Malignant stromal tumor, so called "gastrointestinal stromal tumor", with rhabdomyomatous differentiation occurring in the gallbladder.

Gastrointestinal stromal tumors (GISTs) constitute the largest category of primary nonepithelial neoplasms of the gastrointestinal tract. It is extremely rare that this tumor occurs in the bile tract, and only a few cases have been reported. Immunohistochemically, the tumor cells revealed a phenotype similar to Cajal cells, occasionally with differentiation to smooth muscle cells or neural cells. We present a case of malignant stromal tumor similar to GISTs with rhabdomyomatous differentiation of the gallbladder in a 68-year-old woman. The resected tumor was predominantly composed of spindle cells with rhabdomyomatous differentiation. Immunohistochemical study revealed diffuse staining of tumor cells using vimentin despite negative staining for desmin or S-100. This indicated a mesenchymal origin of the cells without smooth muscle or neuronal differentiation. Myoglobin-positive cells, in which phosphotungstic acid hematoxylin staining revealed cross striations of the cytoplasm, suggested rhabdomyomatous differentiation. Diffuse positivity for KIT in the cells suggested that the pathogenesis of this tumor may resemble that of GIST. The tumor may have derived from a mesenchymal stem cell that had undergone partial rhabdomyomatous differentiation.