Wilms tumor is usually detected incidentally as a large abdominal mass by the patient’s parents or during a physical examination performed for other medical reasons. The mass is usually painless, hard, smooth, and confined to the flank or one side of the abdomen. Other symptoms and signs that may occur include abdominal pain (in approximately 30 % of patients), hematuria (12–25 % of patients), and hypertension (25 % of cases) [1]. No serum markers to diagnose the tumor are used in clinical practice.

Once the child is diagnosed with a large abdominal mass, further investigations are needed in order to reach the diagnosis. During the initial clinical examination one should look for possible anomalies associated with nephroblastoma (Fig. 24.2).

It is important to prevent any abdominal trauma by temporarily prohibiting risky activities (jump-bike) and to refer the child as soon as possible to a team specialized in the management of childhood tumors.

This team will confirm the diagnosis by imaging and will complete the staging.

There is no laboratory test that can be done in order to diagnose nephroblastoma. Urinary catecholamine metabolites are performed in the differential diagnosis of neuroblastoma; they are normal in nephroblastoma. The preoperative diagnosis of nephroblastoma is based on imaging studies [9] including:

Imaging studies are helpful in the differential diagnosis of other tumors, cystic malformations, and hydronephrosis. In the differential diagnosis other retroperitoneal solid masses that should be considered include:

Wilms tumor is usually a large, grossly distorting tumor of the kidney located anywhere within the organ, and very rarely may be even extrarenal. The average weight is about 500 g (300 g after preoperative chemotherapy), but it may range from 60 g to 6.5 kg [5]. The tumor is usually single or rarely multifocal, heterogeneous, often with solid and soft areas due to hemorrhage and/or necrosis, especially in cases treated with preoperative chemotherapy when it may also show a prominent cystic pattern. It may invade renal pelvis, which explains hematuria. Renal vein may be occupied by a tumor thrombus that sometimes grows into the inferior vena cava and reaches the right atrium. Regional lymph nodes are often bulky but are invaded by a tumor only in 15 % of cases [5].

Primarily operated Wilms tumor is a fragile tumor that can rupture and bleed easily and disseminate in the retroperitoneum. If surgery is done following preoperative chemotherapy, tumor’s capsule is much thicker reducing a risk of intraoperative ruptures.

Nephroblastoma is typically composed of different histological components including blastemal, epithelial, and stromal elements which may be represented in any proportion [5, 10]. If none of these components is predominant, tumor is subclassified as triphasic or mixed type, but biphasic or even monophasic type is not rare. The blastemal component may show numerous patterns, and the epithelial and stromal components may show different lines and different degrees of differentiation resulting in innumerable histological patterns. The epithelial component may show primitive epithelial structures such as rosettes, poorly to moderately differentiated tubules, or well-differentiated glomeruli structures. The stromal component may contain a hypocellular, primitive stroma, or a well-differentiated stroma with rhabdomyoblastic differentiation, fat and cartilage. Each tumor component may exhibit anaplasia (found in 5–8 % of nephroblastomas) which is defined as the presence of enlarged, atypical, multipolar mitoses, nuclear enlargement, and hyperchromasia. Anaplasia is further classified as focal and diffuse. Preoperative chemotherapy destroys tumor’s components and changes its histological features making histological subclassification more challenging [11]. Histological criteria in the International Society of Pediatric Oncology (SIOP) and Children’s Oncology Group (COG) classifications differ and should be applied according to preoperative treatment (i.e., if no preoperative chemotherapy was given, the COG criteria should be used [5]; if preoperative chemotherapy was given, the SIOP criteria should be applied) [11]. In both classifications, tumors are stratified into three prognostic groups [5, 12] (Table 24.2).