Thymic lesions in patients with myasthenia gravis: Characterization with thallium 201 scintigraphy

PURPOSE: To assess thallium 201 (²⁰¹Tl) single photon emission computed tomography (SPECT) for evaluation of thymic lesions associated with myasthenia gravis (MG), including lymphoid follicular hyperplasia (LFH) and thymoma. MATERIALS AND METHODS: ²⁰¹Tl SPECT and computed tomography (CT) were performed preoperatively in 46 patients with MG who had undergone thymectomy. SPECT was conducted 15 (early image) and 180 (delayed image) minutes after ²⁰¹Tl injection. Results were visually assessed, and ²⁰¹Tl uptake ratios (thymic lesion count density/lung count density) were measured for quantitative analysis. Uptake was analyzed among the normal thymus, LFH, and thymoma patient groups. RESULTS: Histopathologic results indicated a normal thymus, LFH, and thymoma in 19, 16, and 11 patients, respectively. Mean uptake ratios in the normal thymus, LFH, and thymoma were 0.96 (95% Cl: 0.90, 1.03), 1.14 (95% Cl: 1.04, 1.25), and 1.87 (95% Cl: 1.56, 2.25), respectively, on early images and 1.09 (95% Cl: 1.00, 1.18), 1.65 (95% Cl: 1.48, 1.85), and 2.03 (95% Cl: 1.65, 2.50), respectively, on delayed images. Thymoma showed more intense ²⁰¹Tl accumulation than did the normal thymus (P < .001) and LFH (P < .001) on early images. Both thymoma (P < .001) and LFH (P < .001) displayed more intense uptake than did the normal thymus on delayed images. CONCLUSION: ²⁰¹Tl SPECT can enable differentiation between normal thymus, LFH, and thymoma in patients with MG.