**1. Introduction**

The problems of pediatric oncology are quite urgent, since malignant neoplasms are leading in the structure of child mortality worldwide, second only to external causes. The incidence of cancer cases is estimated by the World Health Organization (WHO) as about 100 per million children. These are rare diseases among the pediatric population, ranging from 0.5 to 4.6% of all cancers [1, 2].

Neuroblastoma (NB) and nephroblastoma, or Wilms' tumor (WT), are the two most common extracranial solid tumors in childhood [3, 4]. Both tumors are most often located in the retroperitoneal space, develop from embryonic cells, and in most cases are diagnosed in children younger than 5 years [4]. NB is an embryonic

malignant tumor arising from the ganglia of the borderline sympathetic trunk and chromaffin tissue. The prevalence of NB is estimated as 1 case per 7000–10,000 live births, it accounts for 6–10% of all malignancies in children [5, 6]. The most common localizations of this tumor are adrenal glands (40%) and retroperitoneal space (25%), less often NB develops in the posterior mediastinum (15%), in the neck (5%) and pelvis minor (5%). The prevalence of WT, which is the most common kidney tumor in childhood, is 1 case per 10,000 children under the age of 15 [7]. About 10% of cases of the disease are associated with the presence in patients of such congenital syndromes as WAGR syndrome, Beckwith-Wiedemann syndrome, Denys-Drash syndrome, and hemihypertrophy [8]. From the point of view of embryogenesis, WT is a solid malignant tumor consisting of derivatives of nephrogenic tissue at different degrees of differentiation. These tumors may contain not only a variety of tissue elements present in a normal kidney, but also skeletal muscles, cartilages, mucosal and stratified squamous epithelium. In typical cases, WT is a large solid growth, often with areas of necrosis, hemorrhages, and cysts. However, in about 7% of cases, WT is a multifocal growth. The tumor can spread to the pelvis and ureter, causing obstruction of the urinary tract. In addition, it can invade the intrarenal blood and lymph vessels, penetrate the renal capsule, and grow into the paranephric and other adjacent tissues, spread from the renal vein into the inferior vena cava [4].
