Oncology Imaging: Light Work
By Rebekah Moan
Radiology Today
Vol. 26 No. 3 P. 6

A multinational research team is helping surgeons make more precise surgical decisions using a method that’s easier to operate and more cost-effective than MRI or PET. The method centers on the fluorescence lifetime (FLT) of fluorescent dyes. FLT measures how long dyes emit light after being excited by a pulse of light rather than relying on binding to cancer-specific molecules as in traditional fluorescence imaging. Unlike fluorescence intensity, which varies with dye concentration, FLT is an inherent property of the dye that depends only on its environment and remains unaffected by concentration or imaging conditions.

Anand Kumar from the radiology department at Massachusetts General Hospital (MGH) and Harvard Medical School, et al, discovered that when mice were injected with the FDA-approved dye, indocyanine green (ICG), tumors had a longer FLT than normal tissue, even when the dye accumulated in both. This allowed them to accurately distinguish tumors from healthy tissue based on FLT alone.

The researchers sought to determine whether the FLT enhancement of ICG observed in animal tumor models also applied to humans. They recruited patients undergoing liver surgery at MGH and head and neck surgery at Mass Eye and Ear. The findings confirmed that tumors had a longer FLT than normal liver or oral tissue, even when more ICG was present outside the tumor. Typically, fluorescence dyes often accumulate in normal tissue and show variability across cancer types, which limits their effectiveness and makes clinical translation costly and time-consuming.

“Instead of developing new dyes, our approach uses FLT changes in already FDA-approved dyes like ICG,” Kumar and first author Rahul Pal tell Radiology Today. “This makes clinical adoption faster, more efficient, and less expensive while also improving tumor detection accuracy over standard fluorescence intensity-based techniques.”

Intense Reaction
In a Nature Biomedical Engineering study, they described using confocal FLT imaging microscopy and histopathology on thin tissue sections, when available. They further observed that FLT enhancement occurred within small clusters of cancer cells, which suggested it may be a fundamental characteristic of cancers. They expanded their research to include 71 patients across multiple institutions, including MGH, Mass Eye and Ear, the University of Pennsylvania, the University of Newcastle in the United Kingdom, and Leiden University in the Netherlands. They included various cancer types— liver, brain, tongue, skin, bone, and soft tissue—all of which consistently had longer FLT than healthy tissue.

For example, in moderately differentiated hepatocellular carcinoma, the fluorescence intensity within the tumor was heterogeneous with bright and dark regions and considerably overlapped with the normal tissue fluorescence. The FLTs within the tumor were significantly longer (P = 0.007) than the FLTs in the surrounding normal liver parenchyma.

In poorly differentiated metastatic colorectal cancer, the fluorescence intensity was high along the rim of the grossly identified tumor boundary, which indicated strong nonspecific ICG accumulation and no apparent ICG uptake within the tumor. Again, the research team found a considerably longer FLT within the tumor compared with the FLTs within the bright, normal liver parenchyma.

To confirm that the long FLT arises from intratumoural ICG and not tissue autofluorescence, they measured liver tumors resected from eight patients who were not injected with ICG. The autofluorescence intensity in non-ICG-injected tumors was nearly 20 times lower than the fluorescence intensity observed in ICG-injected tumors and had very short lifetimes (~0.2– 0.3 ns). This confirmed that the long tumor FLTs were from ICG present in the tumor.

The average sensitivity and specificity were 95.3 ± 3.2% and 95.5 ± 3.2%, respectively, for FLT, and 51.7 ± 33.9% and 53.9 ± 28.8%, respectively, for fluorescence intensity. In terms of accuracy, the classification of tumors from normal tissue in larger resection specimens was more than 97% in all the tumor types.

“Our work with FLT provides a unique contrast mechanism that clearly highlights tumors during surgery,” Kumar and Pal say. “Because FLT is an inherent property of fluorescent dyes, this approach could potentially work with other FDA-approved dyes beyond ICG.”

Surgical Applications
Since the Nature Biomedical Engineering study was published, the research team has been actively working on advancing and clinically validating FLT imaging systems for surgical applications. They have developed fast FLT imaging algorithms that generate real-time images of the surgical field that provide immediate feedback to surgeons. They also built the first prototype of an intraoperative imaging system that can be used directly in the operating room to image patients during surgery.

In terms of research, Kumar and his colleagues are currently obtaining approvals for a larger clinical study to evaluate FLT imaging’s accuracy in distinguishing tumor from normal tissue and detecting residual cancer after surgery. They want to test the intraoperative FLT imaging prototype and are collaborating with clinicians to refine and validate the technology for realtime surgical margin guidance. That also means using motion and tissue-depth correction to ensure accuracy during procedures.

They are also investigating the biological mechanisms behind the FLT increase of ICG in cancer cells. “It is known that ICG accumulates in cancer cell lysosomes after internalization, and we suspect that the FLT shift is influenced by environmental changes such as polarity, viscosity, and interactions with proteins like albumin in the blood,” they say.

They are now actively testing those factors to identify what drives the FLT change in tumors. Understanding the mechanism is key to designing new fluorescent dyes that are brighter than ICG and can undergo a faster FLT shift, which would allow patients to be injected with the dye just before surgery rather than hours in advance.

“With all these advancements in progress, the future of FLT imaging is incredibly promising,” Kumar and Pal say. “As we continue refining the technology and making it more user-friendly and accessible, we believe it will revolutionize cancer surgery by enabling more precise tumor removal and improving patient care worldwide.”