Introduction: In order to resect brain tumors while preserving the patient’s brain functions, neurosurgeons apply intraoperative direct cortical stimulation (DCS) during awake surgery. Apart from language mapping, some highly specialized centers also map further higher cortical functions such as calculation function. Recently, the feasibility of mapping calculation function by repetitive navigated transcranial magnetic stimulation (rTMS) in healthy volunteers has been shown. With this in mind the present study aims to correlate the resection of calculation-positive sites in terms of rTMS with functional patient outcome of calculation performance.
Methods: Nine patients with right- or left-sided (7 right-, 2 left-sided) parietal brain tumors (7 gliomas, 2 metastases) underwent preoperative rTMS calculation mapping. We stimulated 52 previously determined cortical sites over the whole tumor hemisphere while patients performed a calculation task with 80 simple arithmetic operations. Pre- and postoperatively, patients performed a standardized number processing and calculation test (NPCT).
Results: The overall error rate (ER = calculation errors per stimulations) for tumor hemispheres was 7.3%. The resection of calculation-positive sites in terms of rTMS worsened the postoperative NPCT result in 6 cases. In 2 cases no calculation-positive sites were resected and the postoperative NPCT result was similar or better than preoperatively. In one case the postoperative NPCT result was better than preoperatively although calculation-positive sites were resected. According to these results sensitivity and negative predictive value are 100%, specificity is 66.7%, and positive predictive value is 85.7%.
Conclusions: The resection of calculation-positive sites in terms of rTMS correlated with postoperative NPCT results in 8 of 9 cases. Despite only presenting a low number of cases, rTMS might be a useful tool for preoperative mapping of calculation function. However, the reliability of the present results has to be evaluated in a larger series.
Patient Care: By the present study's results we provide an approach for the preoperative mapping of calculation function by rTMS.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of mapping further higher cortical functions. 2) Discuss, in small groups, the preoperative mapping of calculation function by rTMS. 3) Identify an effective treatment for brain tumors within eloquent regions.