Structure of Cancer Incidence in Children
The spectrum of malignancies in childhood is radically different from that in adult patients. The majority consist of systemic disorders and tumors originating from embryonal tissues.
Expressed as a percentage of all pediatric malignant tumors, the distribution is as follows:
- Leukemias — the undisputed leaders, accounting for 30% of all cases.
- Central nervous system (CNS) tumors — occur in 21% of cases.
- Lymphomas (including Hodgkin lymphoma) — account for 14%.
- Neuroblastoma — diagnosed in 6.8% of cases.
- Rhabdomyosarcoma — detected with a frequency of 6.5%.
- Wilms tumor — accounts for 5.2%.
- Retinoblastoma — occurs in 2.7% of cases.
- Hepatoblastoma — the rarest of those listed, accounting for only 0.9%.
Key Features of Pediatric Neoplasms
Pediatric malignancies possess a number of unique biological and clinical characteristics that require a special approach to classification and treatment.
The main features include:
- Organ nonspecificity. Unlike adult patients, whose tumors most frequently affect specific organs (e.g., lung, breast, prostate, colon), childhood neoplasms are largely not restricted to a specific organ. A typical example is rhabdomyosarcomas — the most common soft-tissue sarcomas in children, which lack an exclusive predisposition to arise solely within striated skeletal muscle tissue.
- Rarity of carcinomas. True cancer (tumors of epithelial origin) in children is extremely rare, comprising no more than 6% of all cases. In adults, carcinomas are the dominant type.
- Age dependence. Incidence rates and tumor types vary significantly depending on the child's age.
- Classification challenges. The standard histogenetic classification principle that works exceptionally well in adults is not always applicable in pediatric practice. Dysontogenetic tumors may include elements of multiple germ layers simultaneously.
- Prognosis and survival. Overall, the prognosis for malignant tumors in children is more favorable. The 5-year survival rate in children exceeds 60%, whereas in adults this figure is less than 50%.
- Therapeutic sensitivity. Pediatric tumors demonstrate high sensitivity to chemotherapy, whereas adult carcinomas are often poorly sensitive or entirely resistant to it.
Tumor Reversion Phenomenon
One of the most striking properties of certain pediatric neoplasms is tumor reversion — the ability for spontaneous or induced transition (maturation) from a malignant form to a benign one.
This phenomenon is explained by the fact that such tumors arise either from stem undifferentiated cells or from embryonal tissues that have lagged in their development relative to the surrounding normal tissues of the child.
Known examples of reversion:
- Malignant neuroblastoma can mature into a benign ganglioneuroma.
- Malignant hepatoblastoma can transform into a benign adenoma.
- Immature teratoma transitions into a mature teratoma via differentiation.
Histological Presentation and IHC Diagnosis
On standard hematoxylin and eosin (H&E) staining, the microscopic appearance of many pediatric tumors is characterized by the predominance of primitive (embryonal) features over pleomorphic-anaplastic ones. The histological structure often mimics the processes of specific organogenesis of the organ where the tumor arose. To designate such embryonal tumors, the suffix "-blastoma" is frequently used (e.g., nephroblastoma, retinoblastoma).
Many neoplasms consist of primitive cells possessing small hyperchromatic nuclei and extremely scant cytoplasm. The stroma in such tumors is practically absent, and they are presented as dense clusters of cells. In pathomorphology, the terms solid tumors or small round blue cell tumors are used to describe them.
The differential diagnosis for these neoplasms includes lymphoma, neuroblastoma, rhabdomyosarcoma, and primitive neuroectodermal tumor. The use of immunohistochemical (IHC) markers is critically important for an accurate diagnosis:
- CD45 (leukocyte common antigen) and other cluster of differentiation (CD) markers — used to identify hematopoietic cells.
- Neuron-specific enolase and synaptophysin — serve as neuronal markers.
- Myogenin, desmin, actin — expressed during myogenic differentiation.
- Vimentin — acts as a universal marker of mesenchymal differentiation.
- CD99 — a characteristic marker for primitive neuroectodermal tumors.
Comparative Characteristics: Children vs. Adults
Differences between pediatric and adult oncology are systemic in nature, affecting both tumor biology and approaches to diagnosis and prevention.
| Feature | Children | Adults |
|---|---|---|
| Primary target | Predominantly tissues (lymphoid, neural, hematopoietic) | Predominantly organs |
| Histological type | Mostly sarcomas | 87% are carcinomas |
| Stage at diagnosis | About 80% are already disseminated | Often localized or regional |
| Screening & prevention | Screening not established, prevention unlikely | Screening well-established, prevention possible in 80% of cases |
| Early diagnosis | Often accidental | Constantly improving |