Tumors composed of skeletal muscle are the most common soft tissue sarcomas in the pediatric age group but make up only a small percentage of sarcomas in adults. Multicenter studies by the Intergroup Rhabdomyosarcoma Study/Children's Oncology Group and others have made it possible to study these tumors extensively. The focus of this chapter is benign and malignant soft tissue tumors that have skeletal muscle differentiation as a principal component (Table 20.1). There are several other tumors that display heterologous skeletal muscle differentiation. For this reason, when one encounters rhabdomyosarcomatous differentiation, these diagnoses must be ruled out, especially in adults.
Unlike tumors of adipose tissue and blood vessels, and fibrous lesions, malignant tumors with skeletal muscle differentiation are more common than benign ones. A prevalent theory is that an undifferentiated mesenchymal cell becomes malignant and then differentiates along several tissue pathways, giving rise to sarcomas having various histologic characteristics. Some evidence exists for the origination of some rhabdomyosarcomas (RMSs) from differentiated myocytes, however. Preexisting skeletal muscle is not a prerequisite for their development, however, as is proved by RMSs arising in sites such as the urinary bladder.
The reason for the predominance of RMS in childhood and teenage years and their rarity in adults remains a mystery. During fetal development, normal myogenesis is complete by early in the second trimester. The association of RMS with younger age groups suggests that primitive myogenic stem cells are perturbed by proliferation factors, genetic associations, and tissue differentiation processes rather than external carcinogenic factors.