Parosteal Osteosarcoma

Sites of Involvement

  • The vast majority involve the metaphysis of the femur, humerus, or tibia.

  • The most common location is the posterior distal femur.

  • Rare sites included the craniofacial bones, clavicle, pelvis, and spine.

Signs and Symptoms

  • Slowly enlarging swelling or mass

  • Occasionally pain in the region of the mass

Imaging Findings

Radiographic Features

  • 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.

CT Features

  • 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.

MRI Features

  • 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.

Bone Scan

  • Increased uptake

Imaging Differential Diagnosis

Myositis Ossificans

  • 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


Gross Features

  • 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.

Histological Features

  • 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.

Histologic Differential Diagnosis


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

  • Benign cartilage cap in all tumors

Heterotopic Ossification

  • 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

Ancillary Studies


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


  • Cytogenetic karyotype shows one or more supernumerary ring chromosomes.

  • Ring chromosomes contain amplified material from chromosomal region 12q13-15.

  • Target genes MDM2 and CDK4 amplified in up to 85 % of tumors.


  • Almost no capacity for metastasis

  • Medullary involvement not associated with a poorer prognosis

  • Metastatic rate of 28–47 % for dedifferentiated tumors

Jan 2, 2017 | Posted by in ORTHOPEDIC | Comments Off on Parosteal Osteosarcoma
Premium Wordpress Themes by UFO Themes