Answer: Myofibroblastoma

Histology: These tumors, like solitary fibrous tumors, are composed of bland spindle cells and contain wiry collagen, are well-marginated grossly, and uniformly and strongly express CD34 such that some observers believe they are the same tumor [1]. They differ from solitary fibrous tumors in that malignant examples are not described and by their expression of muscle markers immunohistochemically and by ultrastructural demonstration of myofibroblastic features. They have been described first in the breast [2] and lymph nodes [3, 4], and subsequently a variety of other sites [5-22] and are benign. One of their characteristic features is the presence of wiry collagen fibers called “amianthoid fibers” by some authors [4]. These tumors may also show areas of typical smooth muscle differentiation [23-25], a finding not reported in solitary fibrous tumors.

Discussion: As above, these tumors are quite similar to solitary fibrous tumors but have a different immunohistochemical profile and they are somewhat unique in that usually tumors that express CD34 are negative for muscle markers. Leiomyosarcomas, for example, seldom express CD34 and nodular fasciitis, which is myofibroblastic, lacks CD34. Fibrosarcomas are also typically CD34 negative. Leiomyosarcomas differ on morphologic grounds as well, composed of perpendicularly oriented fascucles of cells with blunt-ended nuclei and para-nuclear vacuoles. Both fibrosarcomas and leiomyosarcomas usually have at least scattered mitotic activity whereas myofibroblastomas have only rare if any mitoses.