AFSTs are slow-growing, often painless tumors composed primarily of spindle-shaped cells and a prominent vascular network.
[4] The tumors are most common in the lower extremities but uncommonly occur in the back, chest wall, iliac crest, groin and nearby lower lateral abdominal region,[4] abdominal cavity, pelvic cavity,[5] breast,[3] cheek, temporal region of the head and, in a report on 24 AFST cases done in Shanghai, the upper limb in 3 cases and, in 1 case each, the retroperitoneum and liver.
[5] Histopathological microscopic analyses of hematoxylin and eosin-stained AFST tissues generally reveal bland appearing spindle-shaped cells and a prominent small, thin-walled blood vessels network in a background of alternating myxoid connective tissue areas and more highly cellular collagen fiber-rich connective tissue areas.
(Myxoid indicates areas that appear more blue or purple than normal due to their high uptake of the hematoxylin stain.)
Cellular angiofibroma differs from AFST in its typical location (i.e. inguinal, scrotal, and vulva areas), its distinct histology of rounded, non-branching vessels, high cellularity, and cells with small nuclei, and its tumor cells' loss of the RB1 gene.
Myxofibrosarcoma tumors commonly show overt malignant features such as highly infiltrating margins, tumor cells with eosinophilic cytoplasm, atypical nuclei, and rapid proliferation rates as evidenced by their high mitotic indexes.
[3] In all of these cases, the presence of one of the AFST-associated fusion genes cited in the previous section lends support for the diagnosis of AFST.