Mesoblastic nephroma of adulthood. Report of three cases.

Mesoblastic nephroma is an uncommon congenital tumor of infancy that rarely occurs in adults. We report three patients (two were female, one was male) who had mesoblastic nephroma of adulthood and who presented at 45, 64, and 66 years of age with hematuria, flank mass, and pain. All underwent nephrectomy without postoperative adjuvant therapy. The tumors were solitary yellow-tan masses with solid and cystic areas involving the renal cortex (three cases) with extension into the renal pelvis and calyces (two) and ureter (one). Microscopically, all consisted of uniform spindle cell proliferations with entrapped dilated renal tubules. Focal necrosis was present in two, but no atypia or mitoses were identified in any case. The spindle cells displayed cytoplasmic immunoreactivity for vimentin, desmin, panmuscle actin (HHF-35), and alpha-smooth-muscle actin, but were nonreactive for keratin (AE1/AE3), epithelial membrane antigen, and S-100 protein. Electron microscopy revealed the presence of smooth-muscle differentiation in two cases and undifferentiated mesenchyme in one. All tumors were DNA diploid by flow cytometry. The patients were free of recurrence 8 months-2 years postoperatively. Because surgical excision may be curative, mesoblastic nephroma in adult patients must be differentiated from spindle cell neoplasms of the kidney that require additional therapy.