Matthew Elliot

Quantification of protoporphyrin IX (PpIX) fluorescence in human brain tumours has the potential to significantly improve patient outcomes in neuro-oncology, but represents a formidable imaging challenge. Protoporphyrin is a biological molecule which interacts with the tissue micro-environment to form two photochemical states in glioma. Each exhibits markedly different quantum efficiencies, with distinct but overlapping emission spectra that also overlap with tissue autofluorescence. Fluorescence emission is known to be distorted by the intrinsic optical properties of tissue, coupled with marked intra-tumoural heterogeneity as a hallmark of glioma tumours. Existing quantitative fluorescence systems are developed and validated using simplified phantoms that do not simultaneously mimic the complex interactions between fluorophores and tissue optical properties or micro-environment. Consequently, existing systems risk introducing systematic errors into PpIX quantification when used in tissue. In this work, we introduce a novel pipeline for quantification of PpIX in glioma, which robustly differentiates both emission states from background autofluorescence without reliance on a priori spectral information, and accounts for variations in their quantum efficiency. Unmixed PpIX emission forms are then corrected for wavelength-dependent optical distortions and weighted for accurate quantification. Significantly, this pipeline is developed and validated using novel tissue-mimicking phantoms replicating the optical properties of glioma tissues and photochemical variability of PpIX fluorescence in glioma. Our workflow achieves strong correlation with ground-truth PpIX concentrations ( R 2 = 0.918 ± 0.002 ), demonstrating its potential for robust, quantitative PpIX fluorescence imaging in clinical settings.

Fluorescence-guided surgery (FGS) was pioneered for glioma and is now established as the standard of care. Gliomas are infiltrative tumours with diffuse margins. FGS provides improved intra-operative identification of tumour margins based on tumour-specific emission visible to the operating surgeon, resulting in increased rates of gross total resection. Multiple fluorescence agents may be used including 5-ALA, fluorescein sodium, and indocyanine green (ICG). This review details the indication, required equipment, mechanism of action, evidence base, limitations, and regulatory issues for each fluorophore as utilised in current clinical practice. FGS for glioma is limited by a reliance on subjective interpretation of visible fluorescence, which is often not present in low-grade glioma (LGG) or at the infiltrative tumour margin. Consequently, there has been a drive to develop enhanced, objective FGS techniques utilising both quantitative fluorescence (QF) imaging systems and novel fluorophores. This review provides an overview of emerging QF imaging systems for FGS. The pipeline for novel fluorophore development is also summarised.