Pathology: myositis ossificans & Fibro-osseous pseudotumour of digit

Fascicles of spindle-stellate cells

Pale eosinophilic cytoplasm

Ovoid euchromatic nuclei with milt atypia; mitotic activity is inconspicuous 

Trabeculae of woven bone with osteoblastic rimming 

Chondrocytes set within lacunae in a hyaline matrix

Spindle-stellate cells with fascicular-storiform pattern

Abundant pale amphophilic cytoplasm 

Round-ovoid nuclei with prominent small nucleoli; occasional mitotic activity

Interspersed "ganglion-like" epithelioid cells; abundant amphophilic cytoplasm; eccentric round-ovoid nuclei with prominent nucleoli and occasional bi-nucleation

Incomplete rim of metaplastic bone; mild chronic inflammatory infiltrate 

Spindle-stellate cells with vague fascicular pattern

Abundant pale amphophilic cytoplasm 

Round-ovoid nuclei with prominent small nucleoli; occasional mitotic activity

Myxoid-collagenous stroma;  seams of osteoid with varialble mineralization; scattered osteoclast-like giant cells

RNA-seq: COL1A1::USP6 gene fusion