Grade 4 unclassified renal cell carcinoma with sarcomatoid component expressing S-100 protein. A case report with peculiar diagnostic and therapeutic implications.

Grade 4 unclassified renal cell carcinoma, with a sarcomatoid component (URCCSC) is a rare high grade tumor presumptively derived from all histological subtypes of renal cell carcinoma (RCC). Even though rare, URCCSC generates a great deal of interest, as it is a particularly aggressive variant of RCC, that is poorly responsive to chemo-immunotherapy. Whether it originates from a separate sarcomatoid cell clone within the tumor or from true cell dedifferentiation from RCC has yet to be established. The diagnosis of URCCSC is usually based on morphological and immunohistochemical characteristics of the neoplastic cells which show transitional epithelial/mesenchymal features. In fact, the frequent loss of epithelial markers and gain of mesenchymal phenotypes, can result in difficulties in interpreting diagnostic data. Consequently assigning the optimal therapeutic treatments can be hindered due to this biological "complexity." Here we present the clinicopathological records of a 51 year-old patient who underwent an excision of a periureteral retroperitoneal mass, and whose first pathological diagnosis was malignant peripheral nerve sheath tumor (MPNST). Eleven months after surgery, a CT-scan revealed a local recurrence of the disease. Later on the patient was admitted to our hospital and a systemic, sarcoma-oriented, treatment was initiated. A partial remission was observed but only with a dacarbazine based regimen administered as a third line therapy, after which a second surgery took place. The removed tumor was diagnosed as URCCSC based on the peculiar morphologic and immunohistochemical characteristics of the cells. Pathological assessment of the first intervention was re-evaluated, resulting in a diagnosis of URCCSC. This case-report therefore highlights the implications that an erroneous pathologic diagnosis can have for the clinical management of this disease. Furthermore, the unexpected response to a dacarbazine based regimen, indicates that this drug should be included among the therapeutic options available against this type of renal carcinoma.