Atypical mesoblastic nephroma. Pathologic characterization of a potentially aggressive variant of conventional congenital mesoblastic nephroma.

A case of an aggressive variant of conventional congenital mesoblastic nephroma (CMN) occurred in a 10-month-old male infant. The tumor proved to be fatal, with two abdominal recurrences, but no metastases were found. Of the various terms used to designate the tumor, atypical mesoblastic nephroma (AMN) is preferred. We reviewed 17 previously reported cases to attempt pathologic characterization of AMN. The features that distinguish AMN from CMN are as follows: (1) atypical gross features consisting of one or more of the following: fleshy areas, foci of hemorrhage, necrosis, involvement of adjacent structures other than connective tissue; and (2) high cellularity and mitotic index. Aggressive behavior was noted at surgery or on follow-up in seven of the 18 cases. It was characterized by one or more of the following: (1) invasion of adjacent structures, such as adrenal gland, spleen, colon, and diaphragm (three cases); (2) one or more recurrences (four cases); and (3) metastasis (two cases). Atypical mesoblastic nephroma should be recognized as a potentially aggressive lesion separate from CMN.