Fat necrosis might mimic metastasis on 18F-FDG PET/CT

Learning Objectives 1. To understand that there are many nonmalignant causes of increased FDG activity including some rare causes like fat necrosis. 2. To know that fat necrosis can mimic metastasis/recurrence on breast cancer patients. 3. To understand the pathophysiology of fat necrosis.

The clinical use of 18F-FDG PET/CT for staging and follow- up of a wide range of cancers has increased significantly recently. Even though FDG PET is a very sensitive technique for the recognition of malignant tumors, FDG is not tumor-specific. Well-known pitfalls in PET/CT with FDG imaging include infection, inflammation, physiological variants, and benign pathologic conditions. There have been very few reports, suggesting that fat necrosis can show focal FDG activity in PET/CT. Fat necrosis can be result of previous fine needle biopsy or excisional biopsy of trauma during surgery. We describe a case of false positive fat necrosis in FDG PET/CT scan in a patient with right breast multifocal ductal carcinoma s/p bilateral mastectomy, with history of chest wall radiation. Patient had 3 prior recurrences, last one requiring a radical resection of the anterior chest wall. At her last follow up PET/CT showed hypermetabolic soft tissue density nodules in the left anterior chest, which were worrisome for a recurrence of her breast cancer. CT-guided core needle biopsy was performed and showed fat necrosis with sclerosis, fibrosis, and macrophages. Conclusion: fat necrosis may falsely be interpreted as a local tumor recurrence or as metastatic breast cancer. We hypothesize inflammatory process causing fibrosis and sclerosis by macrophages adjacent to fat necrosis might correlate with increased FDG activity seen on fat necrosis. Hence, caution must be applied in interpretation of PET/CT in patients with multiple procedures and surgeries to exclude false positive results