Periosteal (also called parosteal or juxtacortical) chondrosarcoma (periosteal CHS) is a malignant cartilaginous neoplasm of the outer cortical surface of long bones. It is rare and it has a predilection for the male sex. Its incidence has two peaks, one in the 2 nd and the other in the 4 th decade and it is exceptional before puberty.
It is usually located in the meta-diaphyseal areas around the knee, but also in the proximal humerus. When located in the posterior aspect of the distal femur, it must be differentiated from parosteal osteosarcoma, as will be discussed later.
Clinically , it has a rather slow (14m on average) asymptomatic course, with little or no pain and/ or swelling.
It presents as a rounded lucency sitting on the outer cortex and usually it contains granular or ring opacities due to calcification (“pop-corn” pattern), depending on the age of the lesion. The underlying cortex appears thickened, sclerotic, with a sharply defined border and occasionally, periosteal “spurring” at the periphery. Typically the medullary canal is uninvolved, as can be demonstrated by CT. MRI is only useful if no calcifications are present.
Histology reveals lobular well differentiated cartilage, usually grade 1 or 2. In fact, the grade 1 features are in the spectrum of periosteal chondroma. Therefore, to differentiate periosteal CHS from periosteal chondroma, one should take into account the following: Periosteal CHS has a larger size (> 5 cm.), it occurs after puberty, it usually shows signs of grade 2-3 malignancy and the cortical erosion is more prominent, eventually with permeation and medullary canal involvement. As a result, grade 1 periosteal CHS should be considered only in an adult patient with clear permeation of the cortex.
It is not clear whether periosteal CHS can originate from a periosteal chondroma.
Prognosis and Treatment
Local recurrences are rare if wide resection is performed and lung metastases are very rare. No chemotherapy is required.
Differential diagnosis alsoincludes periosteal osteosarcoma, high-grade surface osteosarcoma and parosteal osteosarcoma.
Periosteal osteosarcoma is less malignant than classical osteosarcoma, but definitively more aggressive than periosteal CHS. Contrary to periosteal CHS, it can occur also in children before puberty and usually has a shorter (6m on average) clinical course with moderate pain and palpable mass. The radiography shows a lucent diaphyseal mass on the outer cortical surface. The periosteum is elevated and fusiform, and the lesion contains spiculated radiodensities due to ossification (“sunburst” pattern), which are perpendicular to the cortex. The cortex is not as thickened and sclerotic as in periosteal CHS, Codman’s triangles may be seen and the medullary canal is typically uninvolved. On histology, periosteal osteosarcoma is a chondroblastic tumor with some osteoid among malignant osteoblasts and the grade is 2-3. Local recurrences are rare if wide resection is performed and lung metastases are reported in 15% of cases.
Occasionally, high-grade (3 or 4) surface osteosarcoma and parosteal osteosarcoma may enter the differential diagnosis. However, high-grade OgS of the bone surface is very rare, not mainly chondroblastic, and it is an aggressive malignancy with histological and prognostic features as those of classical osteosarcoma, requiring chemotherapy. Finally, parosteal osteosarcoma is a metaphyseal sclerotic lesion, consisting of spindle cells grade 1-2, therefore diagnosis is more straightforward.
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