- Lumbar cerebrospinal fluid (CSF) cytology examination must be obtained pre-operatively or within 31 days following surgery; the optimal time for obtaining CSF is prior to surgery or 1-3 weeks following surgery; ventricular CSF (either pre- or post-op) may be used only if a post-operative spinal tap is contraindicated; if a spinal tap is contraindicated and there is no ventricular CSF available, then CSF cytology can be waived for patients with supratentorial tumors or if there is documentation of spinal subarachnoid metastases (M3); patients who are categorized as M1 must have either an intra-operative positive CSF (via lumbar puncture at the end of the procedure) or a positive lumbar CSF obtained > 7 days post-operatively