Tab. II.

Key diagnostic features.

Reactive Spindle Cell Nodule/Exuberant Scar: circumscribed and, at least focally, infiltrative margins; previous biopsy/FNA; α-smooth muscle actin-positive spindle cells; foamy and hemosiderin-laden macrophages, lymphocytes and foreign body giant cells; fat necrosis

Nodular Fasciitis: circumscribed and, at least focally, infiltrative margins; α-smooth muscle actin-positive spindle cells; fibro-myxoid stroma, at least focally, with tissue culture-like appearance

Inflammatory Pseudotumor: circumscribed and, at least focally, infiltrative margins; α-smooth muscle actin-positive spindle cells closely intermingling with lymphocytes and plasma cells; previous history of local trauma/stimuli; ALK-1 is negative

Myofibroblastoma, Classic-type: circumscribed margins; desmin/CD34/α-smooth muscle actin-positive spindle cells; short intersecting fascicles interrupted by keloid-like collagen fibers

Lipomatous Myofibroblastoma: circumscribed margins; desmin/CD34/α-smooth muscle actin-positive spindle cells with finger-like pseudo-infiltration into an intratumoral lipomatous component

Palisaded/Schwannian Myofibroblastoma: circumscribed margins; desmin/CD34/α-smooth muscle actin-positive spindle cells with formation of Verocay-like bodies; S100 protein is negative

Benign Fibroblastic Spindle Cell Tumor: circumscribed margins; CD34-positive fibroblastic-like spindle cells; short intersecting fascicles; thick keloid-like collagen fibers; variable additional lipomatous component

Spindle Cell Lipoma: circumscribed margins; CD34-positive short spindle cells with bipolar cytoplasmic processes; variably admixed mature lipomatous component; at least focally, myxoid stroma with ropey collagen fibers

Fibroma: circumscribed margins; hypocellular, fibrosclerotic nodule with interspersed CD34-positive fibroblast-like spindle cells

Solitary Fibrous Tumor: circumscribed margins; CD34/STAT6-positive fibroblast-like spindle cells, haphazardly arranged (pattern-less growth pattern); branching vessels, often with perivascular hyalinization

Leiomyoma: circumscribed margins; interlacing fascicles of desmin/α-smooth muscle actin/h-caldesmon-positive spindle cells with the features of mature smooth muscle cells

Myxoma: circumscribed margins; vimentin positive spindle to stellate cells embedded in an abundant myxoid stroma; atypical bizarre cells, along with thick keloid-like collagen fibers, can be seen

Schwannoma/Neurofibroma: circumscribed margins; S100-positive spindle cells with formation of Verocay-bodies and alternating Antoni A and Antoni B areas (schwannoma); cells with wavy nuclei set in myxoid stroma with keloid-like collagen fibers (neurofibroma)

Desmoid-type Fibromatosis: finger-like infiltrative margins; α-smooth muscle actin and β-catenin-positive fibroblast/myofibroblast-like spindle cells arranged into long intersecting fascicles; the cells are often aligned parallel and are separated by collagenized stroma

Dermatofibrosarcoma Protuberans: circumscribed and, at least focally, infiltrative margins; CD34-positive fibroblast-like spindle cells; diffuse storiform growth pattern; low mitotic activity; finger-like or honeycomb infiltration of the adjacent fibro-fatty tissue

Low-grade, Fibromatosis-like Spindle Cell Carcinoma: finger-like infiltrative margins; p63/cytokeratin-positive spindle cells with the features of fibroblasts/myofibroblasts; at least focally, small cohesive clusters of cytokeratin/p63-positive epithelioid-polygonal cells

Inflammatory Myofibroblastic Tumor: circumscribed and, at least focally, infiltrative margins; α-smooth muscle actin-positive spindle cells admixed with lymphocytes and plasma cells; no association with previous history of local trauma/stimuli; ALK-1 expression in about 40-50% of cases

Low-grade Myofibroblastic Sarcoma: circumscribed and, at least focally, infiltrative margins; α-smooth muscle actin-positive myofibroblastic-like cells with mild/moderate nuclear pleomorphism and high mitotic activity (7 to 35 mitoses x 10 HPF); fascicular arrangement