Answer: Medullary carcinoma

Histology: The case consists of multiple nodules of tumor infiltrating the surrounding benign renal parenchyma. Tumor predominantly consists of sheets of cells with only focal microcystic formation. Nuclei of the tumor are enlarged with numerous prominent nucleoli. Cytoplasm is densely eosinophilic with in areas having a rhabdoid morphology. A striking feature in the case is the diffuse infiltrate of the tumor by a prominent neutrophilic infiltrate. In areas microabscess formation within the tumor is present. Focally within areas of red cells one can appreciate that the nuclei have a sickeled appearance.

Discussion: This case has many of the typical features of medullary carcinoma of the kidney. The tumor tends to grow in sheets of cells and in other cases may have a prominent somewhat cribriform appearance more than seen in the current case. Cytology tends to be high grade with the characteristic infiltrate of acute inflammatory cells as seen in the current case. Morphologically, the major differential diagnosis is collecting carcinoma. Collecting duct carcinoma tends to have a more tubular formation with prominent desmoplastic stromal reaction. The presence of rhabdoid morphology while not specific is also quite characteristic of medullary carcinoma of the kidney. In order to definitively diagnose medullary carcinoma of the kidney one should also correlate with clinical findings. The vast majority of patients are African Americans with sickle cell trait. The presence of sickeled erythrocytes seen in the current case helps support the diagnosis of medullary carcinoma of the kidney. At the time the case was received there was no information regarding the race of the patient or whether they had sickle cell trait or disease. Upon further workup of the patient, and after a tentative diagnosis was rendered by us in consultation, the patient was noted to have sickle cell trait and we were notified the patient was African American. In the current case stains were performed for INI-1 which showed loss. This finding is not specific for medullary carcinoma although it is present in the majority of cases. It may also be seen in a proportion of collecting duct carcinomas and true rhabdoid tumors. Many poorly differentiated tumors in the kidney may have rhabdoid morphology yet a diagnosis of rhabdoid tumor should only be restricted to a pure rhabdoid tumor occurring in very young infants. Rhabdoid morphology in addition to medullary carcinoma of the kidney may also be seen in urothelial carcinoma and in undifferentiated carcinomas of the kidney as well. Medullary carcinoma of the kidney has a dismal prognosis, one of the worse of any other kidney tumors. Typically patients present with metastatic disease and die within a year or so after diagnosis. Although some patients minimally respond to chemotherapy, currently there is no effective treatment for these patients.