Parosteal osteosarcoma mimicking osteochondroma: A radio-histologic approach on two cases
© Papathanassiou et al; licensee BioMed Central Ltd. 2011
Received: 12 January 2011
Accepted: 25 July 2011
Published: 25 July 2011
Parosteal osteosarcoma is a well-differentiated variant of osteosarcoma that affects the surface of the bone. The imaging pattern is very typical. We report two cases mimicking an osteochondroma, radiologically and histologically and propose an explanation.
The review of 86 parosteal osteosarcomas of bone revealed this atypical pattern only once. A consultation case was received in the same time, and added to ours. Patients were 28 years old and 56 years old females. Imaging studies included two radiographs, two CTscans, one MRI examination and one bone scan and the results were compared to histology.
On imaging, both lesions presented as ossified lobulated masses attached with a broad base to the underlying cortex. No radiolucent cleft separated the masses and the host bone and cortex continuity between the mass and the femur was seen, with medullary communication. The marrow of the mass had a different density and intensity compared to normal marrow. So, there were features of an osteochondroma (cortex and medullary continuity) and of a parosteal osteosarcoma (ossified marrow). Pathological assessment on the final specimen confirmed the presence of low-grade parosteal osteosarcomas, after an erroneous diagnosis of osteochondroma on the initial biopsy.
Parosteal osteosarcoma can be rarely confused with osteochondroma. A radiologic-pathologic correlation is essential. Cortex continuity is the most misleading imaging feature that may occur in parosteal osteosarcomas. A knowledge of this misleading pattern will help diagnose the lesion from the beginning.
Parosteal osteosarcoma (POS) is a slow-growing tumor which originates from the outer layer of the periosteum and represents 65% of surface osteosarcomas  and in our database accounts approximately for 4, 8% of all osteosarcomas . Unlike conventional osteosarcomas, it involves an older age group typically in the 3rd and 4th decades of life and shows a slight female predilection [1, 3, 4]. The most common location of a POS is the posterior aspect of the distal femur accounting for approximately two thirds of all cases . Confusion may rarely occur in differentiating POS from the sessile variant of osteochondroma on imaging studies. Herein, we report two cases of POS with sessile configurations arising from the posterior distal femur that simulated osteochondromas radiologically and histologically on the initial biopsy and emphasize on the imaging features that may lead to an erroneous diagnosis.
Materials and methods
The study included two female patients aged 56 and 24 years old respectively. Both patients referred to our institution for evaluation of a palpable mass located at the postero-lateral aspect of the distal femur. Information regarding clinical history, physical examination and laboratory tests was recorded. Retrospective evaluation of the available imaging studies was carried out. Both cases were assessed with radiographs (antero-posterior and lateral views) and CT scans before and after contrast medium i.v. infusion. Additionally, one patient underwent a bone scan and an MRI study which included T1WSE before and after gadolinium administration and T2W FAT SAT sequences. Imaging results were compared with the initial biopsy findings and final histopathologic diagnosis.
Both patients presented with focal swelling and mechanical blockage of the left knee. The duration of symptoms was 24 and 36 months for each case with no related trauma or infection during this period. Physical examination revealed two hard, immobile tender masses at the postero-lateral aspect of the distal left femur associated with flexion restriction of the joint at 90° degrees. There was no vascular or neurological deficit of the affected limp. The patients did not notice any significant enlargement of the swelling during the period of the symptoms. Routine blood tests were not significant for any pathological findings.
POS is a rare bone-forming malignant surface tumor which carries a much better prognosis than central, high-grade osteosarcoma (approximately 90% of patients after 5 years) . It constitutes about 1, 7% of all benign and malignant neoplasms of bone . Originally it was described as "benign and malignant parosteal osteoma" by Geschickter and Copeland in 1951 . Clinically it presents as a hard, immobile swelling with no or slight pain and frequently associated with loss of motion range of the neighboring joint [2, 8]. The symptoms are often of prolonged duration . Biologically, the tumor is slow-growing. Pulmonary metastases occur late in the course of the disease, usually following one or more local relapses. Therefore, surgical intervention focuses on the local control of the tumor . Early recognition relies upon clinical suspicion and precise radiologic and pathologic evaluation. Apart from the posterior aspect of the distal femur, the tumor secondarily affects the proximal humerus followed by the proximal tibia . Diaphyseal involvement is seen in up to 10% of cases . In reference with the classic characteristics of the tumor, both masses were located at the postero-lateral portion of the distal femur and appeared hard, immobile and tender with associated limitation of the knee flexion on physical examination.
Macroscopically, POS presents as a dense and well-defined ossified mass attached to the underlying cortex . On histologic grounds, the tumor mainly consists of hypocellular fibrous stroma with minimal atypia of spindle cells and extensive osteoid in the form of well-demarcated bony trabeculae, although smaller foci of cartilage are also encountered [1, 2]. A cartilaginous component is observed in more than 50% of all POSs and in approximately 25% of cases this component lies at the periphery of the tumor. Pathologists and surgeons must recognize this cartilaginous component so as not to confuse POS with osteochondroma .
The differential diagnosis of POS may also include other diverse entities such as mature juxtacortical myositis ossificans, parosteal osteoma, fracture callus, Nora's lesion, periosteal osteosarcoma and/or chondrosarcoma that can be easily distinguished [4, 5, 11]. In the study of Lin J et al , the term "osteochondromalike parosteal osteosarcomas" was used to describe six surface bone-forming malignancies characterized by cortical continuity and no evidence of medullary communication or invasion as shown in our cases.
In conclusion, osteochondroma is the main counterpart of POS in terms of imaging differential diagnosis. Cortex continuity and in a lesser degree merge of medullary cavities, which are by definition expected in osteochondromas, may be atypically seen in POS. Awareness of these features and thorough radiologic-pathologic correlation is crucial in avoiding erroneous diagnostic considerations and treatment.
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