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Treatment and prognosis
Conservative surgery with wide margins is usually effective. The prognosis is very good, with local
[20]
recurrence of 13%-28% and metastatic rates of 512.2% . Metastasis primarily involve the lungs and, rarely,
[28]
the lymph nodes . In the Rizzoli case series of periosteal chondrosarcomas, no patient died of tumor [1,21] .
OSTEOGENIC TUMORS
Parosteal osteosarcoma
Definition and clinical features
Parosteal osteosarcoma is a low-grade osteosarcoma with low-metastatic potential that arises on the cortical
[1]
surface of bone. It constitutes 4.3% of all osteosarcomas and, although infrequent, is the most common
osteosarcoma of the bone surface. Its peak of incidence is in young adults between 20 and 40 years old
[29]
(mean age of 31 years) with a slight female predominance . It almost exclusively affects the metaphysis
of long bones of the extremities, with a marked predilection for the distal metaphysis of the femur (60%
cases). Only exceptionally it occurs in flat bones, small bones of hands and feet and vertebrae. Parosteal
osteosarcoma most commonly debuts as a long standing, painless and indurated mass.

Tumoral progression into a high-grade dedifferentiated parosteal osteosarcoma is seen in a percentage
of cases and can occur either at presentation (primary/synchronous type) or at the time of recurrence
(metachronous type). In primary dedifferentiated parosteal osteosarcomas the clinical history is sometimes
more abrupt.

Imaging
Due to its indolent growth, parosteal osteosarcoma is usually discovered when it has achieved large
dimensions. It is a circumferential or mushroom shaped mass of osseous radiodensity fused to the cortex
[Figure 3]. Radiodensity is maximal near the implant base, while the outer margin tends to be blurred.
Detailed radiographies show a mesh of trabecula (“steel-wool” pattern), ranging from ground glass to ivory
[1]
density . Areas of radiolucency on plain films or CT scans and hyperintense on fluid-sensitive MRI can be
[30]
present in approximately 60% of dedifferentiated parostal osteosarcomas .
The combination of MRI and CT allows for high accuracy in the determination of the extent of
intramedullary infiltration of the tumor, thus assisting in the planning of conservative surgeries that avoid
[31]
surgical morbidity .

Macroscopic features
Grossly, parosteal osteosarcoma presents as a firm, exophytic and lobulated mass with a broad base
attachment to the cortex of the host bone. It can grow either longitudinally along the long axis of the
bone or encircle it [Figure 3]. Elevation of periosteum and reactive periosteal bone formation are typically
absent. The cut surface is whitish, hard, or gritty and may contain areas of cartilage. When dedifferentiation
is present, its occurrence is characterized by a tan and fleshy component not infrequently accompanied by
[2]
areas of hemorrhage and fluid cavities that may correlate well with the lucent areas in imaging studies .
Microscopic features
It is composed of long fascicles of deceptively bland spindle cells embedded in a collagenous stroma and
intervening, well-formed bone trabecula. The fibrous fascicles are hypocellular and the spindle cells exhibit
minimal nuclear atypia. Mitotic figures are inconspicuous. The bone trabecula are typically arranged in
parallel streamers but can also be more curvilinear and arranged in an anastomosing fashion [Figure 3].
Osteoblastic riming is inconsistently present.