Metanephric adenoma and papillary carcinoma with sarcomatoid dedifferentiation of kidney. A case report.

We present the case of a 74-year-old Caucasian male with an expansive process of the left kidney. No clinical and laboratory signs of polycytemia or hypertension were present. Microscopically, the spherical main tumor mass was composed of small basophilic cells arranged in longitudinal branching tubules typical of metanephric adenoma (MA). Another component, different from MA, was formed by a tumor in a papillary and tubopapillary arrangement. This component was diagnostic of grade 3 papillary renal cell carcinoma. The third component, which was merging with the papillary one, was composed of sarcomatoid, spindle cell carcinoma with prominent nuclear polymorphism and a high number of mitotic figures, including atypical mitoses. The sarcomatoid component filled the entire cortico-medullary space and infiltrated the surrounding non-neoplastic renal tissue, including the renal pelvis. Areas of necroses, hemorrhages, and mitotic figures were frequent. No structures of Wilm's tumor were seen in our case. One year after the excision, the patient is without recurrence and metastasis. The existence of the above mentioned tumor supports the hypothesis that metanephric adenomas and papillary renal cell carcinomas are interrelated lesions.