Parosteal OSs are uncommon bone tumors and comprise only 4% of all OSs . While the radiological features of parosteal sarcoma of the long bones are well described, they may be difficult to detect on a simple chest radiograph. In the current case, a dense mass could be seen but it is imperative to perform a meticulous diagnostic workup as it can be confused with other benign and malignant lesions. A CT scan is one of the important diagnostic imaging methods as it can accurately delineate the extent of the tumor invasion and cortical integrity of the underlying bone. Magnetic resonance imaging (MRI) is another option to support a CT scan and give information about soft tissue, medullary involvement, and the differential diagnosis.
The differential diagnosis of parosteal OS from other bone-forming benign and malign neoplasms is yet another complexity . Osteochondroma is usually a pedunculated or rarely a sessile mass covered by a thin cartilage cap usually less than 2 cm. As osteochondroma and parosteal OS share some common clinical and morphologic features, differential diagnosis of parosteal OS from osteochondroma is usually the most challenging. Both of them arise from the cortex and have a similar location and age characteristics. The chondrocytes of parosteal OS usually show more pleomorphism and occasional binucleation and a lack of regular columnar arrangement and polarity . Myositis ossificans may be grossly similar to parosteal OS, radiographically, however, the ossification pattern of parosteal OS is generally centrally located whereas the ossification pattern of myositis ossificans is peripherally located . In addition, lesions of parosteal OS on MRI are more hypointense both in T1-weighted and T2-weighted images . A high-grade surface OS is another entity that may be confused with parosteal OS. A differential diagnosis is usually made by histopathological examination. Spindle cells with marked nuclear atypia and lacelike osteoid production, as seen in conventional OS, are usually evident in high-grade surface OS .
The medical history of the patient is also very important because fracture callus and ossifying hematoma can easily simulate bone tumors. So, clinical, radiologic, and pathologic assessments of the lesions are paramount evidence for the parosteal OS differential diagnosis.
Parosteal OS is a locally aggressive malignant tumor, and resection with a wide margin is the most appropriate treatment. In the current case, the posterolateral part of our patient’s ninth rib was resected en bloc with her eighth and tenth ribs. Her chest wall was then reconstructed to avoid future pulmonary compromise. In general, chest wall reconstruction is recommended for defects > 5 cm or when more than two ribs are resected to avoid flail chest. In the current case, the chest wall defect was > 10 cm and three ribs were resected with intercostal muscles and parietal pleura to achieve a clear margin. The chest wall reconstruction was applied with a collagen mesh and locked plate and no complications were observed during follow-up.