15-year-old boy, pain and swelling in the knee area. An X-ray shows osteolytic patches in the distal femur metaphyses with periosteal reaction.
Open biopsy. (refer to specimen S IX 9b: femur resection after chemotherapy).
- Neoplastic cell population with signs of malignancy: cell and nuclear polymorphism, atypical mitoses.
- Direct formation of osteoid or primitive bone substance by neoplastic cells (sine qua non of diagnosis)
Evidence of osteoid formation is crucial for diagnosis, even if another intercellular substance such as cartilage, chondroosetoid or collagen is evident. Histologically, osteoblastic, chondroblastic and fibroblastic osteosarcoma can be distinguished (they can occur in different areas of one tumour).