To characterize the utility of [18F] fluorodeoxyglucose positron emission tomography (FDG-PET) in diagnosing and treating neurological IgG4-related disease (IgG4-RD).

IgG4-RD is an insidious, progressive immune-mediated fibrotic disease that forms tumor-like masses in affected organs. FDG‑PET can detect areas of increased metabolic activity from tumors and inflammation, but its role in neurological IgG4‑RD is unclear.

Twenty-seven patients (60% female, median age 55.4 [18-79]) had neurological IgG4-RD spanning dacryoadenitis (N=11), pachymeningitis (N=11), orbital pseudotumor (N=4), spinal pachymeningitis (N=1), hypophysitis (N=1), and intraparenchymal lesions (N=1). Two patients with pachymeningitis were counted twice because they also had dacryoadenitis or intraparenchymal lesions. Nine patients had a lumbar puncture. Pleocytosis (WBC range 9-53, lymphocyte predominant) was present in 3/9 patients, and elevated CSF protein (range 67-111) in 6/9 patients. Serum IgG4 levels were elevated in 4/11 patients with pachymeningitis and in 7/11 patients with dacryoadenitis. Nine patients had FDG-PET scans, of which 4 were negative. Sixteen patients had isolated neurological IgG4-RD by PET and/or CT chest/abdomen/pelvis (pachymeningitis/spinal/intraparenchymal lesions, N=8; dacryoadenitis, N=5; orbital pseudotumor, N=3). Two patients with negative CT imaging had systemic disease activity on PET. PET revealed FDG-avid axillary lymphadenopathy in one patient who had an axillary lymph node biopsy showing IgG4-RD. One patient with pachymeningitis had hypermetabolic dura that guided treatment escalation, while three others with pachymeningitis had negative FDG-PET imaging. Serial FDG-PET scans in one patient showed improvement in FDG-avid lesions following therapy.

FDG-PET can be a useful tool for assessing areas of active neurologic IgG4-RD with higher sensitivity than CT and for identifying optimal biopsy sites. Furthermore, FDG-PET can assess treatment response and help guide decisions regarding therapy escalation.