¹⁸F-FDG PET/CT findings of panniculitis and its feasibility for distinguishing active inflammation from stable conditions

Objectives: Panniculitis is a rare disorder characterized by chronic inflammation of the adipose tissue. Most present as an incidental finding in asymptomatic patients underwent modern cross-sectional imaging for other reasons. Since it’s often associated with malignancy, when the symptom appears, it’s hard to establish a precise diagnosis. The aim of this study is to analyze the ¹⁸F-FDG PET/CT findings of panniculitis, and its feasibility for distinguishing active inflammation from stable conditions.

Methods: From March 2012 and December 2015, a total of 14 consecutive patients (12 females and 2 males with ages raging from 46 to 82 years) with panniculitis founded in ¹⁸F-FDG PET/CT and confirmed by clinical or histopathological diagnosis were retrospectively reviewed. The ¹⁸F-FDG PET/CT scans were taken in the recommended standard condition with a dedicated PET/ CT scanner (Discovery VCT, GE, American). Three experienced nuclear medical physicians read the images, and diagnosis was made by consensus. PET/CT diagnosis of panniculitis was based on the observation of higher attenuation or FDG uptake than adjacent adipose tissue. Region of interest (ROI) was drawn from the abnormal adipose tissue, and the maximum standardized uptake value (SUV_max) represented metabolic activity of the ROI. SPSS 20.0 software was applied for statistical analyses. Categorical variables were compared using the Mann-Whitney U-test. P-values less than 0.05 were considered significant.

Results: In this group of patients with panniculitis, 9 /14 (64.3%) had a history of malignancy, including lung cancer (2), colorectal cancer (2), lymphoma (2), breast cancer (1), liver cancer (1) and thymic carcinoma (1). Neither of them showed any signs of recurrence nor metastasis. ¹⁸F-FDG PET/CT found panniculitis in mesentery of small intestine in 11 cases, and subcutaneous fat tissue in 3 cases. All 11 cases with mesentery of small intestine involvement demonstrated hyper attenuation of mesenteric fat tissue with well-defined nodules in it, besides, 8 cases showed “pseudo capsule sign” and 5 cases “halo sign”. Among the 11 cases, only one with intermittent fever showed high FDG uptake in the mesentery while the other ten without obvious clinical symptoms showed none or mild FDG uptake. In other 3 patients with subcutaneous fat tissue involvement, all had multiple lesions with CT performance of hyper attenuation of subcutaneous fat tissue, accompanied by strip or cloud-like patch in it, and PET performance of variable FDG uptakes. Among the 3 cases, 2 had clinical symptoms, including a long term of intermittent fever, multiple skin painful nodules and skin numbness. The other one had no obvious clinical symptoms. Lesion’s SUV_max of 3 symptomatic cases (1 involved mesentery of small intestine and 2 subcutaneous fat tissue) was significantly higher than those 11 asymptomatic cases ( 4.29±0.68 vs. 1.84±1.00 , p < 0.01) .

Conclusion: Panniculitis tend to be occurred in patients with malignancy. Most patients with positive findings on PET/CT are clinically asymptomatic. But when lesions show high uptake of FDG, it is usually related to an active inflammation with clinical symptoms. Therefore, ¹⁸F-FDG PET/CT is not only helpful in detecting panniculitis, but also feasible for identifying active state of the disease. Research Support: none