INTRANODAL MYOFIBROBLASTOMA - REPORT OF A CASE

Palisaded myofibroblastoma (hemorrhagic spindle cell tumor) is a recen t addition to the group of benign primary spindle cell lesions of lymp h nodes. These tumors are characterized histologically by hemorrhage, palisading, and foci of collagen called amianthoid fibers. We report a further typical example with the aim of discussing its differentiatio n. Tumor cells were positive for smooth-muscle actin and vimentin. The cytoplasm contained moderate numbers of rough endoplasmic reticulum c isternae and some smooth-muscle type myofilaments. Subplasmalemmal den sities and plasmalemmal caveolae, as well as material interpreted as e xternal lamina, were identified at the cell surface, whereas the fibro nexus junctions typical of myofibroblasts were not seen. Immunostainin g for type IV collagen was positive. Intranodal myofibroblastomas have largely been considered as myofibroblastic, but the observations pres ented here raise the alternative possibility of simple smooth-muscle d ifferentiation. The foci of collagen widely referred to as amianthoid fibers contained fibrils mostly of conventional diameter, 50-83 nm. Th e giant collagen fibrils typical of true amianthoid change were absent . It is suggested that the term amianthoid be used only after ultrastr uctural confirmation of the presence of giant collagen fibrils.