This work is devoted to a first exploration of Mueller polarimetric imaging for the detection of residual cancer after neoadjuvant treatment for the rectum. Three samples of colorectal carcinomas treated by radiochemotherapy together with one untreated sample are analyzed ex vivo before fixation in formalin by using a multispectral Mueller polarimetric imaging system operated from 500 to 700 nm. The Mueller images, analyzed using the Lu-Chipmann decomposition, show negligible diattenuation and retardation. The nonirradiated rectum exhibits a variation of depolarization with cancer evolution stage. At all wavelengths on irradiated samples, the contrast between the footprint of the initial tumor and surrounding healthy tissue is found to be much smaller for complete tumor regression than when a residual tumor is present, even at volume fractions of the order of 5%. This high sensitivity is attributed to the modification of stromal collagen induced by the cancer. The depolarization contrast between treated cancer and healthy tissue is found to increase monotonously with the volume fraction of residual cancer in the red part of the spectrum. Polarimetric imaging is a promising technique for detecting short-time small residual cancers, which is valuable information for pathological diagnosis and patient management by clinicians.
Healthy human colon samples were analyzed ex vivo with a multispectral imaging Mueller polarimeter operating from 500 to 700 nm in a backscattering configuration with diffuse light illumination impinging on the innermost tissue layer, the mucosa. The intensity and polarimetric responses were taken on whole tissues first and after progressive exfoliation of the outer layers afterwards. Moreover, these measurements were carried out with two different substrates (one bright and the other dark) successively placed beneath each sample, allowing a reasonably accurate evaluation of the contributions to the overall backscattered light by the various layers. For the shorter investigated wavelengths (500 to 550 nm) the major contribution comes from mucosa and submucosa, while for the longer wavelengths (650 to 700 nm) muscular tissue and fat also contribute significantly. The depolarization has also been studied and is found to be stronger in the red part of the spectrum, mainly due to the highly depolarizing power of the muscular and fat layers.
We present Monte Carlo simulations of the backscattering of polarized light by colon tissue in terms of Mueller matrix.
We validated the Monte Carlo code with measurements on aqueous suspensions of polystyrene spheres of different sizes.
In a first instance we have modeled a tissue as a monodisperse scattering medium representing the nuclei in cytoplasm;
then we included a second layer with monodisperse scatterers to represent the most superficial layers (mucosa and
submucosa) while the deeper layers (muscularis and pericolic tissue) were "lumped" into a totally depolarizing
lambertian. These simulations failed to reproduce the Rayleigh type scattering (larger depolarization for circular vs.
linear incident polarization) systematically observed on all experimentally studied tissue samples. This issue has been
solved by modelling tissues as a single layer of bimodal mixtures including large and small scatterers over a lambertian.
In this paper we show the results of multi-spectral Mueller Imaging applied to the analysis of human colon cancer in a
backscattering configuration with diffuse light illumination. The analyzed sample behaves as a pure depolarizer. The
depolarization power, for both healthy and cancerous zones, is lower for linearly than for circularly polarized incident
light for all used wavelengths and increases with increasing wavelength. Based on their visual staging and polarimetric
responses, we chose specific zones which we correlated to the histology of the corresponding cuts. The histological
examination shows that we see a multilayer interaction in both healthy and abnormal zones, if the light penetration depth
is sufficient. The measured depolarization depends on several factors: the presence or absence of tumor, the microscopic
structure of cancer (ratio between cellular density and stroma), its exophytic (budding) or endophytic (penetrating)
nature, its thickness, the degree of cancer penetration in deeper layers and the nature of healthy tissue left under
abnormal layers. These results demonstrate that multi-spectral Mueller imaging can provide useful contrasts for the quick
staging of human colon cancer ex-vivo, with additional information about cancerous zones with different microscopic
structures.
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