Necrosis is the most frequent RPLND finding. The necrotic areas are often surrounded by a prominent infiltrate of foamy macrophages, scattered hemosiderin, and an active fibroblastic proliferation. Within the necrotic foci, pyknotic nuclei in the ghost-like outlines of tumor cells may be seen, often surrounding vessels (Fig. 36.2). These foci often retain their immunophenotype (Fig. 36.3). These semideposits or unviable deposits should not be considered evidence of persistent germ cell tumor. Only cells with intact nuclei and well-preserved chromatin pattern and cytoplasm should be considered as viable.

This is the second most frequent finding at RPLND. The presence of teratoma is often expected on macroscopy as the lymph nodes are often multicystic (Fig. 36.4) and filled with clear or brown fluid. On close inspection some solid areas can have the glistening surface of cartilage.

Microscopically, the tumor shows a typical mixed organoid pattern with epithelial and stroma elements, as seen in testicular teratoma (Fig. 36.5).

Persistent teratoma often shows significant cytological atypia in both mesenchymal and epithelial components. In the absence of stromal invasion or overgrowth, with preservation of a typical organoid appearance by the teratoma the presence of cytologic atypia has no adverse significance and can be ignored [4].

This is a relatively unusual finding and is often suspected due to raised serum markers. Most cases will only have small amounts of tumor present, as cases where the serum markers are very high will not be considered for resection.

Embryonal carcinoma is similar in appearance to that seen in the testis (Fig. 36.6), though it should be noted that CD30 is often negative in these tumors after chemotherapy [5]. OCT3/4 is positive, however, and morphology should be sufficient to differentiate embryonal carcinoma from seminoma.

Seminoma in RPLNDs is unusual: often seminoma is highly infiltrative in the retroperitoneum, and resection is surgically challenging, and imaging often shows tumor wrapped around major vessels. In any chemo- or radioresistant seminoma it is worth taking a second look at the tumor to exclude the possibility of a malignant Sertoli cell tumor that has been erroneously diagnosed.

Yolk sac tumor has a protean nature, and in late recurrences, it tends to show even more unusual patterns, especially the glandular form which may resemble an adenocarcinoma and lead to misdiagnosis (Fig. 36.7). As long as the pathologist is aware of a high serum AFP, this should not occur; however, the morphological appearances may be misleading and immunohistochemistry for AFP may be patchy, though glipican-3 now provides a more sensitive immunomarker [6]. The glandular type of yolk sac tumor appears to be particularly resistant to chemotherapy while the hepatoid form is more common in metastases (Fig. 36.8).

Choriocarcinomas may be difficult to diagnose without knowledge of the serum markers. They may lack a well-defined biphasic pattern and mainly consist of mononucleated trophoblast cells alone. As they frequently metastasize to unusual sites, they may be seen by non-genito-urinary pathologists, and if a history is not available, misdiagnosis is not infrequent. We have seen choriocarcinoma misdiagnosed as squamous cell carcinoma on a number of occasions.