At the forefront of this effort is the work of Prof. Alex Vitkin of the University of Toronto and his collaborators, who are exploring how polarized light can serve as a powerful new tool for understanding biological tissues. Their research spans multiple experimental approaches and scales, from microscopic examination of thin tissue samples to non-invasive probing of bulk tissue. Taken together, these studies suggest that light’s often disregarded wave properties may open new pathways for diagnosing cancer, guiding treatment, and uncovering the physical changes that accompany disease.

To appreciate the significance of this work, it helps to start with a simple idea. When light interacts with tissue, it does not simply pass through unchanged. Instead, it is scattered, absorbed, and otherwise altered in ways that depend on the microscopic structure and composition of the material. Cellular components, fibers, and other structures act as tiny scatterers, shaping how light behaves. By studying these interactions carefully, scientists can extract clues about the underlying structure and composition of the tissue itself.

Polarization represents a particularly rich source in this process. When light is polarized, its electric field oscillates in a specific direction or pattern. As this structured light travels through tissue, it is gradually scrambled by scattering events. The extent and nature of this scrambling depends on properties such as the size, shape, and organization of the tissue’s microscopic components. This allows polarization to carry signatures of the tissue’s internal micro architecture.

One branch of the Vitkin lab’s research focuses on transmission geometry, where light passes through thin tissue sections, typically around 4 to 20 microns thick, similar to those used in standard pathology. Here, advanced techniques such as Mueller matrix microscopy are used to capture detailed polarization information from each point in the sample. These measurements produce a set of quantitative biomarkers that describe how the tissue modifies polarized light.

On their own, these biomarkers can be difficult to interpret. Biological tissues are complex and heterogeneous, and their optical signatures often overlap. Machine learning is therefore used to identify combinations of features that best discriminate between tissue states. In the case of breast cancer, this approach has been used to differentiate between luminal subtypes that appear nearly identical under a conventional microscope. The results are promising, showing that polarization parameters may help reveal subtle structural differences that are not resolved by conventional histopathology.

This ability to extract meaningful information from light-tissue interactions has important practical implications. Traditional pathology relies heavily on staining and visual inspection, processes that can be subjective and time-consuming. Polarimetric methods, by contrast, offer a quantitative and potentially faster alternative. They can analyze entire tissue sections without the need for selective sampling, providing a more comprehensive view of the specimen. In doing so, they may help clinicians make more informed decisions about diagnosis and treatment.

While transmission studies offer insights at the microscopic level, much of the Vitkin lab’s work also focuses on probing bulk tissue non-invasively. In this case, light is directed onto intact tissue and the backscattered signal is measured. This reflection-based approach is essential for non-invasive applications, such as imaging tumors in living organisms.

However, backscattering geometry – whereby thick tissues are examined – introduces a new level of complexity. As light penetrates deeper into tissue, it undergoes multiple scattering events, bouncing from one bio-structure to another in a seemingly chaotic manner. This process alters the original polarization state, posing the central challenge of understanding how polarization evolves under highly scattering conditions.

To tackle this problem, researchers use optical phantoms, carefully designed materials that mimic the optical properties of biological tissue. By studying how polarized light behaves in these controlled environments, they can isolate the effects of specific variables, such as the size and concentration of scatterers in the phantom, on the optical measurement.

One intriguing finding from this work is that linearly polarized light can sometimes be preserved more effectively than circularly polarized light in highly scattering environments. This result may seem counterintuitive at first. Circular polarization has long been associated with a phenomenon known as polarization memory, where its polarization pattern is somewhat retained over multiple scattering events. Yet experiments show that when the scattering particles are very small, in what is known as the Rayleigh regime, linear polarization can dominate.

This behavior is closely tied to the geometry of scattering pathways in turbid media such as tissue. Small particles tend to redirect light in ways that favor certain polarization states, leading to distinctive patterns in the backscattered signal. These patterns provide valuable clues about the size and distribution of scatterers within the tissue. In the context of cancer, this is particularly important, as tumor development often involves changes in cellular structure and organization.

Indeed, studies of tumor tissue have revealed notable differences in polarization signatures compared to healthy tissue. For example, tumors can exhibit higher degrees of polarization preservation and stronger contrast in certain measurements. These effects may be linked to an increased presence of small scatterers, reflecting the altered microstructure of cancerous tissue.

Such findings highlight the potential of polarimetric imaging for detecting and characterizing tumors. Unlike many conventional optical techniques, this approach is non-invasive, does not involve ionizing radiation and does not require the use of dyes or contrast agents. It can also be integrated with existing imaging systems, making it a versatile addition to the clinical toolkit. Although further research is needed to fully understand the underlying mechanisms, polarimetry promises an alternative way of seeing disease.

More recently, the Vitkin lab has expanded its focus to include another subtle property of light: its phase. When coherent light waves interact with a scattering medium, they interfere with one another. This produces a granular interference pattern known as laser speckle. At first glance, speckles may appear random, and are often even considered as noise, but are in fact tightly connected to the medium properties that generated them.

By analyzing features such as speckle size and contrast, researchers can infer properties like scatterer size, turbidity and asymmetry. Experiments using controlled phantoms have shown that these metrics respond systematically to changes in the medium, providing a quantitative link between optical measurements and physical structure.

This approach has also been applied to biological tissues, where initial measurements on melanoma tissue yield differences in speckle patterns between normal and diseased states, reflecting underlying changes in cellular organization.

By combining speckle analysis with polarimetry, the researchers now aim to assess an even broader range of parameters. The concept of polarization speckle, which merges the two aforementioned domains, represents an exciting frontier. As multiple scattering in tissue significantly disrupts the phase properties of light while also altering and scrambling its polarization, the researchers hypothesize that investigating polarization speckles offers a new pathway to sense the complex and often subtle changes in tissue architecture during cancer progression.

What makes this body of work especially compelling is its integration across scales. Ranging from microscopic slides to whole tissues, and from polarization to phase, each approach contributes a piece of the puzzle. Together, they form a comprehensive framework for understanding how light interacts with biological matter.

Prof. Alex Vitkin’s work in this field has been instrumental in advancing these ideas from theory to application. By combining rigorous physics with innovative engineering and computational techniques, this research is helping to bridge the gap between laboratory science and clinical practice. The goal is to improve diagnostic accuracy and assess treatment efficacy by using light to provide deeper insights into architectural remodelling in diseased tissue.

Looking ahead, the potential applications of these methods are vast. In pathology, they could enhance the analysis of biopsy samples, reducing reliance on subjective interpretation. In surgery, they might guide the removal of tumors by providing real-time feedback on tissue boundaries. In radiation therapy, they may inform inter-fraction treatment effects towards adaptive therapeutics. In research, they shed light on quantitative links between tissue characteristics and light properties.

Of course, challenges remain. The complexity of light-tissue interactions means that careful calibration and interpretation are essential. Translating laboratory techniques into robust clinical tools will require further development and validation. Yet the progress made so far is very promising.

What emerges from this research (and from other labs around the world) is a new way of thinking about light, in which it is no longer just a means of illumination: it becomes a probe, a messenger, and a diagnostic tool. By unlocking its hidden dimensions, scientists are gaining access to a wealth of information that was previously out of reach.

The work of Prof. Alex Vitkin and his colleagues demonstrates that even in something as familiar as light, there are still discoveries to be made. And as these discoveries continue to unfold, they hold the promise of transforming how we see, understand, and ultimately treat disease.