# Pediatric Solid Tumors: Wilms, Neuroblastoma, and Hepatoblastoma

## Introduction

Pediatric solid tumors differ fundamentally from adult malignancies in their biology, histopathology, and clinical behavior. Wilms tumor, neuroblastoma, and hepatoblastoma represent three of the most common extracranial solid tumors of childhood. Accurate pathologic classification, staging, and risk stratification are critical for guiding therapy and predicting outcomes.

## Wilms Tumor (Nephroblastoma)

### Epidemiology and Clinical Features

Wilms tumor is the most common **renal malignancy of childhood**, with peak incidence at **3-4 years** of age. It typically presents as a palpable abdominal mass, often discovered incidentally. Associated syndromes include **WAGR** (WT1 deletion: Wilms tumor, aniridia, genitourinary anomalies, intellectual disability), **Denys-Drash** (WT1 point mutation), and **Beckwith-Wiedemann** (11p15 imprinting abnormality). Bilateral tumors occur in approximately 5-7% of cases.

### Histopathology

The classic appearance is a **triphasic pattern** with blastemal, epithelial (tubules, glomeruloid bodies), and stromal (fibrous, skeletal muscle, cartilage) components in varying proportions. Blastemal-predominant tumors are associated with poorer response to chemotherapy. **Anaplasia** is defined by marked nuclear enlargement (more than 3 times adjacent nuclei), hyperchromasia, and multipolar mitotic figures. **Diffuse anaplasia** constitutes unfavorable histology, is present throughout the tumor, and is associated with TP53 mutations and chemoresistance. **Focal anaplasia** is confined to discrete foci and carries a better prognosis.

### Nephrogenic Rests

Nephrogenic rests are **precursor lesions** of Wilms tumor, found in approximately 1% of infant autopsies. **Perilobar nephrogenic rests** are located peripherally and associated with Beckwith-Wiedemann syndrome. **Intralobar nephrogenic rests** occur within the renal lobe and are associated with WAGR and Denys-Drash syndromes. **Nephroblastomatosis** refers to diffuse or multifocal nephrogenic rests and increases the risk of bilateral Wilms tumor.

### Staging and Risk Stratification

**COG (Children's Oncology Group) staging** ranges from Stage I (confined to kidney, completely resected) through Stage V (bilateral tumors). Risk is stratified into favorable histology (no anaplasia) versus unfavorable histology (diffuse anaplasia).

| Stage | Definition |
|---|---|
| I | Confined to kidney, completely resected with intact capsule |
| II | Extends beyond kidney but completely resected (renal sinus, vessels) |
| III | Residual tumor confined to abdomen (positive margins, LN, peritoneal implants, spillage) |
| IV | Hematogenous metastases (lung, liver, bone, brain) |
| V | Bilateral renal involvement at diagnosis  |  Treatment consists of nephrectomy followed by chemotherapy (vincristine, actinomycin D, with or without doxorubicin), with radiation for higher stages. Overall survival exceeds **90%** for favorable histology. |

![Wilms tumor triphasic histology showing blastemal, epithelial, and stromal components](images/wilms-triphasic.jpg)

## Neuroblastoma

### Epidemiology and Clinical Features

Neuroblastoma is the most common **extracranial solid tumor** of childhood and the most common malignancy of infancy. It arises from **neural crest cells** along the sympathetic chain, most commonly from the **adrenal medulla** (40%). Other sites include the posterior mediastinum, retroperitoneum, pelvis, and neck. The median age at diagnosis is **18 months**. Clinical presentations include abdominal mass, periorbital ecchymosis ("raccoon eyes"), opsoclonus-myoclonus, hypertension, and watery diarrhea (from VIP secretion). Elevated urine catecholamines (**VMA, HVA**) are found in over 90% of cases.

### Histopathology and Classification

The **International Neuroblastoma Pathology Classification (INPC/Shimada)** guides histologic assessment. Neuroblastoma is a small round blue cell tumor with Homer Wright rosettes (neuroblastic cells surrounding neuropil). The spectrum of differentiation ranges from **neuroblastoma** (undifferentiated or poorly differentiated) to **ganglioneuroblastoma** (intermixed or nodular) to **ganglioneuroma** (mature, benign). Key features for classification include the **degree of differentiation**, **MKI (mitosis-karyorrhexis index)**, and patient age. Favorable versus unfavorable histology is determined by INPC criteria incorporating these variables.

### Molecular and Genetic Features

**MYCN amplification** is the most important adverse prognostic factor, present in approximately 20% of cases and defining high-risk disease. **Segmental chromosomal aberrations** including 1p deletion, 11q deletion, and 17q gain are adverse. **Whole chromosome gains** (hyperdiploidy) are favorable, particularly in infants. **ALK mutations and amplification** represent a therapeutic target (crizotinib, lorlatinib). Spontaneous regression is possible in **stage 4S**, a special stage in infants under 18 months with liver, skin, and bone marrow metastases.

