head and neck region in children, it has also been documented in extraskeletal muscle locations, especially the female genital tract. Despite extensive investigation, the histogenesis of ASPS continues to be an unsettled issue a half century after its description. The most significant morphologic features are the organoid pattern and the ultrastructural demonstration of a secretory activity that ends in the formation of the characteristic crystals. Early suggestions that it was related to paragangliomas or had a Schwann cell derivation have been discounted. More recently, a possible skeletal muscle origin has been favored based on the immunohistochemical demonstration of various muscle-associated proteins, especially desmin. However, a report of immunoreactivity for the myogenic regulatory protein MyoD1 has not been confirmed in subsequent studies. Although some immunohistochemical studies have recently indicated that ASPS may have muscle differentiation, there is presently no conclusive evidence that it represents a unique type of muscle-derived tumor.