Microscopic description and IHC:

The morphology of the FNA biopsy is non-specific. There are clusters of relatively large epithelioid and pleomorphic cells forming very loose glands. No obvious eosinophilic hyaline inclusions or Schiller-Duval bodies are seen.

Immunohistochemical stains performed on cell block section show the tumor cells are positive for AE1/3, Glypican 3, and SALL4 (focal); and negative for PLAP, CD117, CD30, OCT3/4, Hepatocyte, Arginase, SOX10, CK7, CK20, TTF1, P40, calretinin, and GATA3.

Discussion:

Also known as endodermal sinus tumor, testicular yolk sac tumor more frequently occur in children. In adults, they usually present as a part of mix germ cell tumor. AFP is usually elevated in these cases the degree of elevation is correlated with the severity of disease.

While the IHC stains and clinical history are all suggestive of yolk sac tumor. The morphology of this lesion is atypical. However, yolk sac tumors do have numerous microscopic patterns. Classically, there are eleven: reticular (microcystic), macrocystic, endodermal sinus, papillary, solid, glandular-alveolar, myxomatous, sarcomatoid, polyvesicular vitelline, hepatoid, and parietal.

Schiller-Duval bodies are frequently not present. They are glomeruloid structures with a central vessel and papillary projections within a cystic space. The perivascular space is usually edematous. Another diagnostically useful feature is the eosinophilic hyaline inclusions. They are alpha-1-antitrypsin+ and PAS diastase+. These features, when present, can be used to distinguish the lesion from embryonal carcinoma and seminoma.

table showing different stain measurements
References:

Shah, Rajal B., Mahul B. Amin, and Ming Zhou. "Genitourinary Pathology: A Volume in the Series Foundations in Diagnostic Pathology." (2007).

Ulbright TM, Tickoo SK, Berney DM, Srigley JR; Members of the ISUP Immunohistochemistry in Diagnostic Urologic Pathology Group. Best practices recommendations in the application of immunohistochemistry in testicular tumors: report from the international society of urological pathology consensus conference. Am J Surg Pathol. 2014 Aug;38(8):e50-9.

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