Hyalinizing spindle cell tumor with giant rosettes--a soft tissue tumor with mesenchymal and neuroendocrine features. An immunohistochemical, ultrastructural, and cytogenetic analysis

Hyalinizing spindle cell tumor with giant rosettes is a recently described biphasic neoplasm of soft tissues that shares mesenchymal and neuroendocrine features. Its morphologic structure is distinctive, with the presence of hyalinized paucicellular foci that are termed rosettes. The cells around the latter display positive immunoreactivity for neuroendocrine markers. The small number of cases described to date indicates that they tend to be localized in the extremities. To describe the clinicopathologic features of 2 unusual cases of hyalinizing spindle cell tumor with giant rosettes. One tumor was located in the prestyloid parapharyngeal space and the second in the left thigh. Both tumors were well circumscribed and surrounded by a thin capsule-like fibrous band without infiltrating projections. The rosettes were embedded in a spindle cell proliferation. Immunohistochemical stains showed positive results for S100 protein, synaptophysin, CD57, protein gene product 9.5, and neuron-specific enolase exclusively in the cells palisading the rosettes. These markers were negative in the spindle cell portions of the tumor. The latter were immunoreactive for factor XIIIa, vimentin, HAM56, collagen IV, and CD68. Vimentin was the only marker shared by the rosette-forming cells and the spindle cells. Ultrastructurally, the rosette-forming cells contained neurosecretory granules. This study describes the first cytogenetic analysis in this type of tumor revealing 2 cell lines, both containing a balanced translocation between chromosomes 7 and 16. Follow-up of the patients at 16 and 8 months did not disclose evidence of recurrence. These 2 new cases increase the awareness of hyalinizing spindle cell tumor with giant rosettes and demonstrate that it is a spindle cell neoplasm of unique cytogenetic rearrangements composed of dendritic, histiocytic, and fibroblastic cells admixed with cells that have neuroendocrine differentiation.