Secondary osteosarcoma can be associated with various congenital syndromes, as well as local or systemic inflammatory abnormalities. Areas where the bone is altered from prior trauma or ischemia, as well as from underlying neoplastic or growth anomaly, can also predispose the bone to the development of osteosarcoma (Table 1) [1-3]. As many of these lesions are most common in older age groups, the peak incidence for secondary osteosarcoma is late adulthood.
The most common predisposing factor to secondary osteosarcoma is Paget's disease. The frequency, however, for malignant transformation of Paget's to osteosarcoma is low, less than 1% [4].
As Pagetoid changes are most frequently seen in the pelvis, so too are the secondary osteosarcomas which typically show an ill-defined aggressive lytic lesion with soft tissue mass (
Figure 1a, 1b-c,
Figure 2). Clinically, pain is often a common presenting symptom. Approximately 25% of these patients will develop a pathologic fracture. Areas of increased density and spiculated periosteal reactions are reported but are less frequent than in conventional intramedullary osteosarcoma. We typically perform MR imaging to assess tumor size and relation to surrounding soft tissue structures.
The second most common predisposing factor to secondary osteosarcoma is radiation therapy. Radiation-induced osteosarcoma has a latency period of approximately 5 to 13 years, although shorter latency periods have been reported. These tumors are dose related, usually being detected in patients receiving 100 Gy; however, tumors have been reported in patients receiving 18 Gy [5]. Common sites of involvement are the scapula and humerus (Figure 3a, 3b-c), as well as pelvis, reflecting the high number of patients who undergo breast irradiation for treatment of breast cancer, as well as those receiving pelvic radiation for cervical carcinoma. However, any bone exposed to radiation is at risk. Although malignant fibrous histiocytoma is the most common soft tissue sarcoma associated with radiation therapy, osteosarcoma is the most common bone sarcoma related to radiation [6]. Clinically, patients usually present with new onset of pain or swelling. Children are more susceptible to tumor formation following radiation exposure. These lesions are radiographically similar to other secondary osteosarcomas in which an aggressive lytic lesion with soft tissue extension can be seen. However these lesions can be mixed lytic and blastic in appearance, as well as completely blastic. MR imaging is helpful in differentiating radiation osteitis, which is low in signal intensity on both T1 and T2 weighted images compared to osteosarcoma, which typically has increased signal intensity on T2 weighted images.
Although osteochondroma is usually considered to be a neoplasm, there is some debate that this is an abnormality of growth affecting both the cortex and medullary portions continuous with underlying bone [1]. This lesion is most commonly found in the knee and arises in the metaphysis and grows away from the epiphysis.
A hyaline cartilaginous cap that involutes after growth can undergo malignant degeneration, resulting in lesions such as osteosarcoma. The cartilage cap thickness is related to malignant degeneration. Although the cap is usually less than 1 cm thick, but can be thicker in skeletally immature patients. An area of cartilage cap thickness greater than 1 cm in a skeletally mature patient should be suspect for malignant degeneration [7]. Additionally, new onset of pain, bone destruction or growth should be considered as potential indicators of malignant change as well.
Although routine radiography will show bone destruction, osteoid matrix and possibly periosteal reaction, only MR imaging allows for a sensitive measurement of the hyaline cartilage cap thickness [6].
