Multifocal nephroblastic neoplasia.

Among 23 patients with multifocal metanephric neoplasia were 14 patients with 43 grossly visible subcapsular tumorlets, 0.3-3.5 cm in diameter, that were apparently derived from nodular renal blastema. Tumorlets are defined as circumscribed lesions that are visible on the exterior surface of the kidney, produce either a raised nodule or, in a few instances, a shallow depression, and are independent of the main tumor(s). Most tumorlets displayed homogeneous epithelial maturation. A substantial minority of tumorlets retained a blastemic character and a few were histologically malignant. Tumorlets tended to be multiple and bilateral but were absent in 9 patients with multifocal disease, including 4 of 10 with bilateral tumors. About half the lesions that achieved tumorlet status, including those that gave rise to clinical tumors, underwent malignant transformation. Contralateral tumorlets should not be mistaken for metastasis and do not adversely affect prognosis. The incidence of neoplasia among young, close relatives of patients with multifocal disease appeared to be excessive, but no clear excess of major anomalies, hemihypertrophy, or minor genitourinary tract anomalies was revealed by a retrospective survey of hospital charts.