Progressive Multifocal Leukoencephalopathy or Lymphoma? A Massive Unilateral Hemispheric Mimicker in a Patient Undergoing Lymphoma Treatment

An 81-year-old woman with follicular lymphoma treated with obinutuzumab and bendamustine developed cognitive impairment and dysarthria. Three months before death, neurological exams showed dysarthria, right hemiparesis, and gait disturbance. Blood tests showed lymphocytopenia (lymphocyte 10.4%). Cerebrospinal fluid (CSF) findings were unremarkable, including with respect to cytology. Brain MRI demonstrated a mass-like hyperintense lesion in the left parietal lobe and band-like abnormalities in the left fronto-temporal white matter that lacked contrast enhancement. Symptoms progressed to hemiplegia and mutism; severe dysphagia eventually necessitated intravenous fluid management. Follow-up MRI one month before death revealed a lesion encompassing nearly the entire left hemisphere, with hyperperfusion observed during arterial spin labeling (ASL). JC virus was detected in CSF (221 copy/mL), confirming that the patient had progressive multifocal leukoencephalopathy (PML). Subsequently, she exhibited poor arousal, followed by death. Here, lymphoma recurrence or PML was suspected due to a post-chemotherapy unilateral expanding brain lesion. These conditions are usually differentiated by contrast-enhancement patterns, but PML can also enhance during immune reconstitution. Moreover, lesions rarely cause a mass effect and more often exhibit hyperperfusion, which may aid in diagnosis. While unilateral PML has been reported, especially in the early stage, such an extensive lesion involving nearly an entire single hemisphere, as seen in our case, is rare.