Orbital granulomatosis with polyangiitis: MRI discriminators from IgG4-related disease and lymphoma.

OBJECTIVES: To evaluate the distinguishing MRI features of orbital granulomatosis with polyangiitis (GPA) from IgG4-related orbital disease (IgG4-ROD) and lymphoma. METHODS: This retrospective study included 53 patients (10 GPA, 21 IgG4-ROD, 22 lymphoma) who underwent MRI at disease onset. Two blinded radiologists independently evaluated morphological characteristics including; laterality, location, margins, T2 signal, contrast enhancement, necrosis, and extraorbital findings. Quantitative analysis involved ADC measurements and cortex-normalized ADC ratios. Statistical analysis comprised analysis of variance with post-hoc testing, receiver operating characteristic analysis, and intraclass correlation coefficients (ICC). RESULTS: Bilaterality was more common in GPA than other cohorts (p = 0.038). GPA was significantly differentiated from lymphoma by a higher frequency of ill-defined margins and extraconal fat infiltration (p = 0.001 and p = 0.004, respectively), as well as by marked T2 hypointensity (p < 0.001). Contrast enhancement further distinguished the cohorts (p = 0.017), with hyperintense enhancement (relative to muscle) restricted to the inflammatory cohorts. Necrosis was exclusive to GPA (20%, p = 0.033). Extraorbital findings including sinonasal neoosteogenesis were significantly more frequent in GPA than other cohorts (p = 0.001). Both GPA and IgG4-ROD showed significantly higher mean ADC values/ratios than lymphoma (p < 0.001), with excellent ICC (>0.95). ADC measurements did not differ significantly between GPA and IgG4-ROD. CONCLUSION: MRI reliably discriminates orbital GPA from lymphoma through multiple descriptive features and ADC measurements. Although IgG4-ROD shares overlapping imaging characteristics with GPA, necrosis and extraorbital findings serve as highly suggestive differentiating markers. ADVANCES IN KNOWLEDGE: This study offers a detailed MRI description of orbital GPA and its first comparative analysis with IgG4-ROD and lymphoma.