Answer: Metastatic adenoid cystic carcinoma

Histology: The entire kidney is replaced by nests and tubules with the characteristic morphology of adenoid cystic carcinoma. Large areas of the tumor are associated with fibrosis and necrosis. The tumor extends out into perinephric adipose tissue.

Discussion: This patient 8 years ago had a primary adenoid cystic carcinoma of the lung. Currently, the patient has a large metastatic nodule within the kidney as well as metastases to the heart. This prolonged clinical course is typical of adenoid cystic carcinoma. This is the second example that we have seen of a salivary gland tumor metastatic to the kidney. In this case, the diagnosis is straight forward both based on the morphology as well as the pre-existing clinical history. However, in the other case, there was no known history of a salivary gland tumor and a nephrectomy was performed for a renal mass showing what appeared to be benign mixed tumor. Eventually, a small primary salivary gland tumor was identified and the diagnosis of metastatic mixed tumor from the salivary gland to the kidney was diagnosed. Both the current case and the previous case illustrate that when a tumor is present in the kidney with the morphology of a salivary gland tumor one should always consider a metastasis even if there is no known prior history or obvious salivary gland tumor. Other tumors in the kidney which may have a cribriform appearance include medullary carcinoma of the kidney and end stage renal disease associated renal cell carcinoma. However, both of these tumors have high grade cytology and lack the relatively bland appearance of adenoid cystic carcinoma. Furthermore, both of these neoplasms lack the blue-tinged mucinous secretions and small bland tubular components seen within adenoid cystic carcinoma. The tumor in this case also shows a dimorphic appearance with central tubules having abundant eosinophilic cytoplasm surrounded by more basaloid cells, typical of salivary gland differentiation.