Post-radiation osteosarcoma

Clinical History

Middle-aged woman presented at the Emergency Department after minor trauma of the left leg, experiencing pain.
Her past medical history revealed a previous well-differentiated bone lymphoma which was treated with high dose radiotherapy in that leg 30 years before.
On the physical examination a cauliflower-like soft tissue mass was noted (Fig. 1).

Imaging Findings

Radiographs showed heterogeneously increased bone density and adjacent soft tissue calcifications (Fig.2). In comparison to previous radiographs (taken 9 years before), these findings were overall stable, except for a slight increase in soft tissue calcifications, and were considered radiotherapy-related changes (Fig.3). Additionally, a proximal tibial fracture was suspected and a CT examination was requested.
CT demonstrated similar findings, including a mixed lytic and sclerotic pattern and soft tissue calcifications (Fig.4), and confirmed the presence of a pathologic fracture. Also, ill-defined areas of cortical destruction and amorphous calcifications, less dense than the ones readily seen in the radiographs, were evident.
Since these findings were suspicious in this post-radiation background, MR imaging was performed, showing a destructive infiltrative lesion of the tibia, with an associated soft tissue mass extending to the skin surface. A skip lesion was found in the medial femoral condyle (Fig.5).
A soft tissue biopsy was performed, revealing infiltration by osteosarcoma.

Discussion

Post-radiation osteosarcoma is a type of secondary osteosarcoma that occurs in the tissues treated with radiation for some other disease process, with an evaluated risk of 0.01-0.03% of all irradiated patients [1]. Moreover, radiation induced osteosarcomas account for about 5.5% of all osteosarcomas [1].
The tumour occurs after a mean latency period of 14.5-15.5 years, with a range of 3-65 years after treatment [2, 3].
Radiation-induced osteosarcomas do not usually occur in heavily damaged areas of bone because these lack the ability to regenerate. Instead, they develop in areas where the dose has been sufficient to cause cell mutation, but not complete sterilization. For this reason, post-radiation osteosarcomas tend to occur in the periphery of the radiation field [3].
To some extent, the improved health care quality has likely contributed to the increase in the life expectancy of these patients, allowing them to live longer and experience the oncogenic effects of radiation therapy.
Malignant osteoid formation with dense sclerosis is a feature of radiation-induced osteosarcoma. Co-existing radiation bone changes are seen in up to 50% of patients. The cardinal features of cortical destruction and soft tissue extension are optimally demonstrated with MR imaging, but CT can also be used [4, 5].
Imaging features of bone healing and remodelling during the process of radiation osteonecrosis are dynamic, and it can be difficult to rule out malignancy. Relative lack of change on serial radiographs favours radiation changes, whereas pain, presence of a soft tissue mass, increasing lytic lesions and active enhancement on imaging favour the diagnosis of recurrent tumour or radiation-associated sarcoma.
Treatment consists of surgery and chemotherapy [6]. Surgical options include wide resection with limb salvage, or wide resection with amputation. The latter option was the one performed in this patient (Fig.6).

Differential Diagnosis List

Final Diagnosis

Radiation induced osteosarcoma

URL:	https://www.eurorad.org/case/11555
DOI:	10.1594/EURORAD/CASE.11555
ISSN:	1563-4086