An ordinary flashlight emits light whose waves oscillate in all directions at once. But pass that light through a special filter, and you get a beam that oscillates strictly in one plane: that's polarized light. When such a beam travels through biological tissue, the tissue's internal structure slightly alters the orientation of the oscillations. Previously, these changes were described using simple quantities like average twist or loss of polarization. The authors of the new study went further and discovered that in some spots within the tissue, light doesn't just smoothly change polarization: it forms stable vortices and even more complex structures.

The researchers took thin slices of breast tissue, both healthy and affected by ductal carcinoma, and illuminated them with polarized light, measuring how the orientation of oscillations changed at each point. In healthy tissue, the polarization field remained nearly uniform, with no singular points. But in malignant tissue, points appeared around which the polarization twisted into a vortex, such points are called polarization vortices. Where the tissue structure created both radial and azimuthal variations in properties, more complex formations emerged — structures resembling tiny tornadoes, which physicists call Néel-type skyrmions. Each such vortex or skyrmion is characterized by an integer, the topological charge, which remains unchanged under small deformations and unambiguously indicates the presence of inhomogeneity.

This method provides contrast without any dyes: healthy tissue is topologically "empty," while cancerous tissue is "charged." This paves the way for a new type of diagnostics, where disease is judged not by average brightness or color, but by the very geometry of the polarization field — by how light twists as it passes through tissue.