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) [2]. It constitutes about 1, 7% of all benign and malignant neoplasms of bone [6]. Originally it was described as "benign and malignant parosteal osteoma" by Geschickter and Copeland in 1951 [7]. 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 [4]. 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 [8]. 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 [9]. Diaphyseal involvement is seen in up to 10% of cases [8]. 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 [2]. 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 [4].
Radiographically, POS presents as a densely mineralized lobulated mass with irregular margins and attached to the subjacent cortex [2, 9]. A characteristic finding is a linear radiolucent zone, separating the lesion from the host bone, except for the site of attachment, called the cleavage plane which represents the uncalcified thickened periosteum. However, this radiolucent cleft may be obliterated with advancing tumoral growth [3, 9]. CT scans define accurately the extent of the tumor and cortical integrity [8]. MRI images vary in relation to tumors size as well as the presence of dense osteoid, cartilage, hemorrhage, necrosis or areas of high grade tumor or dedifferentiation. MRI is optimal for exhibiting the appropriate biopsy site and potential medullary invasion prior intervention [8]. Cortex continuity with some peripheral erosions is a useful diagnosis key. Conversely, cortex and medullary continuity are diagnostic features of osteochondroma. Perhaps the different appearances of the medullary cavities of the lesions compared to the host bone should have raised suspicion. Additionally a high grade osteosarcoma developed inside an osteochondroma is very rare, but possible and has a very similar pattern [10]. A peripheral cartilaginous cap may be visible on both parosteal osteosarcoma and osteochondroma. A biopsy made at the periphery of the lesion may reinforce the diagnosis of osteochondroma, as in one of our cases. The final correct diagnosis was established after resection, leading to inadequate margins. The disappearance of the cleavage plane and the preservation of a continuous femoral cortex encircling the tumors are probably attributed to the gradual destruction of the cortex by the slowly growing tumors (Figure 13, 14, 15, & 16).
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 [11], 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.