A 61-year-old female who had bilateral cystic lesions in her mandible was diagnosed ten months ago, at a local hospital. She complained that she had felt no pain or numbness in her teeth and mandible. Also, she claimed that she had suffered from no tumours or systemic diseases before. Afterwards, a panoramic X-ray was taken, which revealed some radiolucent lesions that were located bilaterally in her posterior mandible. The result showed that the cystic expansion was surrounded by well-defined cortical margins Each cystic lesion contained an un-erupted mandibular third molar. There was no evidence of root-restoration in the adjacent molar teeth .According to the first panoramic X-ray examination, the presumptive result of clinical diagnosis was a dentigerous cyst. Then, the patient was initially treated with bilateral curettage in the local hospital. Meanwhile, an incisional biopsy was taken during the first surgery. The histology of the first biopsy showed a piece of cyst-like tissue, which was lined by non-keratinizing, squamous epithelium [Table/ Fig-1b]. But the stromal necrosis was apparent. The histologic diagnosis of the first biopsy was a cystic lesion which was lined by non-keratinizing, squamous epithelium.Two months after the curettage, the patient was referred to Peking University School and Hospital of Stomatology, after she complained of a mass on the right inferior gingiva that bled painlessly. Another panoramic X-ray was taken, which demonstrated a well-healed left lesion while the right lesion had become a mixed, radiolucentradio opaque mass with a diffuse and irregular borderline [Table/ Fig-2]. Therefore, a second incisional biopsy was performed. The second biopsy specimen was a piece of bone-like hard tissue. Histopathology showed many atypical cells which were embedded in poorly calcified region and the nuclear fission had increased. This time, a histologic diagnosis of a low-differentiated sarcoma was made preoperatively. In order to investigate the original site of the sarcoma, further Positron Emission Computed Tomography (PET/ CT) and Emission Computed Tomography (ECT) were performed. PET/CT demonstrated an enhancing red mass, both in the right mandible and in the left pelvis. The left pelvis had multiple abnormal masses in the left ilium, pubis, ischial and acetabular regions . ECT showed that an imaging agent was deposited in the right mandible and in the left pelvis. Both the PET/CT and the ECT indicated that the patient had a malignant tumour in her left pelvis. After the general physical checkup, the patient recalled that she had been suffering from hip pain since past two years. This "indisposition" had been ignored by her earlier, as she had been apprehensive that it had been post-menopausal osteoporosis. However, it could have been reasonably attributable to the tumour which had been identified. The radical operation of hemi-dilectomy was performed [Table/ Fig-4a,4b]. The specimen showed that the tumour had originated from the bone. It was invasive, pale and expanded to the surfa...