Case Report: Concurrence of cerebral nocardiosis and central nervous system B-cell lymphoma in a patient with myasthenia gravis.

We describe a case of simultaneous development of two opportunistic diseases presenting as similar-looking mass lesions in the cerebrum. A woman in her late 40s was admitted in May 2022 complaining of a sudden onset of right-side weakness (motor grade IV). Previously, she was diagnosed with myasthenia gravis in February 2021 and underwent a thymectomy (thymoma WHO type B2, lung invasion) in May 2021. She has since been taking azathioprine and prednisolone. The magnetic resonance imaging (MRI) of the brain showed masses on the left thalamus and right occipital lobe. Other masses were observed in the anterior neck and left lower lung field. Mass lesions were removed from the occipital lobe of the brain and anterior neck. They were abscesses and Nocardia Veterana was isolated. She was started on intravenous trimethoprim-sulfamethoxazole and carbapenem based on the result of the antimicrobial susceptibility suggested by the Clinical and Laboratory Standards Institute. After one month of treatment, most lesions of the lung and soft tissue disappeared, and her right-side weakness improved (motor grade near V). However, weakness worsened again after 2.5 months of treatment. On repeated MRI, the existing lesions improved, but new multiple central necrotic lesions with ring enhancement after gadolinium injection were seen. This observation made it unclear whether the antibiotic treatment had failed, or a new disease had developed. Tumor removal was repeated from the frontal lobe of the brain, and Epstein-Barr virus-positive diffuse large B-cell lymphoma was diagnosed. Since then, the patient has been receiving additional chemotherapy against lymphoma along with antibacterial treatment. To the best of our knowledge, this is the first reported case of the co-occurrence of cerebral nocardiosis and lymphoma. In conclusion, brain mass lesions in immunodeficient hosts are a diagnostic challenge and biopsy should be considered in cases of uncertainty. [ABSTRACT FROM AUTHOR]