Childhood cancer is not a single disease but a complex group of conditions characterized by the uncontrolled growth of abnormal cells. Determining the precise number of childhood cancer types is difficult because the answer depends on the level of classification used for diagnosis. At the broadest level, there are about a dozen major categories of pediatric malignancy. However, when considering the distinct cellular and molecular features that define modern diagnoses, the number of unique diseases rises into the hundreds. The ongoing advancements in genetic sequencing continue to reveal new, highly specific subtypes.
Key Differences Between Pediatric and Adult Cancers
Pediatric cancers are fundamentally different from those typically seen in adults, which affects how they are categorized and treated. Cancers in children, generally defined as those occurring from birth through age 14 or 19, often arise from embryonic or mesenchymal tissues, meaning they originate in developing cells like those that form blood, bone, or the nervous system. Adult malignancies, in contrast, are overwhelmingly carcinomas that develop from epithelial cells, such as breast, lung, or prostate cancer.
The underlying causes also show a significant divergence. Most adult cancers accumulate genetic damage over decades due to environmental exposures and lifestyle choices. Childhood cancers rarely relate to these external factors; instead, they are primarily driven by spontaneous, random genetic changes or specific chromosomal rearrangements that occur during early development. Pediatric tumors typically have a much lower number of total mutations compared to adult cancers.
How Childhood Cancers Are Classified
To standardize the study of these diverse diseases, the medical community relies on the International Classification of Childhood Cancer, 3rd edition (ICCC-3). This framework organizes tumors based on their presumed tissue of origin and their microscopic appearance, known as histology. This approach differs from the classification of many adult cancers, which are often grouped by the primary site where the tumor was first found.
The ICCC-3 divides all pediatric malignancies into 12 main diagnostic groups. These main groups serve as broad categories for epidemiological surveillance and reporting. Within these 12 main groups, the classification is further refined into 47 distinct subgroups. A “type” of childhood cancer generally refers to one of these 12 main categories, while a “subtype” refers to the more specific diagnosis within the subgroups.
The system further breaks down some of the most complex subgroups into highly specific divisions, aiming to categorize tumors based on their cytogenetic and molecular characteristics. While there are 12 main groups, the total number of molecularly distinct diseases is well over 100 and continues to grow as research advances.
The Most Common Major Diagnostic Groups
The 12 major ICCC-3 groups are not evenly represented, as just a few categories account for the majority of all pediatric cancer diagnoses. Leukemias, which are cancers of the blood and bone marrow, constitute the single most frequent category in children. Acute lymphoblastic leukemia (ALL) is the most common form, accounting for approximately three-quarters of all childhood leukemia cases.
Central Nervous System (CNS) tumors represent the most common type of solid tumor in children, including malignancies of the brain and spinal cord. These tumors are highly diverse and include various forms of astrocytomas, medulloblastomas, and ependymomas. Lymphomas, which arise in the lymphatic system, are the third most frequently diagnosed group, including both Hodgkin and Non-Hodgkin forms.
Neuroblastoma is a distinct tumor that originates in the developing nerve cells of the sympathetic nervous system, often appearing in the abdomen, chest, neck, or pelvis. Retinoblastoma is another unique pediatric tumor that develops in the retina of the eye and is often diagnosed in infants and very young children.
Understanding Subtypes and Rarer Forms
The complexity of childhood cancer extends beyond the most common types and includes many less frequent and highly specialized tumor groups. These include renal tumors, such as Wilms tumor, which is the most common kidney cancer in children. Other groups are soft tissue sarcomas, like rhabdomyosarcoma, and malignant bone tumors, which include osteosarcoma and Ewing sarcoma. Germ cell tumors, which originate from reproductive cells, and hepatic tumors, like hepatoblastoma, also fall into the remaining ICCC-3 categories.
The true numerical count of childhood cancers is dramatically increased by the ongoing molecular sub-typing within each major group. Advances in genomic analysis have shown that what was once considered a single disease, such as B-cell acute lymphoblastic leukemia (B-ALL), is actually a collection of many distinct diseases. Researchers have identified dozens of unique molecular subtypes within B-ALL alone, each defined by specific genomic alterations. This genetic stratification is necessary because these subtypes have different risk profiles and require tailored treatments. The refinement of diagnosis down to these specific genetic lesions moves the total well beyond the initial 12 broad categories.