Endoscopic-assisted Visualization of 5-aminolevulinic Acid-induced Fluorescence in Malignant Glioma Surgery: a Technical Note

Objective: With the use of fluorescence-guided resection with 5-aminolevulinic acid (5-ALA), the rate of complete resection of the contrast-enhancing part of malignant gliomas could be increased from 36% to 65%. Because the visualization of 5-ALA-induced fluorescence depends on a sufficient exposure to fluorescent light, residual tumor tissue in deep-seated resection cavities might not be detected. In addition, subcortical parts of a large spherical tumor might not be visualized, owing to a tangential position at the periphery of the microscopic field. With the availability of a specially designed endoscope with the capability to visualize 5-ALA fluorescence, we investigated the impact of this new technique on the visualization of residual glioma tissue.

Methods: A standard dose of 5-ALA 20 mg/kg was administered to 9 patients with deep-seated contrast-enhancing brain tumors 3 hours before surgery. A standard surgical exposure was performed and supplemented by the use of a specially designed endoscope with an option of 5-ALA fluorescence guidance. After microscopic visualization of the surgical cavity, endoscopic visualization was employed. If additional fluorescence tissue was detected, microscopic visualization was performed. Detected remnants of the tumor were removed and evaluated by histologic examination.

Results: In all cases, fluorescence-guided endoscopic visualization identified 5-ALA-positive tissue not sufficiently exposed by conventional microscopic visualization. In 8 patients, histopathologic examination confirmed residual tumor tissue; in 1 patient, the endoscopic visualized tissue was classified as radiation necrosis. In this patient, the tumor was completely ALA negative microscopically.

Conclusions: As an additional instrument, fluorescence-guided endoscopic visualization might help to overcome technical limitations of the conventional microscopic exposure of 5-ALA-positive tumor tissue. The false-positive 5-ALA tissue indicates that endoscopic visualization may overestimate the amount of tumor, so further analyses to ascertain the sensitivity and specificity of this technique are required.