The latest developments in fluorescence imaging

Developments in fluorescence imaging: the pathway to standard clinical care.

Roughly, fluorescence imaging can be divided in non-specific and disease-specific fluorescence imaging. Non-specific imaging is mostly performed using the EMA and FDA-approved fluorescent tracer called indocyanine green (ICG). The implementation of ICG imaging is performed in all specializations of surgery, ranging from neurosurgery, plastic surgery towards gastrointestinal and vascular surgery. As the identification of ICG is possible using widely available commercial fluorescent cameras, both in open laparoscopic and robotic settings, the clinical use is increasing fast.

In laparoscopic and robotic modalities, only visual information can be obtained. Therefore, improved visualization of vital structures like vessels, nerves and ureters is becoming increasingly important. The assessment of the perfusion status is possible by visualization of intravascular flow using ICG, which binds to albumin. This technique is among others used during reconstructive and colorectal surgery to evaluate the vascular supply towards the anastomosis and its end-organ. The goal is to improve tissue viability, and therefore, decrease the incidence of anastomotic leakage, which is a well-known complication.  Anastomotic leakage is associated with significant morbidity (and even mortality), sometimes requiring additional surgical interventions.  Further, in endocrine surgery, the identification of the parathyroid and thyroid gland and assessing its vascularization with fluorescence imaging can decrease postoperative morbidity significantly. Currently, national and international clinical studies are conducted to perform perfusion assessment in multiple disciplines and disease areas. Nevertheless, it appears that these studies lack standardization and uniformity of outcome parameters. Consequently, this complicates broad clinical implementation into standard of care.

Perfusion parameters in clinical care.

Recently, Goncalves et al. published an extensive review discussing the most important perfusion parameters to introduce ICG imaging into standard clinical care.¹ A variety of perfusion parameters are described in this review. First, intensity-related parameters such as absolute fluorescence intensity are used. This is a well-known parameter in fluorescence-guided imaging. However, intensity parameters are prone to intra- and interpatient bias and dependent on the used fluorescence camera. Further, imaging outcomes are dependent on different imaging properties such as the distance between the camera and the tissue, acquisition time and tissue composition. Due to these factors, only semi-quantification is possible, as absolute quantification is technically not feasible.

Second, the authors discuss time-dependent parameters. For example, time intensity and time to peak intensity. These parameters consist of timing values and changes in intensity over time. Moreover, inflow and outflow parameters such as peak intensity and 50% of the maximum intensity can be calculated. The advantage is that there is no influence of the measured intensity itself. Nevertheless, it is a limited parameter in terms of useability which might be influenced by intra-operative camera positioning relative to the surgical field of interest and imaging angle. Outflow parameters, such as intrinsic transit time, which addresses the vascular elimination of ICG in time, are also widely used. Third, the review states that the evaluation of ICG intensity comparing two or more regions of interest is therefore a so-called relative parameter. This static strategy is also widely used in tumor-specific imaging. However, the reference region might not be representative as it is prone for selection bias.

Goncalves et al. conclude that the field of non-specific fluorescence imaging using ICG is proceeding in the last few years. Despite major advancements, they strictly emphasize the need for standardization of parameter settings prior to the start of clinical trials. Moreover, the standardization of camera types and ICG dose should be discussed in (inter-)national committees of experts in the field of fluorescence imaging. They should come together to unify clinical studies, methodology and outcome parameters.

Developments in fluorescence imaging: a call for standardization in tumor-specific imaging.

In March 2021, Louwerends et al. published a great review on optical imaging in cancer surgery.² The authors provide a comprehensive manuscript containing all clinical studies performed in the field of tumor-specific fluorescence imaging. They indicate that fluorescence imaging can be performed,

1. to visualize tumor tissue and perform oncological excision with a sufficient margin
2. during debulking surgery
3. to identify occult disease before and after surgical excision and;
4. to improve the identification of vital structures like nerve and vascular perfusion.

For the first indication, they address a variety of outcome limitations, primary focusing on the use of the tumor-to-background ratio (TBR). The TBR is a broadly used outcome parameter to measure the optimal dose of a fluorescent tracer in Phase 1/2 clinical trials. However, Louwerends addresses the heterogeneity of the TBR calculation. Selection bias, differences in optical properties of tissue, and differences in fluorescent cameras vary between centers. Nevertheless, all factors influence the final study outcome. As a result, adequate, reliable, and consistent quantification is difficult and hard-to-interpret. Therefore, the authors advocate for the use of clinical outcome parameters to prove efficacy of fluorescence-guided surgery rather than ex vivo imaging outcomes. For example, how many times does fluorescence imaging change the initial and intentional surgical plan? Nevertheless, it can be stated that standard imaging protocols are still needed in tumor-specific imaging. Preferably ex vivo, in combination with outcome parameters to distinguish clinically relevant and non-clinically relevant excisions.

Recommended workflow for standardization.

The authors recommend that researchers should evaluate prior and during resection whether the initial resection was altered based on fluorescence imaging. Moreover, fluorescence imaging should be performed between initial delineation and actual resection of the tumor. During resection, fluorescence imaging can be performed to guide the surgeon towards the novel resection. Thereafter, fluorescence imaging of the surgical cavity is performed to detect potential residual disease. Afterwards, evaluation of the detection rate of true positives, false positives and false negatives is advised.

Developments in fluorescence imaging: promising fluorescent tracers in clinical trials.

The best readable update on all new and promising tracers is merged in a review of Azari et al.³ Multiple keynote fluorescence experts joined their forces to summarize all pilot and Phase 1 to Phase 3 studies. The final goal is to implement a fluorescent tracer into standard of surgical care, as for example 5-ALA, which is standard-of-care and used to improve the detection of glioblastoma in neurosurgical care. Currently, there are several fluorescent tracers already in Phase 3 trials: the article highlights SGM-101 targeting CEA in colorectal cancer and OTL138 targeting folate receptor-alpha to detect ovarian cancer and lung cancer as promising for the upcoming years.

At TRACER, we are keen on standardization, focusing on understandable and easy-to-execute imaging protocols, so clearly highlighted in the above scientific papers. Moreover, our team members are part of the national fluorescence imaging taskforce with the primary goal to evaluate all research and clinical outcomes of fluorescence-imaging studies performed in the Netherlands. With this taskforce, we are confident to standardize imaging and work together towards the final clinical introduction of fluorescence imaging towards standard of care. Read more about our optical fluorescent molecular imaging expertise here.

References.

1. Goncalves, L. N. et al. Perfusion Parameters in Near-Infrared Fluorescence Imaging with Indocyanine Green: A Systematic Review of the Literature. Life 11, (2021).
2. Lauwerends, L. J. et al. Real-time fluorescence imaging in intraoperative decision making for cancer surgery. The Lancet Oncology vol. 22 (2021).
3. Azari, F. et al. Intraoperative molecular imaging clinical trials: a review of 2020 conference proceedings. Journal of Biomedical Optics 26, (2021).