Dedifferentiated high-grade osteosarcoma originating from low-grade central osteosarcoma of the fibula.

Low grade central osteosarcoma is a distinct, rare low grade malignant neoplasm characterized histologically by a spindle cell proliferation associated with trabecular bone formation. This tumor usually carries a good prognosis. However, it has the potential to recur, dedifferentiate, and metastasize subsequent to surgical treatment. In extremely rare occasions, LGCOS presents with areas of dedifferentiation upon initial patient evaluation. In this report we describe one such case affecting the fibula of a 39-year-old female, presenting with pain. Initial biopsy demonstrated a high-grade osteosarcoma. No areas of LGCOS were seen histologically. Following biopsy the patient received 3 months of chemotherapy at a different institution. However, the patient continued to exhibit symptoms. X-rays indicated a very aggressive tumor in the diaphysis of the proximal fibula, MRI revealed soft tissue involvement. The tumor and surrounding soft-tissues were excised en bloc at our institution. Microscopically, the lesion consisted of high-grade osteosarcoma as well as an underlying LGCOS that involved the cortex and medullary cavity of the fibula. The two histological components of the tumor were sharply delineated from one another. No chemotherapy effect was appreciated histologically or clinically. Eight months following the surgery, the patient died from lung metastases. This is the third documented case of dedifferentiated LGCOS at initial presentation. This case reveals that the low-grade component was also evident on preoperative radiographs.