In the future, the determination of CTCs in the CSF could be a new quantitative method for the anti-tumor response assessment of systemic or intrathecal therapy (as opposed to CSF cytology, which is subjective and not a quantitative method). If the method shows greater sensitivity than CSF cytology and can reliably measure single tumor cells, the sensitivity of CSF examination in patients with a clinical suspicion of LM will increase. Possibly, this method can also be used to detect micrometastases in the CSF in patients without neurological symptoms, but with a high risk of CNS metastases.