[Optical coherence tomography in neurosurgery]

High Precision Bone Cutting by Er: YAG Lasers Might Minimize the Invasiveness of Navigated Brain Biopsies

Biopsies of brain tissue are sampled and examined to establish a diagnosis and to plan further treatment, e.g. for brain tumors. The neurosurgical procedure of sampling brain tissue for histologic examination is still a relatively invasive procedure that carries several disadvantages. The “proof of concept”-objective of this study is to answer the question if laser technology might be a potential tool to make brain biopsies less invasive, faster and safer. Laser technology might carry the opportunity to miniaturize the necessary burr hole and also to angulate the burr hole much more tangential in relation to the bone surface in order to take biopsies from brain regions that are usually only difficult and hazardous to access. We examined if it is possible to miniaturize the hole in the skull bone to such a high extent that potentially the laser-created canal itself may guide the biopsy needle with sufficient accuracy. The 2-dimensional, i.e. radial tolerance of the tip of biopsy needles inserted in these canals was measured under defined lateral loads which mimic mechanical forces applied by a surgeon. The canals through the skull bones were planned in angles of 90° (perpendicular) and 45° relative to the bone surface. We created a total of 33 holes with an Er : YAG laser in human skull bones. We could demonstrate that the achievable radial tolerance concerning the guidance of a biopsy needle by a laser created bone canal is within the range of the actual accuracy of a usual navigated device if the canal is at least 4 mm in length. Lateral mechanical loads applied to the biopsy needle had only minor impact on the measurable radial tolerance. Furthermore, in contrast to mechanical drilling systems, laser technology enables the creation of bone canals in pointed angles to the skull bone surface. The latter opens the perspective to sample biopsies in brain areas that are usually not or only hazardous to access.

[Intraoperative vascular fluorescence in cerebral glioblastomas and vascular histological features]

5-ALA intraoperative fluorescence is widely used in surgery of brain tumors for intraoperative demarcation of boundaries and more total resection because 5-ALA metabolites are not accumulated in the intact brain and vascular tissues. Given this fact, it was hypothesized that fluorescence of vessels in the immediate vicinity of a brain tumor may indicate their infiltration by tumor cells as a potential pathway for their dissemination and as a factor for continued tumor growth after surgery and adjuvant therapy.

PURPOSE

Identification of fluorescent vessels located near cerebral gliomas, with a histological description of their structure, relationships with the tumor, and potential invasion of the walls by tumor cells.

MATERIAL AND METHODS

A prospective cohort study included 14 patients with malignant supratentorial gliomas, aged 20 to 78 years. Five patients were operated on due to continued tumor growth. Two hours before surgery, all patients received 5-ALA orally. During surgery, a microscope (Carl Zeiss OPMI Pentero, Germany) with a fluorescent module (BLUE-400) was used. In all cases, molecular-genetic and immunohistochemical examinations of the tumor material were performed. During surgery, fluorescent vessels, after evaluating their functional significance, were also resected for histological examination.

RESULTS

Glioblastoma and anaplastic astrocytoma were verified in 10 and 4 patients, respectively. In 4 out of 10 glioblastoma cases, vessels with homogeneous or fragmentary fluorescent walls were detected in the tumor bed after resection of most of the tumor; in patients with anaplastic astrocytomas, vascular fluorescence was not observed. In the four vascular samples with intraoperatively detected wall fluorescence, tumor invasion into the vascular layers was revealed in all cases. These patients underwent an immunohistochemical examination with monoclonal antibodies to the glial GFAP marker, which clearly identified areas of ingrowth of tumor cells into the vascular wall.

CONCLUSION

5-ALA intraoperative fluorescence is a fundamentally new approach in the rapid diagnosis of tumor-infiltrated blood vessels. Invasion of tumor cells to intact vessels may be a mechanism of tumor progression and dissemination. Additional resection of fluorescent vessels may affect the radicalness of surgical treatment, but requires a mandatory assessment of their functional significance.

Cross-Polarization Optical Coherence Tomography for Brain Tumor Imaging

This paper considers valuable visual assessment criteria for distinguishing between tumorous and non-tumorous tissues, intraoperatively, using cross-polarization OCT (CP OCT)—OCT with a functional extension, that enables detection of the polarization properties of the tissues in addition to their conventional light scattering.

Materials and Methods: The study was performed on 176 ex vivo human specimens obtained from 30 glioma patients. To measure the degree to which the typical parameters of CP OCT images can be matched to the actual histology, 100 images of tumors and white matter were selected for visual analysis to be undertaken by three “single-blinded” investigators. An evaluation of the inter-rater reliability between the investigators was performed. Application of the identified visual CP OCT criteria for intraoperative use was performed during brain tumor resection in 17 patients.

Results: The CP OCT image parameters that can typically be used for visual assessment were separated: (1) signal intensity; (2) homogeneity of intensity; (3) attenuation rate; (4) uniformity of attenuation. The degree of match between the CP OCT images and the histology of the specimens was significant for the parameters “signal intensity” in both polarizations, and “homogeneity of intensity” as well as the “uniformity of attenuation” in co-polarization. A test based on the identified criteria showed a diagnostic accuracy of 87–88%. Intraoperative in vivo CP OCT images of white matter and tumors have similar signals to ex vivo ones, whereas the cortex in vivo is characterized by indicative vertical striations arising from the “shadows” of the blood vessels; these are not seen in ex vivo images or in the case of tumor invasion.

Conclusion: Visual assessment of CP OCT images enables tumorous and non-tumorous tissues to be distinguished. The most powerful aspect of CP OCT images that can be used as a criterion for differentiation between tumorous tissue and white matter is the signal intensity. In distinguishing white matter from tumors the diagnostic accuracy using the identified visual CP OCT criteria was 87–88%. As the CP OCT data is easily associated with intraoperative neurophysiological and neuronavigation findings this can provide valuable complementary information for the neurosurgeon tumor resection.

Quantitative nontumorous and tumorous human brain tissue assessment using microstructural co- and cross-polarized optical coherence tomography

Optical coherence tomography (OCT) is a promising method for detecting cancer margins during tumor resection. This study focused on differentiating tumorous from nontumorous tissues in human brain tissues using cross-polarization OCT (CP OCT). The study was performed on fresh ex vivo human brain tissues from 30 patients with high- and low-grade gliomas. Different tissue types that neurosurgeons should clearly distinguish during surgery, such as the cortex, white matter, necrosis and tumorous tissue, were separately analyzed. Based on volumetric CP OCT data, tumorous and normal brain tissue were differentiated using two optical coefficients — attenuation and forward cross-scattering. Compared with white matter, tumorous tissue without necrotic areas had significantly lower optical attenuation and forward cross-scattering values. The presence of particular morphological patterns, such as necrosis and injured myelinated fibers, can lead to dramatic changes in coefficient values and create some difficulties in differentiating between tissues. Color-coded CP OCT maps based on optical coefficients provided a visual assessment of the tissue. This study demonstrated the high translational potential of CP OCT in differentiating tumorous tissue from white matter. The clinical use of CP OCT during surgery in patients with gliomas could increase the extent of tumor resection and improve overall and progression-free survival.