Case Report: Breaking the diagnostic dilemma of "lymphoma-like" presentation: a case of adult-onset Still's disease characterized by generalized lymphadenopathy and extremely elevated ferritin.

Adult-onset Still's disease (AOSD) is a rare autoinflammatory disease, the clinical manifestations of which often overlap with other diseases such as lymphoma. This overlap makes differential diagnosis difficult, especially in patients with generalized lymphadenopathy, splenomegaly, and systemic metabolic hyperactivity. In this article, we discuss a 53-year-old East Asian (Chinese) female patient who had a fever, transient rash, generalized lymphadenopathy, and splenomegaly. An 18F-FDG PET/CT scan showed diffuse hypermetabolism in multiple lymph nodes, her spleen, and her bone marrow, and her serum ferritin level was elevated at 15,485 ng/mL, leading to strong suspicion of lymphoma. However, an excisional biopsy of the right inguinal lymph nodes with pathological, immunohistochemical, and T-cell receptor gene rearrangement analyses showed no evidence of clonality or lymphoma. Given the typical clinical presentation and laboratory findings, the patient was diagnosed with AOSD according to the Yamaguchi classification criteria. Glucocorticoid therapy rapidly normalized the patient's temperature, the rash disappeared, her lymph nodes decreased in size, and her ferritin and inflammation levels significantly decreased, suggesting a good therapeutic response. This case illustrates the value of lymph node biopsy in AOSD with "lymphoma-like" imaging features. Moreover, ferritin is a critical biomarker and is crucial for early identification of such difficult cases.