bubble_chart Overview

Peripheral soft osteosarcoma is a type of soft osteosarcoma that originates outside the bone and implants onto the bone. Its incidence is lower than that of central soft osteosarcoma. It is more common in males, with a male-to-female ratio of 2:1. The disease typically occurs in adulthood, usually after the age of 20, and does not manifest before puberty.

bubble_chart Pathological Changes

1. Gross Appearance: Peripheral chondrosarcomas are generally large when surgically resected, with a typical cauliflower-like uneven surface, often enveloped by a thin fibrous pseudocapsule.

In the initial stage of malignant transformation from osteochondroma to peripheral chondrosarcoma, the originally thin and discontinuous cartilage cap becomes thickened (in childhood, the normal thickness of the cartilage cap can reach 2 cm). Therefore, in adults, if the cartilage cap exceeds 1–2 cm in thickness, malignancy should be suspected. Further, the superficial and deep cartilage tends to grow in a convex, lobulated manner, invading the cancellous bone of the osteochondroma. Eventually, this cartilage loses the characteristics of normal hyaline cartilage, becoming soft, fluid-rich, gray, and translucent tumor cartilage.

If the peripheral chondrosarcoma progresses further, its surface expands into a cauliflower or mushroom-like shape, with tumor cartilage extending into the base of the osteochondroma and the host bone. Tumor cartilage lobules invade the cancellous bone of the osteochondroma and may eventually reach the host bone. The tumor cartilage has a strong tendency to calcify and ossify, with this tendency being more pronounced in deeper layers. Calcification appears as yellow-white granular, ring-like, or irregular spots, with a hard, chalky or gritty texture. Ossification manifests as proliferative cancellous bone or white, ivory-like bone.

2. Microscopic Appearance: The malignancy of peripheral chondrosarcoma is classified into three grades.

Grade I: The most common, accounting for about two-thirds of cases. Sometimes, the early malignant transformation of osteochondroma can be observed, but in most cases, the tumor is large, and the original osteochondroma is no longer visible. The tumor exhibits marked and diffuse calcification-ossification, and due to necrosis, chondrocytes are scarce. In these areas, the cells often lack malignant features. In the superficial and non-calcified regions of the tumor, the cartilage contains abundant cells, which are larger, exhibit grade II pleomorphism, and often have binucleated cells.

Grade II: Relatively common, accounting for about one-third of cases. The intensity and extent of calcification-ossification are less than in grade I peripheral chondrosarcoma, and imaging shows an aggressive tumor. The cells have large, deeply stained nuclei of varying sizes, with pronounced pleomorphism, and binucleated cells are common.

Grade III: Rare. Imaging shows an aggressive or highly aggressive tumor with minimal calcification. The tumor is composed of well-differentiated cartilage, with markedly pleomorphic nuclei that are deeply stained, sometimes including giant or bizarre nuclei. Trinucleated or multinucleated cells are common.

Malignant Progression (Dedifferentiated Chondrosarcoma)

The progression from grade I to grade II peripheral chondrosarcoma takes many years and is rarely observed. The progression from grade II to grade III peripheral chondrosarcoma occurs over a shorter period. It is common for grade I or grade II peripheral chondrosarcomas to recur multiple times over 10 or more years without an increase in malignancy.

Similar to central chondrosarcoma, peripheral chondrosarcoma can also develop into a high-grade non-cartilaginous sarcoma (malignant fibrous histiocytoma, fibrosarcoma, or osteosarcoma), though this is rare (about 4%), and is termed dedifferentiated chondrosarcoma. In such cases, a highly aggressive, osteolytic sarcoma may arise, erasing the radiopaque image of the peripheral chondrosarcoma and invading the soft tissues. Pathologically, evidence of a pre-existing peripheral chondrosarcoma and/or osteochondroma is found adjacent to the high-grade non-cartilaginous sarcoma, with no transitional features.

bubble_chart Clinical Manifestations

The incidence is less than that of central chondrosarcoma. It is more common in males, with a male-to-female ratio of 2:1. It occurs in adulthood, generally after the age of 20, and does not develop before puberty.

It frequently occurs in the pelvis, followed by the proximal femur, spine, sacrum, proximal humerus, ribs, clavicle, distal femur, and proximal tibia. Compared to central chondrosarcoma, peripheral chondrosarcoma is more likely to occur in the pelvis and axial skeleton and is almost never found distal to the knee or elbow joints.

In the spine and sacrum, peripheral chondrosarcoma originates from the posterior vertebral arch; in the pelvis, it arises from the iliac wing or anterior arch; in the clavicle, it originates from the shaft; and in long bones, it arises from the metaphysis near the epiphysis (the same location as osteochondroma).

Peripheral chondrosarcoma grows expansively inward and outward, invading a pre-existing osteochondroma. If it progresses further, peripheral chondrosarcoma can implant onto the bone.

The main symptom is a slow-growing mass on the bone surface. The mass is surrounded by soft tissue but is not adherent to it. The mass is often round or spherical, with a hard texture resembling bone.

Some patients may have a history of multiple hereditary osteochondromas or solitary osteochondroma, but most patients have no such history. A history of multiple surgeries and recurrent tumors is not uncommon. In such cases, one or more soft tissue masses or nodules completely separated from the bone may be palpable.

The mass may be painless, but more than half of the patients experience mild pain, which may develop secondary to swelling. If peripheral chondrosarcoma is located in the pelvis, it may cause symptoms of lumbosacral plexus or peripheral nerve compression; if located in the vertebral body, it may lead to paraplegia.

