FDG‐PET in macrophage activation syndrome associated with systemic juvenile idiopathic arthritis

We herein describe a case of systemic juvenile idiopathic arthritis (s‐JIA)‐associated macrophage activation syndrome (MAS) in which the ^18Ffluorodeoxyglucose positron emission tomography (^18FFDG‐PET) findings were characteristic. The pattern of greater ^18FFDG accumulation into the spleen compared with the liver was more remarkable in this patient compared with s‐JIA. This pattern, however, was also observed in cases of acute leukemia. In the present patient, serum interleukin (IL)‐18 was extremely elevated (255 000 pg/mL), whereas in leukemia patients it is mildly elevated (360–1480 pg/mL). ^18FFDG‐PET might be a useful indicator of s‐JIA and MAS in patients with fever of unknown origin. The pattern of ^18FFDG accumulation, however, can also be observed in acute leukemia. The combination of ^18FFDG‐PET and serum IL‐18 might be useful for the diagnosis of s‐JIA and MAS.