Correct Answer:
C. Alveolar soft part sarcoma

Discussion:

Multiple elements inform a differential diagnosis, including: patient demographics (e.g., pediatric vs. adult, male vs. female), anatomic location, and tumor morphology. Here, we have a young adult, female patient with lesions in both lungs, suggesting infectious, systemic, or metastatic etiologies. The lesion is composed of cells with abundant eosinophilic cytoplasm and fairly uniform nuclei, arranged in nests with central discohesion (i.e., an “alveolar” pattern). The morphology raises the differential diagnosis of “pink cell tumors,” and immiunohistochemistry is often necessary to resolve the diagnosis. This tumor is immunoreactive for cathepsin K and TFE3, and is negative for S-100, desmin, myogenin, and cytokeratin. The morphologic and immunophenotypic features are in keeping with metastatic alveolar soft part sarcoma, of which the patient had a known history. Alveolar soft part sarcomas are translocation sarcomas, driven by translocations between the ASPL gene on chromosome 17 and the TFE3 gene on the X chromosome (Xp11). Tumors with recurrent genomic translocations are often deceptively bland cytologically, despite their aggressive clinical behavior; that is, they comprise nuclei of uniform size and shape, thereby lacking pleomorphism. To quote a beloved professor, “Translocation tumors are like designer shoes…they all look the same!” Each cell is driven by the exact same genomic alteration, thus each cell looks relatively the same. This can be a useful clue when formulating a differential diagnosis on the basis of the cellular morphology.