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[Dedifferentiated diaphyseal juxtacortical osteosarcoma of ulna with metastasis to the lung].

Dedifferentiated parosteal osteosarcoma is a variant in which a high grade osteosarcoma coexists with a parosteal osteosarcoma. We report the case of a 20-year-old female patient who presented with six months of evolution of pain and functional limitation of the right forearm, with no apparent cause; radiographs were performed, observing a tumoral lesion in the diaphysis of the right ulna. Physical examination showed pain upon palpation in the diaphysis of the ulna and limitation of prone-supination. Axial computed tomography of the thorax revealed metastatic disease in the upper lobe of the left lung. An incisional biopsy was performed on the right ulna, with a report of dedifferentiated parosteal osteosarcoma. Therefore, the patient was managed with neoadjuvant chemotherapy with cisplatin and doxorubicin until completing three cycles. Surgical treatment consisted of intercalary resection of the diaphysis of the right ulna, plus reconstruction of the microvascularized autologous graft of the right fibular diaphysis and graft stabilization with 3.5 mm dynamic compression plate (DCP) and one-third tubular plate. In the same procedure, pulmonary metastasectomy was performed by thoracoscopy. Post-surgical histopathological report with 100% necrosis. Currently, the patient is asymptomatic, with no evidence of tumor activity. Dedifferentiated parosteal osteosarcoma is a rare pathology, but should be suspected as a differential diagnosis in the presence of a parosteal osteosarcoma; it should be taken into account that this disease can metastasize due to its dedifferentiated pattern. It is important to plan a surgical treatment that allows an adequate functional reconstruction, always taking into account the oncological principle.