Exploiting Optical Contrasts for Cervical Precancer Diagnosis via Diffuse Reflectance Spectroscopy
Among women worldwide, cervical cancer is the third most common cancer with an incidence rate of 15.3 per 100,000 and a mortality rate of 7.8 per 100,000 women. This is largely attributed to the lack of infrastructure and resources in the developing countries to support the organized screening and diagnostic programs that are available to women in developed nations. Hence, there is a critical global need for a screening and diagnostic paradigm that is effective in low-resource settings. Various strategies are described to design an optical spectroscopic sensor capable of collecting reliable diffuse reflectance data to extract quantitative optical contrasts for cervical cancer screening and diagnosis.
A scalable Monte Carlo based optical toolbox can be used to extract absorption and scattering contrasts from diffuse reflectance acquired in the cervix in vivo. [Total Hb] was shown to increase significantly in high-grade cervical intraepithelial neoplasia (CIN 2+), clinically the most important tissue grade to identify, compared to normal and low-grade intraepithelial neoplasia (CIN 1). Scattering was not significantly decreased in CIN 2+ versus normal and CIN 1, but was significantly decreased in CIN relative to normal cervical tissues.
Immunohistochemistry via anti-CD34, which stains the endothelial cells that line blood vessels, was used to validate the observed absorption contrast. The concomitant increase in microvessel density and [total Hb] suggests that both are reactive to angiogenic forces from up-regulated expression of VEGF in CIN 2+. Masson's trichrome stain was used to assess collagen density changes associated with dysplastic transformation of the cervix, hypothesized as the dominant source of decreased scattering observed. Due to mismatch in optical and histological sampling, as well as the small sample size, collagen density and scattering did not change in a similar fashion with tissue grade. Dysplasia may also induce changes in cross-linking of collagen without altering the amount of collagen present. Further work would be required to elucidate the exact sources of scattering contrast observed.
Common confounding variables that limit the accuracy and clinical acceptability of optical spectroscopic systems are calibration requirements and variable probe-tissue contact pressures. Our results suggest that using a real-time self-calibration channel, as opposed to conventional post-experiment diffuse reflectance standard calibration measurements, significantly improved data integrity for the extraction of scattering contrast. Extracted [total Hb] and scattering were also significantly associated with applied contact probe pressure in colposcopically normal sites. Hence, future contact probe spectroscopy or imaging systems should incorporate a self-calibration channel and ensure spectral acquisition at a consistent contact pressure to collect reliable data with enhanced absorption and scattering contrasts.
Another method to enhance optical contrast is to selectively interrogate different depths in the dysplastic cervix. For instance, scattering has been shown to increase in the epithelium (increase in nuclear-to-cytoplasmic ratio) while decrease in the stroma (re-organization of the extra-cellular matrix and changes in of collagen fiber cross-links). A fiber-optic probe with 45° illumination and collection fibers with a separation distance of 330 μm was designed and constructed to selectively interrogate the cervical epithelium. Mean extraction errors from liquid phantoms with optical properties mimicking the cervical epithelium for μa and μs' were 11.3 % and 12.7 %, respectively. Diffuse reflectance spectra from 9 sites in four loop electrosurgical excision procedure (LEEP) patients were analyzed. Preliminary data demonstrate the utility of the oblique fiber geometry in extracting scattering contrast in the cervical epithelium. Further work is needed to study the systematic error in optical property extraction and to incorporate simultaneous extraction of epithelial and stromal contrasts using both flat and oblique illumination and collection fibers.
Various strategies, namely self-calibration, consistent contact pressure, and the incorporation of depth-selective sensing, have been proposed to improve the data integrity of an optical spectroscopic system for maximal contrast. In addition to addressing field operation requirements (such as power and operator training requirement), these improvements should enable the collection of reliable spectral data to aid in the adoption of optical smart sensors in the screening and diagnosis of cervical precancer, especially in a global health setting.
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