Near Infrared Autofluorescence Imaging in Minimally-Invasive Video-Assisted Parathyroidectomy (NIRAF MIVAP Study)

Native fluorescence or autofluorescence (AF) is defined as the emission of light by a naturally occurring fluorophore, on excitation by incident light of a specific wavelength. Accordingly, fluorescence produced by an endogenous fluorophore differs from that resulting from the application of a synthetic fluorescent marker and is, therefore, considered "contrast-free fluorescence." Maximal AF from both thyroid and parathyroid tissue occurs at the near-infrared spectrum (820-830 nm wavelength) with parathyroid tissue maximal emission intensity being 2-20 times greater than that of thyroid tissue and lasts at least for one hour in vivo. This is probably attributed to that the fluorophore assumed to be responsible for thyroid and parathyroid tissue AF is the calcium-sensing receptor that is present in parafollicular cells (C-cells), but to a much higher extent in parathyroid tissue. This allows for PTGs to be visualized as bright spots on a background of low fluorescence (thyroid gland). The fact that AF persists for at least an hour even after gland devascularization or unintentional resection, implies that NIRAF cannot confirm PTG viability. Viability depends on adequate tissue perfusion. This in turn requires assessing tissue vascularity using AF from a synthetic fluorescent marker such as indocyanine green (ICG); the concept of contrast-enhanced AF rather than contrast-free AF.

The property of AF in a tissue depends on the concentration of the endogenous fluorophore and whether it is affected by a pathological process. Hence, its application in clinical practice allowing it to serve as an optical biopsy (a point detection tool that provides real-time, non-invasive diagnostic information without physically exciting tissue). This might explain why unlike normal parathyroid tissue, parathyroid adenomas (PAs) demonstrate a heterogenous pattern of AF. Demarchi et al. reported that most PAs in their study appeared significantly less autofluorescent than the rim of compressed normal parathyroid tissue (at the periphery of a PA) that appeared as bright cap. This was referred to as cap AF. Nevertheless, no noticeable differences in the pattern of AF between adenomatous, hyperplastic, and normal PTGs were observed by others.

Minimally invasive video-assisted parathyroidectomy (MIVAP) is one of the most popular targeted parathyroid procedures performed for primary hyperparathyroidism. It has been demonstrated that the advantages offered by MIVAP enables it to overcome the limitations of other targeted parathyroid procedures. Allowing for full neck exploration and/or performing a concomitant thyroid procedure without the need to convert to a standard cervicotomy.

The objective of this study was to assess the feasibility of applying NIRAF-imaging via a limited direct access and the potential patterns of AF (AF signature) of PAs on contrast-free NIRAF-based imaging, and whether specific patterns of AF correlated with factors intrinsic to PAs such as the dominant cell type of a PA, its size and/or location. The additional operative time, and cost conferred using this technology were also evaluated