### Risk Stratification

**Low risk** patients undergo observation or minimal chemotherapy with greater than 95% survival. **Intermediate risk** patients receive moderate chemotherapy with approximately 90% survival. **High risk** patients require intensive multimodal therapy including induction chemotherapy, surgery, high-dose chemotherapy with autologous stem cell rescue, radiation, and anti-GD2 immunotherapy with dinutuximab, achieving survival of approximately 50-60%.

| Risk Group | Key Features | Treatment | Survival |
|---|---|---|---|
| Low | Favorable biology, localized, no MYCN amp | Observation or minimal chemo | >95% |
| Intermediate | Localized unfavorable or infant metastatic | Moderate chemotherapy | ~90% |
| High | MYCN amplified, age >18 mo with metastases | Intensive multimodal + anti-GD2 immunotherapy | ~50–60% |

![Neuroblastoma with Homer Wright rosettes and neuropil on H&E staining](images/neuroblastoma-rosettes.jpg)

## Hepatoblastoma

### Epidemiology and Clinical Features

Hepatoblastoma is the most common **primary hepatic malignancy of childhood** with peak incidence at **6 months to 3 years** and a male predominance. It presents with abdominal distension, hepatomegaly, and palpable mass. **Alpha-fetoprotein (AFP)** is elevated in over 90% of cases and is used for diagnosis and monitoring. Associations include **Beckwith-Wiedemann syndrome**, familial adenomatous polyposis (FAP/APC germline mutation), and very low birth weight premature infants.

### Histopathology

The **epithelial type** is most common and includes fetal, embryonal, macrotrabecular, small cell undifferentiated, and cholangioblastic patterns. **Pure fetal with low mitotic activity** has the most favorable histology and may be treated with surgery alone. **Embryonal** pattern is less differentiated with higher nuclear-to-cytoplasmic ratio and mitotic rate. **Small cell undifferentiated** is the most aggressive subtype, often lacks AFP elevation, and is associated with INI1 (SMARCB1) loss. **Mixed epithelial and mesenchymal type** contains osteoid, cartilage, skeletal muscle, or other mesenchymal elements. Extramedullary hematopoiesis within the tumor is common.

### Molecular Features

**Wnt/beta-catenin pathway activation** is present in over 80% of cases, with nuclear beta-catenin accumulation detectable on immunohistochemistry. CTNNB1 mutations are most common. **NFE2L2** mutations and **TERT** promoter mutations characterize aggressive subtypes. Molecular risk classification is integrated into CHIC (Children's Hepatic tumors International Collaboration) staging.

### Staging and Treatment

**PRETEXT staging** (PRE-Treatment EXTent of disease) is based on imaging and defines the number of liver sections involved along with key annotation factors (vascular involvement, extrahepatic disease, tumor rupture). **POST-TEXT** restaging occurs after neoadjuvant chemotherapy. Treatment consists of **cisplatin-based chemotherapy** (PLADO: cisplatin plus doxorubicin, or C5V: cisplatin plus 5-FU plus vincristine) followed by surgical resection. **Liver transplantation** is considered for unresectable tumors confined to the liver (POST-TEXT IV or central tumors). Overall survival is **70-80%** with current protocols, exceeding 90% for favorable risk groups.

![Hepatoblastoma showing fetal and embryonal epithelial patterns with extramedullary hematopoiesis](images/hepatoblastoma-histology.jpg)

## Clinical Pearls

Diffuse anaplasia in Wilms tumor, defined by marked nuclear enlargement and multipolar mitotic figures, is the key histologic finding conferring unfavorable prognosis and is associated with TP53 mutations. MYCN amplification is the single most important adverse prognostic factor in neuroblastoma and is essential for risk stratification regardless of stage or age. Small cell undifferentiated hepatoblastoma should prompt evaluation for INI1/SMARCB1 loss, as this aggressive subtype has distinct biology and requires intensified therapy. Alpha-fetoprotein is a critical biomarker in hepatoblastoma for diagnosis, treatment response monitoring, and surveillance for recurrence.

## References

1. Dome JS, et al. Children's Oncology Group's 2013 blueprint for research: renal tumors. *Pediatr Blood Cancer*. 2013;60(6):994-1000.
2. Maris JM. Recent advances in neuroblastoma. *N Engl J Med*. 2010;362(23):2202-2211.
3. Czauderna P, et al. Hepatoblastoma state of the art: pathology, genetics, risk stratification, and chemotherapy. *Curr Opin Pediatr*. 2014;26(1):19-28.
4. Shimada H, et al. The International Neuroblastoma Pathology Classification (the Shimada system). *Cancer*. 1999;86(2):364-372.