The patient’s clinical data of the patient with primary small cell osteosarcoma of coccyx with concurrent metastases were collected， and its clinical characteristics， imaging performance， diagnosis and treatment methods were analyzed through the review of relevant literatures.

A 10-year old girl admitted to hospital due to neck pain for two weeks and fever for one week， and a mass in lower right lung lobe found in lung CT examination. Body examination showed body temperature up to 37.5 ℃， no other discomfort. The laboratory results indicated alkaline phosphatase 221.3 U·L^－1， creatinine 38.1 μmol·L^－1.The ¹⁸F-FDG PET/CT results showed high metabolic mass in the lower lobe of the right lung， considering malignancy； the metabolism of the left side of the cervical 2 vertebrae was increased， maybe metastatic tumors； the tail bone density was not equal， a high metabolism soft tissue nodule protruding into the pelvis， considering malignant neoplasm.The MRI results showed that the axoid morphology was irregular， with long T1 and T2 signals， and the lesion was markedly enhanced； slightly longer T1 and slightly longer T2 signals lesions were found in sacral 5 level spinal canal， sacral 5 vertebral body and caudal vertebrae and around the capsule， and no obvious enhancement； anterior soft tissue around the sacral vertebrae anterior soft tissue was enhanced， space occupying lesion was considered. The postoperative pathology of the lower lobe of the right lung was considered metastatic small cell-type osteosarcoma for the lungs.Based on the above manifestations and pathological analysis， puncture biopsy of soft tissue lesions of the coccyx was performed， and the pathological result indicated small cell osteosarcoma. Finally the patient was diagnosed as primary small cell osteosarcoma of coccyx combined with lung and bone metastasis.

Primary tail bone small cell osteosarcoma is difficult to be diagnosed， but ¹⁸F-FDG PET/CT molecular imaging plays a key role in clarifying the lesion scope，tumor metabolic degree， and guiding the biopsy site.