Classification of Rhabdomyosarcoma

Lesions now classified as rhabdomyosarcoma (RMS) in children are as a rule comprised of very primitive cells which do not show obvious features of fully developed muscle. It is not surprising that the early attempts to classify this group of tumors focused upon tumors that showed some recognizable features of skeletal muscle such as cross-striations. Stout (1946) gave credit to Weber as the first to describe such a tumor in 1854 in his classical description of RMS of the skeletal muscles. Weber described a patient who had a protruding tongue for several years that developed an obvious tumor mass about the size of a hand when it was removed at 21 years of age. The tissue consisted of small round or oval cells or nuclei with only scanty cytoplasm. A large proportion of the cells showed cross-striations. The conclusion of that author was that these cells were growing normal muscle fibers which were in different stages of development. He compared the microscopic appearance of these cells to those of a 4-5-month-old human embryo, but could not explain the tumor cells with scanty cytoplasm. He believed that these tumor cells developed out of inflammatory exudate which was found in the area of the excision. In retrospect, there can be little doubt that this was a neoplasm which would be diagnosed as an RMS today; it had enough cross-striations present to recognize that the tumors cells had a skeletal muscle origin.