[A case of tumor-forming chronic pyelonephritis that was difficult to be differentiated from renal cell carcinoma].

We experienced a rare case of tumor-forming type of chronic pyelonephritis that could hardly be differentiated from renal cell carcinoma. A 62-year-old man was admitted to our department with right renal tumor which was discovered when he underwent examination for gastric cancer. Abdominal ultrasound sonography and computed tomography revealed a mass in the middle portion of the right kidney, measuring 3.8 x 3.5 cm in diameter, with irregular surface and isoechoic and isodense inner mass. Renal angiography demonstrated a hypervascular area in the middle portion. Radical nephrectomy and subtotal gastrectomy were performed under a tentative diagnosis of right renal cell carcinoma and early gastric cancer. The excised specimen macroscopically demonstrated a yellowish mass in the middle portion and microscopically revealed many lymphocytes and few neutrophils, macrophages or foam cells. The patient was diagnosed as tumor-forming type of chronic pyelonephritis. Such obvious inflammatory findings very rarely exist in the tumor-forming type of chronic pyelonephritis. Therefore, caution should be exercised in the differential diagnosis between this disease and renal cell carcinoma.