-aminolevulinic acid-induced protoporphyrin IX concentration correlates with histopathologic markers of malignancy in human gliomas: the need for quantitative fluorescence-guided resection to identify regions of increasing malignancy (original) (raw)

Extent of resection is a major goal and prognostic factor in the treatment of gliomas. In this study we evaluate whether quantitative ex vivo tissue measurements of d-aminolevulinic acid-induced protoporphyrin IX (PpIX) identify regions of increasing malignancy in low-and high-grade gliomas beyond the capabilities of current fluorescence imaging in patients undergoing fluorescence-guided resection (FGR). Surgical specimens were collected from 133 biopsies in 23 patients and processed for ex vivo neuropathological analysis: PpIX fluorimetry to measure PpIX concentrations (C PpIX ) and Ki-67 immunohistochemistry to assess tissue proliferation. Samples displaying visible levels of fluorescence showed significantly higher levels of C PpIX and tissue proliferation. C PpIX was strongly correlated with histopathological score (nonparametric) and tissue proliferation (parametric), such that increasing levels of C PpIX were identified with regions of increasing malignancy. Furthermore, a large percentage of tumor-positive biopsy sites (∼40%) that were not visibly fluorescent under the operating microscope had levels of C PpIX greater than 0.1 mg/mL, which indicates that significant PpIX accumulation exists below the detection threshold of current fluorescence imaging. Although PpIX fluorescence is recognized as a visual biomarker for neurosurgical resection guidance, these data show that it is quantitatively related at the microscopic level to increasing malignancy in both low-and high-grade gliomas. This work suggests a need for improved PpIX fluorescence detection technologies to achieve better sensitivity and quantification of PpIX in tissue during surgery. G liomas account for over 70% of all primary brain tumors. 1 -3 Currently brain tumor research seeks to find diagnostic and prognostic biomarkers for gliomas that would inform both surgical and/or medical treatment. Extent of resection is increasingly accepted as critical to optimal surgical treatment and patient prognosis. 4,5 Image guidance facilitates neurosurgical resection but is subject to intraoperative brain shift, which degrades the accuracy of relating navigational information with the surgical field presented to the surgeon. 6 -12 The use of d-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) fluorescence-guided resection (FGR) of brain tumors has gained increased clinical interest. 13 FGR provides the operating surgeon with an intuitive, real-time surgical guidance tool for delineating tumor tissue that mitigates some of the challenges of image-guided neuro-navigation caused by intraoperative brain shift and deformation. ALA-induced PpIX FGR studies support its utility as a surgical tool for intraoperative delineation of a tumor. For example, the largest study to date, a multicenter, randomized phase III clinical study of ALA-induced PpIX for FGR of malignant gliomas, showed a significant improvement in gross total resection of the contrast-enhancing tumor (65% vs 36%), as well as a higher 6-month progression-free survival (41.0% vs 21.1%), in the FGR group compared with the conventional white-light treatment group. 14 -16 In a recent study of malignant gliomas, we found that visible levels of ALA-induced PpIX fluorescence correlate with tumor burden and World Health Organization (WHO) histopathological score of resected specimens. 17 To date, intraoperative detection of PpIX fluorescence for tumor delineation has largely been subjective (i.e., a modified surgical microscope is used to visualize "pink" fluorescence). 15 -21 This approach is limited in its sensitivity for identifying low (but significant) levels of accumulated PpIX in a tumor, potentially leaving some amount of resectable tumor unidentified. 22 Indeed, we recently reported preliminary in vivo results in human brain tumors 23 that indicate PpIX fluorescence may be a tumor-targeting biomarker with a diagnostic performance that exceeds subjective visible assessments when measured quantitatively with a new fiber optic approach. 24 Thus, even though ALA-induced PpIX fluorescence has been shown to be a successful biomarker for surgical resection of malignant glioma, the subjective assessments used to date do not appear to be sufficiently sensitive or quantitative to exhaust its full potential. As a result, establishing the underlying relationships between ALA-induced PpIX concentration in brain tumor tissues and their histologically determined malignancy profile is critical to further development and optimization of FGR.