BALTIMORE—It can be easy to mistake some metastases to the kidney for primary renal tumors, according to a presentation at the 2013 annual meeting of the U.S. and Canadian Academy of Pathology.

Angela Wu, MD, from the Department of Pathology at the University of Michigan in Ann Arbor, and her team reviewed their institution’s experience with metastases to the kidney—particularly those with unusual or deceptive clinical or histologic features, and that would be encountered in a typical, busy pathology practice—from May 1987 to August 2012. They included only cases that involved a known primary tumor and a definitive final diagnosis of a metastatic tumor, and excluded autopsy cases.

The set of cases in their analysis—15 nephrectomies, 26 core biopsies, and two fine-needle-aspiration cases—comprised less than 1% of all the renal masses they resected or biopsied in the 25-year period.

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The most common primary tumor sites were lung (20), followed by breast, head and neck (four), prostate (two) and colon (two). There were also “a few cases from unusual sites” such as adrenal gland, skin, tibia and testis, Dr. Wu said.

The majority (86%) of the tumors was carcinomas, and most of these were adenocarcinomas. There were also eight squamous-cell carcinomas and one small-cell carcinoma, all from lung primaries. A minority of the cases (14%) were non-carcinomatous tumors, which included a mixture of sarcomas, germ cell tumors and melanoma.

In 88% of the cases (38/43) the primary cancer was diagnosed first, whereas in 2% (1/43) the kidney metastasis was diagnosed first; in the rest, the primary and the metastasis were diagnosed concurrently. The majority (93%) of the patients presented with a renal mass, but one patient presented with a renal cyst, and two with renal failure.

In about 35% of patients, the clinical features favored a primary renal neoplasm over a kidney metastasis.

Traditionally, metastases to the kidney are thought to be multiple and bilateral, Dr. Wu noted, but in this study most cases were solitary (70%) or unilateral (77%). Other unusual features included a greater than 10-year interval between the diagnosis of the primary and metastasis (19% of cases); no other known distant organ metastasis at the time of the kidney metastasis diagnosis (37% of cases); and a medullary rather than a cortical location (11%).

“While there are some unusual and deceptive clinical, radiologic, and histologic features seen in a subset of metastases to the kidney, fortunately—at least in this cohort—a primary tumor had been diagnosed prior to the discover of the metastasis,” Dr. Wu said. “This means that misdiagnosis can be avoided, but it would depend on a thorough investigation of the patient’s clinical history, a high index of suspicion, and diligent comparison between the primary tumor and the metastases. In the end, communication between the urologist and pathologist is key to arriving at the correct diagnosis.”