Originating from the periosteum, it grows outward from the bone but tends to encase the shaft. It is relatively rare, with similar incidence rates in males and females. Although it can occur at any age, over half of the cases are diagnosed in individuals older than 30.

bubble_chart Pathological Changes

The tissue morphology is highly inconsistent, making diagnosis difficult. Some resemble benign tumors, while others closely resemble sclerosing fleshy tumors. The latter can destroy the cortex and invade the medullary cavity, leading to a poor prognosis. Occasionally, the tumor may have a cartilaginous cap, resembling a benign osteochondroma.

bubble_chart Clinical Manifestations

The clinical symptoms are mild and may be painless, presenting only as a mass, which can sometimes be quite large. If located behind the lower end of the femur, it may affect normal joint movement.

The natural course resembles that of a benign tumor, with 75% growing slowly and recurring after repeated resections, yet patients can still survive for more than 5 years. The remaining 25% exhibit aggressive behavior, with biological characteristics similar to sclerosing fleshy tumors.

It primarily occurs in the metaphysis of long bones, especially the distal femur, proximal tibia, and proximal humerus.

X-rays may reveal multiple nests of new bone formation, growing slowly and tending to aggregate into tumor masses. The margins are often lobulated and rounded, with small trabeculae within the tumor. In the early stages, the boundary with the parent bone is clear, and a narrow radiolucent zone may be visible. As the tumor progresses, this boundary disappears. The deep part is dense and homogeneous. In less aggressive cases, the cortex and cancellous bone remain unaffected. In more aggressive cases, the cortex is destroyed, the medullary cavity is eroded, and osteolytic areas appear, interspersed with foci of new bone formation. CT is valuable in determining whether the medullary cavity is involved. Metastasis occurs late, but a few highly aggressive cases may metastasize early.

Depending on the location, size, and invasiveness, according to the Farn and Huvos classification, grades I-II can be treated with en bloc resection, ensuring normal surrounding soft tissue and underlying bone margins are preserved. For grades III-IV, amputation combined with high-dose multi-agent chemotherapy is the preferred treatment, with drugs selected primarily targeting the tumor's reproductive cycle.

bubble_chart Differentiation

Myositis ossificans and bone tumors, among others, are difficult to differentiate from post-traumatic calcification and myositis ossificans. The latter exhibits a feathery appearance, tends to deposit in soft tissues, and in rare cases may appear dense and round. The ossified mass is less dense than parosteal osteosarcoma and has a sclerotic margin.