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Transitional cell carcinoma arising in a pyelocaliceal cyst. An unusual cystic renal lesion with cytologic and imaging findings.

The differentiation between benign and malignant cystic lesions of the kidney is a diagnostic challenge. Medical imaging aids in this task, but many cystic renal lesions require further work-up, frequently by computed tomography-guided fine needle aspiration. We report on the pathological findings in a case of moderately differentiated papillary transitional carcinoma, which arose in a pre-existing pyelocaliceal cyst in a 53-year-old man. In the case of this lesion, the distinction between a benign and a malignant renal cyst is blurred. To our knowledge, this is the third such occurrence to be reported and the first to be diagnosed by fine needle aspiration biopsy.