Because peripheral chondrosarcoma grows slowly, the symptoms it causes are often not severe for at least some time. Additionally, since patients have had osteochondroma for many years and consider it harmless, peripheral chondrosarcoma is often very large and oddly shaped at the time of initial diagnosis.

bubble_chart Auxiliary Examination

X-ray findings The imaging is typical and generally not misdiagnosed, even allowing for diagnosis prior to histological confirmation.

The basic imaging shows increased density with radiopaque shadows caused by calcification and ossification within the tumor cartilage. Compared to central chondrosarcoma, peripheral chondrosarcoma exhibits more frequent and extensive calcification, with the increased density often occupying nearly the entire mass in many cases. Therefore, in peripheral chondrosarcoma, an extraosseous mass can be observed, appearing as a poorly defined lobulated structure with an uneven, cauliflower-like surface and very high density. In thicker areas of the tumor, instead of ivory-like high-density shadows, calcifications without a defined structure can be seen, typically presenting as nodular, punctate, and ring-shaped patterns (corresponding to cartilaginous lobules and peripheral formation). Calcification is less pronounced on the surface of the mass, resulting in blurred boundaries between the tumor and soft tissues.

In rare, very early-stage cases, peripheral chondrosarcoma may still appear as an osteochondroma on imaging. Its malignant transformation can be confirmed through CT, MRI, gross pathology, and histopathology, with bone scans showing strong positivity rather than the expected negative result in adults.

When peripheral chondrosarcoma is located in the trunk bones, traces of a pre-existing osteochondroma are rarely observed, possibly due to the osteochondroma being small or unrecognized.

bubble_chart Treatment Measures

Perform a wide resection. In grade I peripheral soft osteosarcoma or borderline tumors, due to their low recurrence rate and the fact that recurrent tumors can still be cured, marginal resection may be used in certain areas, often employing limb-sparing surgery. Intralesional resection is contraindicated, such as opening the pseudocapsule, tearing the pseudocapsule from the cartilage, or fragmenting the tumor.

In the early stages of peripheral soft osteosarcoma, when the tumor is not too large, resection can be performed at the base of the tumor on the host bone. However, in most cases, peripheral soft osteosarcoma is large in size, enveloping its origin on the host bone, necessitating segmental tumor resection. Segmental resection is suitable for peripheral soft osteosarcoma in the limbs, scapula, ribs, and most of the pelvis, but is rarely used for peripheral soft osteosarcoma in the spine.

Amputation is suitable for large peripheral soft osteosarcomas where other surgical options are not feasible, requiring a wide resection margin. Since peripheral soft osteosarcoma is often located in the limb girdle, interscapulothoracic or hindquarter amputation is commonly performed.

Radiotherapy or chemotherapy is ineffective and is not even used in palliative treatment.

In the rare cases of dedifferentiated peripheral soft osteosarcoma, surgery involves wide or radical resection margins, often in the form of amputation, and may be combined with chemotherapy.

bubble_chart Prognosis

The prognosis depends on the malignant grade of peripheral soft osteosarcoma and the possibility of extensive resection. Grade I peripheral soft osteosarcoma rarely metastasizes; Grade II peripheral soft osteosarcoma can metastasize, but it is uncommon within 5 years of symptom onset; Grade III peripheral soft osteosarcoma often metastasizes, but early occurrence is also rare.

The malignancy of peripheral soft osteosarcoma is lower than that of central soft osteosarcoma, not only because Grade I peripheral soft osteosarcoma is more common while Grade III is extremely rare, but also because, at the same histological grade, the malignancy of peripheral soft osteosarcoma is lower than that of central soft osteosarcoma.

Peripheral soft osteosarcoma located in the trunk (accounting for more than half of all cases), regardless of its malignancy, can lead to death. Peripheral soft osteosarcoma located in the ribs, pelvis, or scapula can be cured through surgical resection, while those in the spine or sacrum are rarely curable.

bubble_chart Differentiation

The diagnosis is generally straightforward and can even be made based solely on imaging findings. Sometimes, differentiation is required between a large, radiopaque osteochondroma and a peripheral chondrosarcoma. The radiopaque image of an osteochondroma is dense and irregular, with a clear boundary from the soft tissue. In contrast, the peripheral chondrosarcoma may have indistinct boundaries with the soft tissue in some areas. CT and MRI can help detect thickening of the cartilage cap in peripheral chondrosarcoma and differentiate it from fluid accumulation at the apex of an osteochondroma. Bone scans show weak positivity in adult osteochondromas but strong positivity in peripheral chondrosarcomas. Additionally, continued tumor growth in adulthood supports the diagnosis of peripheral chondrosarcoma, which can be monitored through serial X-rays.

The differential diagnosis between peripheral chondrosarcoma and parosteal osteosarcoma is relatively easy. Parosteal osteosarcoma may contain cartilaginous areas but has its own predilection sites, and its base on the host bone differs from that of peripheral chondrosarcoma. The radiopaque image of bone-like tissue in non-cartilaginous regions exhibits typical histopathology.

Peripheral chondrosarcoma also needs to be differentiated from synovial chondromatosis. The gross and histopathological features of synovial chondromatosis closely resemble grade I or even grade II peripheral chondrosarcoma. However, synovial chondromatosis originates from the joint cavity or tendon sheath, and its clinical and imaging characteristics differ from those of peripheral chondrosarcoma.

Finally, in special cases, peripheral chondrosarcoma must be distinguished from reactive or pseudotumorous osteochondral callus. Reactive cartilage exhibits features of active proliferation and hyperplastic cells.