Monochromatic light from a laser is used to shine on an object of interest, resulting in a change in vibrational energy states, which can be detected as electromagnetic radiation that is filtered through a monochromator ( 11). Handheld Raman spectroscopic probes harness the traditionally employed Raman spectroscopic technique described decades ago ( 10). The purpose of this review is to examine the technique and evidence for the utility of handheld spectroscopy tools in neuro-oncologic surgery. Other devices aimed at improving visualization in fluorescence guidance surgery have recently been developed.
Among the most studied techniques has been the use of handheld Raman spectroscopy in the identification of cancer cells during surgery. Handheld tools were developed as an adjunct to intraoperative resection, under the premise that real-time use and the ability to adjust the angle of the instrument could facilitate detection of remaining tumor at the time of surgery. However, the infiltrative margin of HGGs and World Health Organization (WHO) grade II low grade gliomas (LGGs) have remained challenging to visualize, since sufficient levels of fluorescence often cannot be detected using traditionally employed visualization technologies ( 9). Fluorescence-guided surgery using 5-aminolevulinic acid (5-ALA) has been a crucial addition to the neurosurgeon's toolkit, allowing for direct fluorescence visualization of HGGs using wide-field surgical microscopy ( 8). Also, non-specific enhancement after iMRI may be confused with residual tumor. Although these instruments have demonstrated utility, intraoperative retraction and tumor resection often result in brain shift, making it challenging to assess the extent of resection in real time. Other techniques, such as neurosurgical virtual reality and simulation have facilitated preoperative visualization and planning ( 7). Multiple studies have highlighted the fact that microscopic involvement of high-grade gliomas (HGGs) extends well-beyond the contrast-enhancing lesion visible on T2-weighted MRI ( 5, 6). More sophisticated MRI techniques, such as whole brain magnetic resonance spectroscopic imaging (MRSI), has allowed for more precise visualization of tumor than would otherwise be possible with standard MRI ( 5). As such, numerous tools have been developed, for both pre- and intra-operative use, in order to assist in the identification and removal of neoplastic tissue ( 4). Despite this, however, complete tumor resection remains challenging due to the presence of microscopic disease and infiltrative tumor tissue that is difficult to visually differentiate from the surrounding brain. Indeed, numerous studies in the literature have demonstrated that the proportion of tumor removed is significantly associated with key postoperative outcome metrics, including overall survival (OS) and progression-free survival (PFS) ( 1– 3). The fundamental goal in the surgical treatment of brain tumors is achieving the greatest possible extent of resection while minimizing injury to surrounding pathways present in the adjacent brain.
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