Presumed Paravertebral Inflammatory Pseudotumor in Relapsing Granulomatosis With Polyangiitis: Fluorodeoxyglucose (FDG) Positron Emission Tomography/Computed Tomography (PET/CT) Findings and Complete Metabolic Response to Rituximab-Based Therapy

We report a case of a 60-year-old woman with biopsy-proven, proteinase-3 (PR3)-antineutrophil cytoplasmic antibody (ANCA)-associated granulomatosis with polyangiitis (GPA), diagnosed six years earlier, who presented with right lower thoracic pain during a disease relapse.The relapse was supported by serological reactivation with a markedly elevated anti-PR3 level, a raised erythrocyte sedimentation rate (ESR) (88 mm/h), and elevated C-reactive protein (CRP) (48 mg/L).Unenhanced computed tomography (CT) showed no pulmonary parenchymal abnormality but revealed soft-tissue infiltration of the anterior and right lateral paravertebral mediastinal fat extending from T4-T5 to T10-T11, without adjacent osseous involvement.Fluorine-18 ( 18 F)-fluorodeoxyglucose (FDG) positron emission tomography/CT (PET/CT) demonstrated intense uptake within the lesion, with a maximum standardized uptake value (SUVmax) of 12.06.Given the differential diagnoses of lymphoma and other granulomatous disease, the lesion was interpreted as a presumed GPA-related paravertebral inflammatory pseudotumor; it was not biopsied.After rituximab and methylprednisolone, the patient achieved complete clinical and biological remission with ANCA seroconversion, and follow-up FDG PET/CT at three months showed complete resolution of the abnormal uptake.Because neither FDG avidity nor treatment response was specific, the presumed diagnosis could not be confirmed histologically in this case.