Childhood rhabdomyosarcoma with anaplastic (pleomorphic) features. A report of the Intergroup Rhabdomyosarcoma Study

The pleomorphic subtype of rhabdomyosarcoma (RMS) is now rarely diagnosed in both children and adults. Most cases previously called pleomorphic RMS are probably diagnosed as something else, most often embryonal RMS in children and malignant fibrous histiocytoma in adults. To analyze the concept of pleomorphic RMS in children, we reviewed the tumors of patients entered on the Inter-group Rhabdomyosarcoma Study (IRS I, II, and III). The presence of cells with lobated, hyperchromatic nuclei at least three times larger than the common tumor cell population (anaplastic cells) was selected as the main criterion. Of about 3,000 cases, 110 showed these types of cells, had sufficient histologic material, and had available follow-up data. These tumors were divided into two subgroups: Subgroup I tumors contained only scattered anaplastic cells, and tumors with foci or large sheets of anaplastic cells were classified as subgroup II. Besides the anaplastic-pleomorphic areas, most of these tumors had distinctive features of embryonal RMS (105 cases) and rarely had characteristics of alveolar RMS (five cases). The age distribution of these patients did not differ significantly from those whose tumors did not show the anaplastic features, the average being 6 years and the median 4 years. Lower extremity, retroperitoneum, and the head and neck region were the most common primary tumor sites. The 5-year survival rate was 60% for subgroup I tumors and 45% for subgroup II tumors compared with the survival rate of 68% for 482 IRS II embryonal RMS cases with no anaplastic-pleomorphic features. The lower survival rate for patients in subgroup II was statistically significant (p = 0.004) and similar to the unfavorable survival of patients with alveolar RMS and undifferentiated sarcoma. Because anaplastic cells are seen in many soft tissue sarcomas and in both embryonal and alveolar RMS in children, this feature is not sufficiently unusual to separate a pleomorphic subtype of RMS. The presence of anaplastic cells in aggregates or diffuse sheets throughout the tumor, however, portends a poor survival for these patients.