Rhabdomyogenesis in renal neoplasia of childhood.

Of 220 consecutive primary renal tumors of childhood, 17 contained substantial amounts of histologically identifiable striated muscle cells (over 10% of sampled tumor parenchyma). These tumors could be further subclassified into two groups: Wilms' tumors with "massive" rhabdomyogenesis (one-third or more of the tumor parenchyma composed of muscle), and Wilms' tumors with "moderate" rhabdomyogenesis (10-30% muscle composition. The former tumors were invariably seen in young children, under 4 years of age; often the patients were infants 1 year of age, or younger, and more than half of the patients in this group had bilateral tumors. Bilaterality was not seen in patients harboring tumors with "moderate" rhabdomyogenesis, whom in addition, were older children. In both groups, there was a tendency for polypoid intrapelvic growth. All but one of the tumors described in this report were classified as Wilms' tumor; the single exception was considered be be a primary rhabdomyosarcoma of the kidney. Patients with congenital malformations related to Wilms' tumor (one aniridia, one hemihypertrophy) were seen only in the group with"massive" rhabdomyogenesis. However, anatomical lesions consistent with neoplastic multifocal origin were present in both groups. Thus, our findings indicate a definite correlation between extensive rhabdomyogenesis and clinical behavior. This relation is expressed in patterns of age distribution, bilaterality and manner of growth, which are sufficiently consistent to individualize this histologic variant as a cytodifferentiated form of nephroblastoma.