Benign nature of incidental orbital uptake on bone scintigraphy: insights from multi- modality imaging and follow-up

Objective To characterize incidental orbital radionuclide uptake on bone scintigraphy using multi-modal imaging and follow-up. Methods This retrospective study included 66 patients with focal orbital uptake on whole-body bone scintigraphy (2019–2022). Subsets underwent follow-up bone scintigraphy (n = 27 patients, median interval 23.0 months), MRI (n = 31 patients, within 14 days of bone scintigraphy), and CT or fluorine-18 fluorodeoxyglucose (FDG) PET/CT (n = 21 patients, median interval from bone scan 8 months). Serial changes in the target-to-nontarget (T/NT) ratio on bone scans were analyzed, while MRI signal/ bone reporting and data system (Bone-RADS)classification, CT morphology/Bone-RADS classification, and metabolic activity (maximum standardized uptake value, SUVmax) were assessed. Results Seventy-six orbital foci were evaluated, predominantly localized to the superior/lateral walls (61/76, 80.3%) with round morphology (72/76, 94.7%). Extra-orbital findings included metastasis (5 patients), degenerative changes (34 patients), and benign lesions (17 patients). Follow-up bone scintigraphy showed non-significant T/NT ratio decrease (median 4.9 to 3.7; P = 0.301), with visual stability in 25/28 lesions (metastatic: 4/5 patients; non-metastatic: 21/23 patients). MRI detected abnormal orbital lesions in 16 patients (16 lesions; mean size 9.0 ± 2.4 mm), involving the frontal bone (n=10) and zygomatic bone (n=6); Bone-RADS categories: 12 category 2/3, 4 category 4. Follow-up MRI (11 lesions) showed stable size; two lesions had signal evolution. CT identified abnormal orbital findings in 20 patients (21 foci), including one osteolytic sphenoid lesion (cortical destruction, Bone-RADS 4, SUVmax 5.1) suggested metastasis. The remaining 20 foci demonstrated benign CT features: 18 Bone-RADS 1 lesions (ground-glass opacities: 16) and 2 Bone-RADS 2/3 lucent lesions. These lesions showed minimal FDG uptake (median SUVmax 1.4) on PET/CT. Conclusions: Incidental focal orbital uptake on bone scintigraphy typically localizes to superior/lateral walls with rounded morphology. Multi-modality imaging, particularly CT demonstrating characteristic benign features (predominantly ground-glass opacities) combined with low FDG avidity and longitudinal stability, confirms benign etiology in most cases. CT provides superior osseous characterization versus MRI. Individualized imaging surveillance is appropriate.