Small round blue cell tumor is the name given to a group of highly malignant neoplasms that occur mostly in the pediatric age group. Although this term could also apply to some adult neoplasms, notably small cell carcinoma of the lung, the term has become widely associated with childhood cancer. The name is derived from the primitive, highly cellular nature of these lesions, which typically present a vast sea of dark-blue nuclei on hematoxylin-based stains. Cytoplasmic abundance roughly correlates with cellular differentiation, which is often modest. Although cytoplasmic landmarks allow for cell type identification in some cases, ancillary techniques, such as immunohistochemistry, electron microscopy, and genetics, have become important in the diagnosis of these tumors.

Although childhood cancer constitutes a relatively small percentage of all malignancies, it causes a large percentage of pediatric mortality, being second only to accidents as a cause of death in industrialized nations. Pediatric cancer also differs from adult cancer in the great percentage of cases treated on multi-institutional protocols. Despite success in treatment, more than 100,000 person-years are lost each year to this group of cancers.

TYPES OF ROUND CELL TUMORS AND THEIR DIFFERENTIAL DIAGNOSES

Leukemias and brain tumors constitute most childhood cancers. Although the former can present as solid masses, these lesions do not typically enter into the differential diagnosis of small round cell tumors. Table 31.1 lists the most common lesions in the latter group. Of these tumors, neuroblastoma, rhabdomyosarcoma, lymphoma, and Wilms' tumor are most common.