RADIOTHERAPY FOR PEDIATRIC GENITOURINARY TUMORS: Its Role and Long-Term Consequences

Radiotherapy has been an important part of the treatment armamentarium for pediatric genitourinary tumors since the early twentieth century. The genitourinary system is one of the more common sites of origin for pediatric solid tumors, including three of the most common pediatric malignancies—Wilms' tumor, neuroblastoma, and several classes of rhabdomyosarcoma. Embryologically, the kidney and the associated collecting system are derived from multiple primordial tissues. The kidney is derived from the metanephros, whereas the excretory system (which gives rise to the ureters and collecting system) is derived from the wolffian duct tissues. Although positioned in close final contact with these tissues, the adrenal gland which gives rise to neuroblastoma seems to have an entirely different embryologic origin, with the adrenal medulla taking its origin from neurocrest cells (essentially a portion of the peripheral nervous system) and the adrenal cortex derived from mesodermal structures. The immature mesenchymal cells that give rise to rhabdomyosarcoma and the various germ cell tumors can be traced back to the yolk sac tissues from which they follow a migratory route into the gonads. The multiple disparate tissue types that give rise to pediatric genitourinary cancers may help explain the dramatic differences in tumor development, physiology, and response to therapy seen in this complex family of malignancies. From the point of view of radiation biology and radiation pathology, the pediatric genitourinary malignancies include some of the most radiosensitive tissues (germ cell tumors and infantile neuroblastoma) and tumors that are more radioresistant (certain rhabdomyosarcomas and undifferentiated sarcomas). Of the various normal tissues located in close proximity to the common genitourinary tumor sites, only the kidney poses major problems with respect to radiosensitivity and collateral normal tissue damage. As is true for all pediatric malignancies, the irradiation of growing children poses special problems with respect to functional organ development and musculoskeletal growth. The design of treatment fields with special attention to the minimization of late growth abnormalities may pose a greater challenge to the radiation oncologist than the simple selection of treatment fields designed to control the tumor.