There is cellular proliferation of spindled myofibroblasts. There is a focal fasciitis-like stroma with mild to marked, mostly moderate, cellularity due to fibroblasts.
The osteoid formation is more evident here. Osteoblastic rimming is prominent. Osteoclasts are also part of the lesion.
Osteoid and more mature woven bone can be appreciated, however, the zoning phenomenon characteristic of myositis ossificans is not usually seen.
Other areas are more spindled and almost nodular, although irregular deposits of osteoid can still be seen.
Fibro-osseous pseudo tumor of digis (FOPD), also known as florid reactive periostitis, falls along the spectrum of nodular or proliferative fasciitis. In other words, this is a myofibroblastic proliferation, but exhibits several distinguishing features: (1) arises from phalanges of heands and feet (2) elaborates osteoid, bone and/or cartilege (Fletcher).
Grossly, FOPD is a small (less than 3 cm) rubbery firm nodule. Histologically, this lesion is similar to nodular or proliferative fasciitis with the addition of irregular produce of osteoid, bone or cartilege. Typically, one sees a proliferation spindled myofibroblasts in a myxoid stroma with areas of woven bone and osteoid.
Myositis ossificans (MO), a related lesion, is similar in that it is a spindled myofibroblastic pseudotumor that produces bone and cartilege, occurring after as a reaction to injury. However, MO occurs in the skeletal muscle and FOPD occurs in the soft tissues of the digits. Furthermore, myositis ossificans exhibits a particular "zoning phenomenon" (not seen in FOPD) in which the osteoid arising from the spindle cells gradually transitions to immature woven bone and mature lamellar bone (Folpe). In one study of FOPD, half showed a zonal organization, with mature woven one peripherally and immature woven bone centrally (Moosavi).
MO and FOPD are important to recognize because may be mistaken for benign and malignant neoplasms such as osteosarcoma or parosteal osteosarcoma. However, keep in mind that osteosarcoma of the hand is exceptionally rare.
It affects a wide age range, but typically arises in young adults, with a peak incidence in the second and third decades. It usually presents as a swelling of the affected part, sometimes accompanied by pain, tenderness, and redness. The lesion usually develops as a result of injury or trauma (Folpe). Often the patient engaged in an occupation requiring repetitive manual use. 75% of cases show calcification on XRay.
Typically self-limiting or cured by simple excision. Recurrence is not common.
Moosavi CA, Al-Nahar LA, Murphey MD, Fanburg-Smith JC.Fibroosseous pseudotumor of the digit: a clinicopathologic study of 43 new cases. Ann Diagn Pathol. 2008 Feb;12(1):21-8.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 15401.
Folpe AL, Inwards CY. Bone and Soft Tissue Pathology: Foundations in Diagnostic Pathology Philadelphia, PA: Elsevier; 2010: 242-3.