Functional activity within brain tumors: a magnetic source imaging study

To determine whether low-grade gliomas contain functional cortical activity more often than high-grade gliomas within radiologically defined abnormal tissue.

Patients with intra-axial cerebral lesions located in the vicinity of eloquent brain cortex preoperatively underwent magnetic source imaging. A dual 37-channel biomagnetometer was used to perform the imaging. Evoked magnetic fields were analyzed using the single-equivalent dipole representation to ascertain the neuronal source. Stimuli included painless tactile somatosensory stimulation of fingers, toes, and lips and auditory presentation of pure sinusoidal tones.

A retrospective analysis of 106 nonconsecutively treated patients, who had undergone preoperative magnetic source imaging between February 1996 and December 1999, revealed that 24.5% of the patients had been at risk for neurological deficits, because functionally active tissue was located within or at the border of the tumor. Functional activity was found within the radiologically defined lesion in 18% of Grade 2 tumors, in 17% of Grade 3 tumors, and in 8% of Grade 4 tumors.

The results confirm that, regardless of tumor grade, intra-axial brain tumors may involve or directly border on functional cortex. The degree of involvement of functionally viable cortex appeared greater for low-grade tumors than for high-grade lesions. On the other hand, high-grade lesions were more likely to be associated with functional cortex at their margins or within peritumoral edema. To safely maximize tumor resection, preoperative functional imaging and intraoperative electrophysiological mapping of the cerebral cortex and the white matter tracts are deemed necessary.