Spontaneous hematoma of the kidney simulating a nephroblastoma. Report of two cases.

The International Society of Pediatric Oncology (SIOP) 6 clinical trial for Wilms' tumor (WT) includes preoperative chemotherapy for all nonmetastatic patients. Approximately 50% of patients treated in this manner can be classed stage 1 after surgery. This strategy requires a positive diagnosis in the absence of histologic data. Diagnostic errors are very rare: Only 1.5% of the first 856 cases entered in the SIOP 6 trial were found to be benign. This article reports two very similar cases of spontaneous hematoma of the kidney that were initially misdiagnosed as hemorrhagic WT. Unusual severe acute anemia was a feature of both cases. Even the most modern state-of-the-art imaging techniques were unable to rule out the possibility of a tumor subjacent to a voluminous renal hematoma. The patients' courses during preoperative chemotherapy were of no formal diagnostic value, and the correct diagnosis was not made until surgery.