Introduction: The extent of surgical resection is an important prognostic factor in high grade gliomas. ALA –induced fluorescence has been used to improve the extent of surgical resection. However, there are variations in colour perception by the human eye that may lead to variation of intraobserver fluorescence detection. Aims: To construct and validate a visual analogue scale to assist in the standardization of ALA-induced fluorescence.

Methods: Scale construction: Colour samples obtained from 108 intraoperative fluorescence images were arranged in 10 panels according to their content of red, green and blue expressed as RGG. Scale construction validation: The chosen scale was validated by testing 56 healthcare professionals and three colour blind individuals. Statistical analysis: Inter-rater and intra-rater correlations were analysed using intra-class correlations. Bias was analysed using Student’s t-test and repeated measures analysis of variance.

Results: The chosen scale was calculated by (Red%-Blue%). Both the inter- and intra-rater analysis showed excellent overall ICC-values (ICC = 0.75) and acceptable levels of bias (<1 step of the scale). The colour blind individuals were unable to recognise fluorescence in any of the 108 images.

Conclusions: The fluorescence image analogue scale has excellent intra-rater and inter-rater reliability as well as acceptable levels of bias when assessing images of fluorescence. It would be of value in standardising fluorescence guided surgery (FGR). With further research, development and tests in clinical practice, it may become a useful tool to improve the results of FGR. Colour blind surgeons will be unable to use FGR without additional help.

Patient Care: Standardization of fluorescence guided surgical resection will improve the extent of surgery leading to significant improvement in time to tumour recurrence and better quality of life of patients.

Learning Objectives: Participants will learn fluorescence guided surgery of high grade gliomas, its principles and limitations.

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