Fig. 4: Two representative cases in which changes in the maxVAF/VAF values of CSF ctDNA reflected intracranial tumor changes.

At baseline, both MRI assessment and cytopathological observation indicated intracranial metastasis in patients P9 (a) and P77 (b). After 2–3 cycles of treatment, MRI assessment and cytopathological observation showed partial response in the lesion of patient P9, and correspondingly, the CSF ctDNA and CEA values gradually decreased compared to the baseline values. In contrast, MRI assessment and cytopathological observation in patient P77 revealed intracranial lesion progression and the corresponding CSF ctDNA and CEA values increased gradually. Abbreviation: CEA carcinoembryonic antigen, CSF cerebrospinal fluid, ctDNA circulating tumor DNA, maxVAF max variant allele frequency, MRI magnetic resonance imaging.