**3.5 Surgery planning**

The plan of the surgery should consider different aspects:

a.The aim of the surgery (resection vs. biopsy).

Most of the awake surgeries are performed to maximize the extent of tumor resection, but, in some cases, an awake surgery may be indicated for a biopsy. This is the case of lesions located in or near eloquent regions and/or the patient may not be in good condition for a long surgery. In those cases, less time will be required for the surgery and probably only direct cortical stimulation will be performed.

b.The clinical status of the patient (including cognitive evaluation).

As it was previously explained, a complete cognitive evaluation is mandatory in any patient considered for awake surgery. This evaluation added to the clinical assessment will draw a precise picture of the clinical situation of the patient, determining the functional and cognitive state of the patient. In our experience, patients, who present any neurological or cognitive deficit, usually present shorter periods of adequate attention and collaboration in performing the selected tasks, independently of precise anesthetic management. In other words, patients with functional or cognitive dysfunction usually show fatigue symptoms before the patients without the neurological impairment. This must be considered in the planification of the procedure, trying to shorten the presurgical period (vascular accesses, material preparation, patient positioning, surgical field preparation), and the surgical approach (cutaneous phase and craniotomy). Bearing this in mind, the first DCS will be performed in a brief period and, if the surgery course is adequately developed, the subcortical stimulation may also start sooner. This can limit the negative effect of fatigue in the development of awake surgery.

c.Structural and functional findings of presurgical studies.

DTI for tractography and fMRI studies have both a significant role in surgical planning. DTI studies are useful to identify the white matter pathways around or in the tumor, while the fMRI allows identifying cortical regions that are functionally involved in specific tasks. Both imaging techniques may help us to decide the size and location of the craniotomy, as well as the place of the corticectomy. They also allow us to predict the result of the direct cortical and subcortical stimulation.

Regarding these considerations, an awake procedure should fulfill the following premises:

