Alveolar soft part sarcoma (ASPS) is a rare, malignant soft tissue tumor of uncertain differentiation. It affects mainly young individuals (15–35 years of age) with a female sex predilection. It is located primarily in the head and neck region in children and lower extremities in adults. Patients often present with distant metastases to lungs and brain.

Grossly, ASPS often presents as a soft, poorly circumscribed, gray tumor with an average size of 6 cm. The larger tumors display areas of necrosis and hemorrhage.

Histologically, ASPS is characterized by variable sized nests, alveolar and pseudoalveolar formations separated by delicate strands of vascularized connective tissue.

Cytologically, the tumor cells have abundant eosinophilic to clear cytoplasm, centrally placed vesicular nuclei and prominent nucleoli. Lymphovascular invasion is commonly present; however, mitoses are inconspicuous.

Immunophenotypically, ASPS show strong nuclear staining for TFE3 (carboxy terminal). Other stains that can be positive include desmin (focal) and S100.

Cytogenetically, it is characterized by the der17t(X;17)(p11;q25) ASPSCR1(ASPL)-TFE3 fusion gene, though very specific for ASPS, can also be found in Xp11.2 renal cell carcinoma.