False-positive 18F-fluorodeoxyglucose positron emission tomography/computed tomography caused by incidental injury in a bulky intracystic carcinoma of the breast

We present a case of right intracystic carcinoma of the breast. An 83-year-old woman was seen at our hospital with a growing bulky mass in the right breast for the last 2 years. Whole-body fluorine-18 fluorodeoxyglucose positron emission tomography and computed tomography (18F-FDG-PET/CT) was performed 15 days after mammography (MMG). 18F-FDGPET/CT showed a right bulky cystic mass and intense FDG uptake in the intracystic nodules as well as in the adjacent extracystic lesion and the underlying pectoralis major muscle. It suggested tumor invasion of the pectoralis major muscle. Right mastectomy combined with partial resection of adherent pectoral muscle was performed. The main cystic lesion was diagnosed to be solid-papillary carcinoma, but the area of the pectoralis muscle was diagnosed to be a “foreign body” inflammation. It was thought that the compression related to MMG and core needle biopsy induced an injury in the bulky breast mass with leakage of contents into adjacent tissue, resulting in the foreign body inflammation. In conclusion, the coexisting foreign body inflammation was responsible for a false-positive 18F-FDG-PET/CT scan in the current case.