Large, heavily mineralized mass with lobulated morphology adherent to the cortex in the metaphyseal region of the underlying bone.

Mass is typically heavily mineralized throughout, including the surface.

Underlying cortex often thickened with thick indolent periosteal new bone formation.

Large tumors encircle the bone.

Large heavily mineralized mass attached to the surface of the bone.

May show clear space between mass and cortex as the mass encircles the bone.

Minimal unmineralized soft tissue component.

Significant unmineralized peripheral soft tissue mass or significant intramedullary component raises concern for dedifferentiation.

Ossified regions have low signal intensity on T1- and T2-weighted images.

Focal unmineralized area with predominantly high signal intensity on T2-weighted images or significant intramedullary involvement raises concern for dedifferentiation.

The most useful modality to assess the status of medullary involvement (approximately 50 % of tumors), usually representing <25 % of the cross-sectional area.

Increased uptake

May or may not be attached to the cortex, but with epicenter in muscle

More mineralized peripherally due to zonation pattern of ossification

Often contain internal fat on MRI or CT indicative of mature benign ossification

Continuity of the cortical and cancellous bone between the lesion and the parent bone

Heavily ossified, hard, tan-white, somewhat lobulated mass attached to the underlying cortical bone.

Occasionally the tumor wraps around underlying bone.

Approximately 50 % contain white-gray areas of hyaline cartilage located either on the surface, a feature mimicking osteochondroma, or within the tumor.

Invasion of medullary cavity in 25–40 % of low-grade tumors and up to 50 % of dedifferentiated tumors.

Soft, fleshy areas may correspond to progression (dedifferentiation) to a high-grade sarcoma.

Size usually ranges between 4 and 10 cm in greatest dimension.

Long trabeculae of woven bone arranged in a parallel-oriented pattern surrounded by a hypocellular population of minimally atypical spindle cells in collagenous stroma.

Rare mitotic figures.

Fascicles of cells with elongated nuclei with little to no abnormal chromatin.

Spindle cell component with moderate cellularity and cytologic atypia in a minority of tumors.

Varied appearances of bone include prominent cement lines (“pagetoid appearance”), irregularly shaped trabeculae (fibrous dysplasia-like), solid areas, and an anastomosing pattern.

Hyaline cartilage with minimal atypia to low-grade malignant features presents as a cap (“osteochondroma-like”) or islands within the tumor.

Adipose tissue, with or without bone marrow, situated between bone trabeculae in 15 % of tumors.

Areas of dedifferentiation to a high-grade sarcoma (osteosarcoma, spindle cell sarcoma, or undifferentiated pleomorphic sarcoma) in 15–43 % of tumors. Occurs at initial diagnosis or more commonly in recurrent tumor.

Intertrabecular spaces filled with fatty hematopoietic marrow devoid of a spindle cell component

Benign cartilage cap in all tumors

Zonated maturation of spindle cell component into the bone

Hypercellular spindle cell component with brisk mitotic activity

Dense, mature cortical-type bone throughout the tumor

No cartilage component

Coupling of MDM2 and CDK4 immunostains reportedly shows variable amounts of nuclear positivity for at least one marker in approximately 90 % of cases.