The tumor originates in the small blood vessels of the oral mucosa, eyes, lips, cheeks, tongues, and subcutaneous tissue of the extremities (78% of all cases). About 18% of cases involve major veins i.e. 5 cases involved the common femoral veins and one case each involved the femoral vein, common iliac vein, inferior vena cava, and subclavian vein. Cases involving the ascending aorta have also been reported. The presenting symptoms of these tumors were strictly dependent on their locations and impacts on the involved vasculature. Most cases involving superficial sites presented with a small (mean diameter of 1.5 cm), painless, slowly growing mass. However, tumors growing in deep tissues went unnoticed until they became large (e.g. 15 cm.) enough to obstruct blood flow: two cases involving the ascending aorta presented with the signs and symptoms of acute aortic dissection (e.g. severe pain, congestive heart failure, cardiac arrest, fainting , stroke, ischemic peripheral neuropathy, and/or paraplegia) while 9 cases involving large veins presented with acute swelling, pain, and tissue/organ dysfunctions in the areas drained by the involved veins. Pathology
The microscopic histopathology of hematoxylin and eosin stained nodular fasciitis tumors (see above and three below figures) consists of spindle-shaped myofibroblastic cells (i.e. cells with features of smooth muscle cells and fibroblasts.). These cells are in a myxoid (i.e. more blue or purple compared to normal connective tissue because of excessive uptake of the hematoxylin stain) or a collagenous (high content of collagen fibers) tissue background. The neoplastic myofibroblasts are arranged in whorls and/or short bundles.
These cells may show high rates of replicating as judged by their mitotic index but these mitoses are normal in appearance. The tumor tissues often contain red blood cells, lymphocytes and giant osteoclast-like giant cells and may contain sites of bone-like tissue. NF is sometimes classified into three subtypes based on its predominant histopathological pattern: myxoid or reactive (type I), cellular (type II), and fibrous (type III). These patterns appear related to the duration of the lesion with the myxoid variant tending to have the shortest duration and the cellular and fibrous variants tending to have progressively longer durations.Immunohistochemical analyses indicate that the cells in NF usually express smooth muscle actin, muscle specific actin, and vimentin proteins but generally do not express CD34, S-100 protein, desmin, trypsin, factor VIII, F4/80 (also termed macrophage-specific antigen), or HLA-DR1 proteins. Uncommonly, the cells in NF tumors also express the CD68 (a histiocyte-specific marker) protein. The histopathology and expressions of marker proteins in cranial fasciitis tumors are similar to NF although its tumors tend to be more organized and have higher levels of inflammatory cell infiltrates, vascularity, and involvements of underlying bone than NF.The histopathology and expressions of marker proteins in intravascular fasciitis tumors are also similar to NF. In comparison to NF, however, its spindle-shaped tumor cells are more often arranged in a storiformed (i.e. whirled) or haphazard pattern and have vesicle-containing nuclei with prominent nucleoli.