Central low-grade osteosarcoma with pagetoid bone formation: a potential diagnostic pitfall.

Central low-grade osteosarcoma is an uncommon form of osteosarcoma, which is often difficult to distinguish from benign bone lesions. We reviewed the radiographic studies, the histologic material and the clinical records of two patients with central low-grade osteosarcoma that closely simulated the histologic appearance of Paget's disease of bone. The patients were two women aged 46 and 53 years. Radiologically, they presented a large ill-defined densely sclerotic lesion involving the proximal tibia. Both lesions only focally presented the conventional histologic appearance of central low-grade osteosarcoma, with a proliferation of fibroblast-like cells embedded in a dense collagenous stroma and irregular anastomosing tumor bone trabeculae. The most striking feature was the presence of extremely thickened irregular plates of bone with an irregular mosaic pattern of cement lines that closely resembled that of Paget's disease of bone. One patient, who had been initially treated for Paget's disease for 7 years, experienced disease progression. At resection of proximal tibia, there was evidence of dedifferentiation to high-grade osteosarcoma. After 2 months, she developed local recurrence that was treated with above-knee amputation, followed by chemotherapy. She died with multiple lung metastases 4 months later. The other patient is alive 9 months after wide tumor resection. These cases further expand the spectrum of central low-grade osteosarcoma, and underscore the diagnostic difficulties in separating central low-grade osteosarcoma from benign bone diseases, which may lead to delay in diagnosis, inadequate treatment, and eventually to dedifferentiation. Recognition of this variant of central low-grade osteosarcoma is based on the aggressive radiologic appearance and on adequate tumor sampling for histologic examination.