Inflammatory myofibroblastic tumors of the kidney: a clinicopathologic and immunohistochemical study of 12 cases.

Inflammatory myofibroblastic tumor is a rare entity composed of spindle cells admixed with variable amounts of extracellular collagen, lymphocytes, and plasma cells. In the genitourinary tract, inflammatory myofibroblastic tumor most commonly occurs in the bladder. Isolated case studies of inflammatory myofibroblastic tumor of the kidney, renal pelvis, and ureter have been previously reported. Our series includes 12 cases of inflammatory myofibroblastic tumor occurring in the renal pelvis (six cases), renal parenchyma (four cases), and immediate perirenal soft tissue (two cases). Clinical presentation included flank pain (two patients), painless gross hematuria (one patient), and ureteropelvic junction stenosis with hydronephrosis (one patient). The remaining eight patients were asymptomatic. All patients underwent nephrectomy. The tumors were characterized by firm white tissue or had a myxoid "gelatinous" appearance. Three histologic patterns were identified in the tumors, including a myxoid vascular pattern, a compact spindle cell pattern, and a hypocellular fibrous pattern. Immunohistochemical and electron microscopic studies supported a myofibroblastic proliferation. All cases were negative for anaplastic lymphoma kinase. Follow-up was available in eight cases and ranged from 1 to 17 years with no evidence of recurrence. Based on this series, renal inflammatory myofibroblastic tumor is a proliferative lesion of myofibroblasts of uncertain pathogenesis with no identified potential for recurrence or metastases.