Residency Program - Case of the Month

Answer:

Alveolar soft-part sarcoma

Discussion:

Alveolar soft part sarcomas have a fairly distinctive appearance with organoid nests of uniform discohesive eosinophilic to clear cells with thin fibrovascular septae giving a pseudoalveolar appearance. Although this tumor has little variation in appearance, the septae may be lost leaving the appearance of sheets of epitheloid cells. In rare cases, myxoid or cystic change, hemorrhage, and lymphocytic inflitrate may be seen.  Intravascular extension is seen in most cases of alveolar soft part sarcoma.

Even though the morphologic appearance is fairly distinctive, confirmation of immunohistochemistry is made with TFE3. The TFE3 antibody highlights malignant cells by binding to the ASPL-TFE3 fusion protein caused by an unbalanced translocation (der(17)t(X:17)(p11;p25) present in the tumor cells. This marker has evolved as a highly sensitive and specific marker for alveolar soft part sarcomas. However, it is important to note that TFE3 can also be positive in granular cell tumors and rare pediatric renal tumors.

Alveolar soft part sarcomas are a rare entity and thought to account for <1% of all soft tissue tumors. There is a slight female predominance with tumors most often involving the deep soft tissues of the buttock or thigh. Alveolar soft part sarcomas have an indolent but persistent course with late metastases. Children tend to have a better prognosis.

The differential diagnosis for alveolar soft part sarcoma includes renal cell carcinomas (RCC positive), adrenal cortical carcinomas (Melan A cross reactivity), hepatocellular carcinomas (HepPar1 positive), malignant melanoma (S100, Melan A, and HMB-45 positive), paragangliomas (chromogranin A and synaptophysin positive), alveolar rhabdomyosarcoma (desmin and myosin positive), and perivascular epitheloid cell neoplasms (usually TFE-3 negative). Immunohistochemistry is helpful in establishing a diagnosis when morphology is chalenging or not classic.

References:

1. Folpe AL, Deyrup, Alveolar soft-part sarcoma: a review and update. J Clin Pathol 2006;59:1127-1132
2. Sternberg’s diagnostic Surgical Patholog, Philadelphia, Lippincott Williams & Wilkins, 2010. Electronically viewed: > Table of Contents > Section I - Skin, Soft Tissue, Bone, and Joints > Chapter 5 - Disorders of Soft Tissue > UNUSUAL LESIONS > ALVEOLAR SOFT PART SARCOMA