Parosteal osteosarcoma mimicking osteochondroma: A radio-histologic approach on two cases

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 [1] and in our database accounts approximately for 4, 8% of all osteosarcomas [2]. 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 [5]. Confusion may rarely occur in differentiating POS from the sessile variant of osteochondroma on imaging studies. Herein, we report two cases of POS wi