Case 1: 15-year-old female patient. (A, B) An incisional biopsy was performed from the lateral side of the distal femur, and a conventional osteosarcoma in the distal part of the left femur was diagnosed. No distant metastasis was found at the time of the initial diagnosis. Preoperative chemotherapy (methotrexate + doxorubicin + cisplatin regimen) was administered. A preoperative computed tomography scan of the chest revealed multiple lung metastases. (C–F) Wide resection including the skin around the biopsy site, reconstruction with a megaprosthesis, and sartorius muscle flap coverage were performed. After the surgery, the lung metastases were inoperable, and ifosfamide + etoposide, pazopanib, and gemcitabine + docetaxel were administered. However, the lung metastasis could not be controlled, and the patient died 1 year 7 months after the diagnosis.