Secondary Osteosarcoma After Carbon‐Ion Radiotherapy for Desmoid‐Type Fibromatosis: A Case Report

ABSTRACT Background Radiotherapy is considered an alternative treatment for unresectable or pharmacologically resistant desmoid‐type fibromatosis. While it results in relatively good local control, the risk of secondary malignancy remains a concern. Case We present a case of secondary osteosarcoma after carbon‐ion radiation therapy (CIRT). A 31‐year‐old male patient presented with left thigh pain. The tumor was located between the left gluteus maximus and gluteus medius and extended to the vastus lateralis and biceps femoris. It was diagnosed as desmoid‐type fibromatosis after needle biopsy. The patient was treated with several medications, including a cyclooxygenase 2 inhibitor and tamoxifen; however, his left thigh pain did not improve. He was treated with CIRT 1 year after diagnosis (67.2 Gy [relative biological effectiveness] 16fr/4wks). He developed osteosarcoma of the left femur 8 years later. He underwent chemotherapy and tumor excision with disarticulation of the left hip. Pulmonary metastasis was detected 6 and 17 months after the definitive surgery and excised using metastasectomy. However, he died due to the recurrence of multiple pulmonary metastases 29 months after the definitive surgery. Conclusions In this case, we believe that the low radiation dose to the femur may have caused secondary malignancy.