soft tissue tumor alveolar soft part sarcoma MPATH:412 Malignant tumor of unknown origin though probably myogenic due to expression of MyoD and other myogenic lineage markers. Consists of grouped polygonal tumor cells with granular eosinophilic cytoplasm arranged in an organ-like configuration separated by thin fibrovascular septa. Central areas within these nests of cells become necrotic, and the loss of architecture produces an alveolar appearance. (M-9581/3). mouse_pathology.ontology sarcoma