Hyalinizing spindle cell tumor with giant rosettes - A report of three cases with ultrastructural analysis

We report the light microscopic, ultrastructural appearance and immunohisto chemical staining profile of three distinctive soft-tissue tumors recently designated hyalinizing spindle cell tumor with giant rosettes. The tumors o ccurred in two men, 41 and 54 years old, and one woman, 62 years old. Two t umors arose in the lower extremities and one in the upper arm. Two tumors w ere resected and measured 3 and 13.2 cm in greatest diameter; a biopsy only was done of the third tumor. Grossly, the tumors had a tan, pink, or white cut surface. The largest tumor exhibited central cystic change. Microscopi cally, they all displayed similar features and were composed of fibromyxoid regions, with areas of hyalinization in two tumors and focal ossification in one tumor. Scattered throughout each of the tumors were rosette-like str uctures in which neoplastic cells were arranged around a central collagenou s core. Ultrastructurally, the neoplastic cells demonstrated the features o f fibroblasts. In all tumors, there was abundant extracellular collagen fib ers and in one there were large aggregates of amorphous extracellular exter nal lamina-like material. The center of the rosette-like structures was com posed of banded collagen fibers and the cells at the periphery of the roset tes had ultrastructural features similar to the neoplastic spindle cells lo cated elsewhere in the tumor. Immunohistochemically, the tumor cells staine d for vimentin. There was focal staining of the widely distributed spindle cells and cells that formed the rosettes for Leu-7, S-100 protein, and CD34 . In one tumor, there was faint diffuse staining of the spindle cells for n euron-specific enolase. One tumor (with the amorphous extracellular materia l) stained for type TV collagen. There was no staining for desmin, muscle a ctin, smooth muscle actin, keratin, or epithelial membrane antigen. These r esults demonstrate that hyalinizing spindle cell tumor with giant rosettes is composed of fibroblasts. We did not demonstrate any ultrastructural or i mmunohistochemical differences between the spindle cells that comprised the majority of the mass and those that surrounded the rosette-like structures . There was no ultrastructural evidence of neural differentiation to explai n the focal S-100 protein and Leu-7 staining of the tumor cells.