I think there are two important differential diagnoses for this case - seminoma (pure) and high grade diffuse large B cell lymphoma. Of the two, I favor the latter because of the diffuse infiltrative pattern, nuclear size (smaller than I would expect for seminoma), vesicular chromatin and prominent nucleoli (seminoma cells have hyperchromatic nuclei, not vesicular chromatin pattern). There are certainly many small lymphocytes that could exist in either neoplasms. A few simple immunohistochemical stains (AE1, PLAP, CD117, CD45, CD20 or CD79a, CD3 or CD43) should resolve the matter. If this is a case of testicular seminoma, it is most likely a pure form and classic. Although the age of the patient is advanced, I do not see a mixture of variably differentiated germs cells seen in spermatocytic seminoma.