Crossed Cerebellar Diaschisis in Thalamic Lymphoma on ¹⁸ F-FDG PET/CT

Primary central nervous system lymphomas (PCNSLs) are extranodal variant forms of non-Hodgkin lymphoma arising within the brain parenchyma, leptomeninges, or spinal cord. PCNSL can present with varied neurological symptoms and imaging findings, making diagnosis without biopsy difficult. PCNSLs are highly aggressive, causing rapid deterioration, but are responsive to chemotherapy and radiotherapy making early diagnosis important. Crossed cerebellar diaschisis (CCD) is mostly seen with cerebral cortex vascular insults and is rarely reported with thalamic lesions and even rarer with thalamic lymphoma. However, CCD has also been described in other brain tumors (including primary glioma), chronic subdural hematoma, congenital insults, intracranial infections, and various dementia subtypes. We present a rare case of thalamic lymphoma evaluated with positron emission tomography/computed tomography that showed hypermetabolism of thalamus and associated hypometabolism in ipsilateral cerebral cortex and contralateral cerebellum representing CCD.