Abstract
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Objectives: This exhibit will familiarize radiologists and nuclear medicine physicians with some FDG-avid benign cutaneous and subcutaneous conditions encountered in daily practice
Methods: We will discuss - 1. 18F-FDG Uptake in inflammation: basic pathophysiology 2. Cutaneous and subcutaneous lesion in systemic diseases 3. Infections 4. Autoimmune diseases involving the skin 5. Subcutaneous lesions 5. Iatrogenic causes of FDG-avid skin lesions 7. Benign tumors
Results: 1. Tumors show FDG uptake both in the tumor cells and in tumor-associated inflammatory cells. These inflammatory cells have been previously shown to have a higher level of FDG uptake than tumor cells. This can therefore facilitate the use of FDG-PET to image inflammatory processes, even without an underlying malignancy. Previous studies have shown that the level of FDG uptake correlates will with the density of inflammatory cells in both acute and chronic inflammatory processes, which in turn allows FDG-PET to track inflammatory disease activity. 2. The hallmark of a systemic disease which can also have cutaneous and subcutaneous manifestations is Sarcoidosis, with different lesions appearing in the acute and chronic stages of the disease. These may be FDG-avid, and detected on FDG-PET. 3. Although FDG-PET is not generally required to diagnose skin infection, these are often incidentally encountered on daily practice. As expected from their clinical appearance, faruncles and abscesses appear as localized foci of FDG uptake and therefore can be differentiated from widespread infections such as cellulitis. Another infection with typical imaging and clinical) appearance is herpes zoster, which shows dermatomal FDG uptake, easily appreciated in the coronal plane. 4. FDG-avid autoimmune processes such as pemphigus (both primary and para-neoplastic) should be part of the differential for any cutaneous lesion in the appropriate clinical setting, depending on patient's history. 5. Fat necrosis, which usually appears secondary to various local tissue insults, show high FDG uptake of PET. This can be usually easily diagnosed owing to typical morphological findings on CT and the clinical setting. A different subcutaneous lesion is panniculitis. It is a group of several pathological processes consisting of fat cell death with adjacent inflammatory cells. These explain for the FDG-avidity of this subcutaneous process. 6. Infected subcutaneous devices and post-operative wound infection are usually FDG avid. However, the most common cause of FDG uptake along a surgical wound is normal inflammatory reaction along healing wounds. 7. Benign tumors of cutaneous and subcutaneous tissues are generally uncommon, and most are not FDG-avid due to their low metabolic activity rate. Some exceptions do exists, such as dermatofibroma and hibernoma.
Conclusion: FDG uptake in the skin or subcutaneous tissues is commonly encountered on PET scans performed for other indications. Although often nonspecific, patient history, clinical setting and additional findings on imaging (CT, MR etc.) could help narrow the differential diagnosis. Research Support: None $$graphic_C1CFD2D7-06B6-4787-8CEC-9B5791E2B7A1$$