Magnetic resonance imaging characteristics of thymoma in Vietnamese patients with myasthenia gravis in relation to histopathological type and disease staging

Introduction
Myasthenia gravis (MG) is a relatively uncommon neuromuscular disease. The thymus plays an important role in the pathogenesis of MG, with 70% of patients presenting with thymic follicular hyperplasia and 20% presenting with thymoma. This study aimed to characterize magnetic resonance (MR) images of thymomas in patients with MG, associated with various stages and types.

Material and methods
A prospective study was performed in 46 MG patients with thymoma, who underwent surgery between August 2014 and September 2018, in Vietnam. Differences in MR imaging (MRI) images, according to type and stage, were assessed by the c2 test or Fisher’s exact test and Student’s t-test. A p-value < 0.05 was considered significant.

Results
The proportions of each type of thymomas found to be invasive were 28.6%, 61.1%, 66.7%, 90.9%, and 100%, for type A, AB, B1, B2, and B3 thymomas, respectively. Heterogeneous signal intensities on T1- and T2-weighted images, necrosis, and cysts were more commonly observed in high-risk thymomas than in low-risk thymomas. Non-invasive thymomas were more likely to display smooth margins than invasive thymomas (p < 0.05). Only 16.7% of invasive thymomas were detected in MR findings.

Conclusions
Only a few characteristic MRI features could be used to differentiate thymomas by stage or type.