New Method Enables Noninvasive Detection of Circulating Tumor Cells in Blood

Metastasis, the process by which cancer cells gain the capacity to spread and create new tumors in different body locations, typically via blood or lymphatic vessels, is a significant marker of advanced cancer and complicates treatment. Early detection is crucial and can be achieved by identifying circulating tumor cells (CTCs) in blood samples. Yet, CTCs can be scarce and might be absent in small blood samples, despite being present in a patient's bloodstream. Researchers have developed a solution to this problem through a technique named diffuse in-vivo flow cytometry (DiFC). This technique involves marking CTCs with a fluorescent agent, focusing a laser onto an artery, and using a detector to capture the resulting fluorescent signals to count CTCs. However, the efficacy of DiFC can be heavily impacted by background noise, primarily from the inherent fluorescence of the surrounding tissue, termed autofluorescence (AF).

A joint research team from Tufts University (Medford, MA, USA) and Northeastern University (Boston, MA, USA) is working to tackle this problem and enhance the DiFC method initially developed by the Northeastern group. Their latest study evaluated the efficiency of a new approach named the dual-ratio (DR) method, designed by the Tufts group, in reducing noise in DiFC and increasing its penetration range. Initially created for spectroscopy techniques, the DR method has now been adopted for DiFC. The new DR DiFC system uses two lasers and two detectors arranged strategically in space. Theoretically, by combining the signals of the two detectors, noise can be eliminated, and the AF contributions from the surface of the measured medium, such as the skin, can be minimized. However, the conditions under which DR DiFC truly outperforms standard DiFC remain uncertain.

The researchers addressed this uncertainty in three ways. They conducted Monte-Carlo simulations with various noise and AF parameters, including different source-detector configurations. They performed DR DiFC experiments on a synthetic tissue-mimicking flow phantom with cell-mimicking fluorescent microspheres. Finally, the team measured the AF of mouse skin and underlying muscle to understand noise variation with tissue type and depth. The experiments showed DR DiFC to be superior to standard DiFC if the noise fraction not canceled by DR was below 10% and if AF contributions were more concentrated near the surface than evenly distributed within the target volume.

However, as the mouse experiments suggested, AF is typically higher in the skin than in the underlying muscle, suggesting that DR DiFC might offer an advantage over standard DiFC in most cases. Notably, if AF was higher near the surface rather than evenly distributed, DR DiFC had a considerably higher penetration range than standard DiFC. Overall, these findings are crucial for the development of DR DiFC as an emerging technique for non-invasively detecting fluorescent molecules in the bloodstream. This method will enable physicians to promptly detect cancer cells in patients' blood without the need to draw samples, making metastasis diagnosis simpler and more precise. In the future, it could be extended to other cell types or systemic molecules of interest.

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