The influence of brain tumor treatment on pathological delta activity in MEG

The goal of the MEG study was to investigate the influence of tumor treatment on pathological delta activity (1-4 Hz). The treatment consisted of neurosurgery, and in some of the patients, additional radiotherapy. MEG and MR recordings were made both before and after the treatment in 17 patients. The signal power in the delta frequency band was determined for each recording. The malignant tumors were associated with large tumor volumes. Furthermore, both malignant tumors and tumor volume were associated with high signal powers in the delta band, indicating a correlation of delta power with the severity of the lesions. In all patients with high grade tumors, the delta power was lower after the treatment. The sources underlying the delta signals were estimated with an automatic single dipole analysis method. Estimated sources were projected onto MR scans. Preoperatively 14 clusters of equivalent sources describing focal activity were found in 12 out of 17 patients. Thirteen of these clusters were located near the tumor, and one cluster near an edema border. The locations near tumors are plausible and suggest that in general the source estimation was reliable. After the operation, 13 such clusters were found in 12 patients. Eleven clusters were located near the lesion border and one cluster near the edema border. Furthermore a cluster contralateral to the lesion in the other hemisphere indicated that brain lesions can affect the functioning of more distant brain areas than just the peritumoral brain tissue. Of the 12 patients who had preoperatively peritumoral clusters, 11 patients had postoperatively perilesional sources. In these cases the shift in source locations was in general considerably smaller than the dimension of the preoperative tumors. This finding indicates that similar areas generate the pre- and postoperative delta activity. Furthermore, focal delta sources were found in a case without tumor recurrence, and also in cases that most tumor tissue was removed. These findings suggest that the pathology underlying the slow waves is not the presence of the tumor bulk but the structural damage done by the tumors on the surrounding white/gray matter.