[67Ga]Ga-citrate SPECT/CT and [18F]FDG-PET/CT for evaluating therapeutic response in necrotizing otitis externa

Background
Necrotizing otitis externa (NOE) is a rare but severe infection of the external ear canal that can progress to osteomyelitis of the temporal bone, often caused by Pseudomonas aeruginosa. Effective management requires prolonged systemic antibiotic therapy, as premature termination increases the risk of relapse. There is ongoing debate regarding the optimal duration of antibiotic treatment and the criteria for discontinuation. Functional imaging techniques, such as [67Ga]Gallium citrate SPECT/CT and [18F]FDG-PET/CT, have shown promise in assessing disease activity, but their roles in therapy evaluation remain underexplored. In this study, we describe the radiological disease activity during treatment and we relate activity at cessation of treatment to disease control in patients scanned with [67 Ga]Ga-citrate SPECT/CT and/or [18F]FDG-PET/CT. Secondly, inter-rater variability is assessed.

Results
Twenty patients with NOE, with a median duration of intravenous treatment of six months (range 1–24 months) were included. Relapse occurred in 3 patients. Qualitative assessment of 95 scans showed slight inter-rater agreement for [67Ga]Ga-citrate SPECT/CT (kappa = 0.167) and fair agreement for [18F]FDG-PET/CT (kappa = 0.400). Relapse was seen in patients both with and without complete resolution on imaging. In the semi-quantitative assessment, [67Ga]Ga-citrate SPECT/CT showed a decrease in countsmax ratio (1.93 to 1.35), but an increase in countsmean ratio (1.56 to 1.73) between baseline and end of treatment. [18F]FDG-PET/CT showed a 50% reduction in both maximal standardized uptake value, corrected for lean body mass (SULmax) and peak standardized uptake value, corrected for lean body mass (SULpeak) ratios and also a decrease in metabolic tumor volume (MTV) in most cases.

Conclusions
Semi-quantitative measurements on [67Ga]Ga-citrate SPECT/CTs and [18F]FDG-PET/CTs demonstrated notable variation in NOE. However, our findings suggest that MTV might be a useful parameter in quantifying the [18F]FDG-PET/CT result during the course of disease. Although [18F]FDG-PET/CT with visual evaluation, potentially alongside MTV, appears to be the most effective, the inter-observer variability emphasizes the necessity for standardized protocols to improve consistency and reliability in clinical practice and to guide clinical decision-making.