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 18F fluorodeoxyglucose positron emission tomography (18F FDG-PET) findings were characteristic. The pattern of greater 18F FDG 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). 18F FDG-PET might be a useful indicator of s-JIA and MAS in patients with fever of unknown origin. The pattern of 18F FDG accumulation, however, can also be observed in acute leukemia. The combination of 18F FDG-PET and serum IL-18 might be useful for the diagnosis of s-JIA and MAS.