Undifferentiated sarcomas of children: pathology and clinical behavior--an Intergroup Rhabdomyosarcoma study.

Undifferentiated soft tissue sarcoma (UND-STS) is the most poorly defined tumor eligible for Intergroup Rhabdomyosarcoma Studies (IRS). Recent IRS UND-STS experience was reviewed to assess the histologic characteristics and clinical behavior of undifferentiated sarcomas. Of the 1,527 patients entered on IRS-III and IRS pilot-IV, 96 had tumors classified by the IRS Pathology Committee as UND-STS. Of these, 52 had adequate histologic material for this study. After application of immunohistochemistry, 18 tumors were reclassified, mostly as embryonal rhabdomyosarcomas (RMS), primitive neuroectodermal tumors, and intra-abdominal desmoplastic small round cell tumors. The remaining 34 UND-STS had a diffuse hypercellular histologic pattern made up of sheets of medium-sized cells. The tumor cells had a minimal to moderate amount of cytoplasm and a variable nuclear morphology, predominately vesicular with finely granular chromatin. Except for reactivity with antibodies against vimentin, most tumors had a negative immunohistochemical profile. The 5 year Kaplan-Meier survival estimate for patients with non-metastatic disease was 72%, a significant improvement when contrasted with patients diagnosed to have UND-STS in IRS-I and IRS-II. Med. Pediatr. Oncol. 29:170–180, 1997. © 1997 Wiley-Liss, Inc.