Renal adenocarcinoma presenting as a solitary metastasis: contribution of electron microscopy to diagnosis.

Two patients, presenting with single mass lesions involving the posterior fossa and temporal bone, respectively, are reported. Clinically, both were thought to have primary tumors in those locations, hemangioblastoma in one and glomus jugulare paraganglioma in the other. Morphologic evaluation was compatible with adenocarcinoma of renal origin and large renal tumors were subsequently found in both patients. Electron microscopy was useful in eliminating the possibility of a primary tumor in those respective locations by demonstrating focal, but conspicuous dense arrays of microvilli at intercellular areas or along the free surfaces of plasma membranes. Microvilli do not, by themselves, signify any specific primary organ site. However, taken in the clinical and histologic contexts of a given case, ultrastructural observations can assist in establishing a definitive diagnosis.