A 63-year-old man was admitted to our hospital for complaints of a painless knot in his right testicle. The patient underwent
orchiectomy for suspicion of
malignancy. Pathologic examination detected a firm, circumscribed mass that consisted histologically of noncohesive, large, undifferentiated
tumor cells diffusely infiltrating the testicular interstice. Because of the patient's advanced age and the structure of the neoplastic cells, the differential diagnosis favored a
lymphoma over a malignant
germ cell tumor but also included a dedifferentiated metastatic
neoplasm. Immunohistochemistry was tailored accordingly and was completed using three successive panels of
antibodies. Immunostaining with the first panel of
antibodies directed against leukocyte
epitopes (CD45, CD20, and CD3) remained negative and made a
lymphoma unlikely. The subsequent panel (
cytokeratin AE1/3,
cytokeratin 18, and HMB-45) helped to rule out a
malignant melanoma and aided to settle the diagnosis of a metastatic
carcinoma. The reaction pattern of the last panel of
antibodies pointed to a pulmonary origin of the putative primary
tumor (PSA-negative, TTF-1-positive, and CK20-negative). The diagnosis of a metastasized poorly differentiated
adenocarcinoma of the lung was confirmed by autopsy 5 months later. This case represents the extremely rare occurrence of a testicular
metastasis as a primary manifestation of an occult
neoplasm and shows the usefulness of an integrated site-specific clinicomorphologic approach that should precede and guide the choice of diagnostic immunoreagents.