Diagnostic utility of cerebrospinal fluid IgG index, 24-hour IgG synthesis rate, and immunophenotyping in primary central nervous system large B-cell lymphoma: A case report

Rationale: Primary central nervous system (CNS) large B-cell lymphoma (PCNS-LBCL) is a rare and aggressive brain malignancy that is challenging to diagnose early. Conventional imaging methods, such as magnetic resonance imaging and PET-CT, may not provide conclusive results in the early stages. This case highlights the diagnostic utility of cerebrospinal fluid (CSF) IgG index, 24-hour IgG synthesis rate, and immunophenotyping, offering a reliable noninvasive approach to improve early diagnosis when imaging results are inconclusive. Patient concerns: A 57-year-old male presented with progressively worsening headache, altered consciousness, and cognitive impairment, which developed gradually over several weeks. These symptoms prompted further diagnostic evaluation to identify the underlying cause. Diagnoses: Magnetic resonance imaging revealed nonspecific abnormalities, while PET-CT showed increased metabolic activity in regions suspicious for malignancy. CSF analysis demonstrated an elevated IgG index and 24-hour IgG synthesis rate, indicating CNS immune activation. Flow cytometry revealed abnormal B-lymphocytes, confirming the diagnosis of PCNS-LBCL. Interventions: The patient received a combined treatment regimen of R-CHOP chemotherapy and intrathecal methotrexate (MTX), chosen due to its ability to penetrate the blood–brain barrier and directly target CNS lesions. This combined approach is essential for treating PCNS-LBCL and preventing further neurological damage. Outcomes: Following treatment, the patient showed significant clinical improvement, with normalization of CSF parameters and recovery of neurological function, including improved cognitive abilities and motor skills. At the 1-year follow-up, imaging studies showed no relapse, and flow cytometry confirmed the absence of abnormal B-lymphocytes. Lessons: This case demonstrates the potential of integrating CSF IgG index, 24-hour IgG synthesis rate, and immunophenotyping as reliable, noninvasive diagnostic tools for the early detection of PCNS-LBCL, especially when imaging findings are inconclusive. The approach may offer valuable insights for improving diagnostic accuracy in clinical practice.