Over half of all human cancers arise in the squamous epithelium and approximately one million patients with non-melanoma cancers of the squamous epithelium are identified each year [1]. Examples of tissues with stratified squamous epithelia include the cervix, skin and oral cavity. The incidence of these cancers can be significantly reduced if they are detected at their earliest stages. Early detection has significant implications. If an epithelial cancer is detected early when it is still in a pre-invasive or pre-cancerous state, it can be easily cured, thus significantly reducing cancer morbidity and mortality.

Currently, the diagnosis of the majority of squamous epithelial cancers is carried out through visual inspection (either through a microscope or endoscope), followed by biopsy, which is then submitted for histopathology to determine if pre-cancer or cancer is present. Visual inspection is subjective and generally, multiple biopsies have to be taken to increase the likelihood of finding a cancerous lesion. Furthermore, early pre-cancers are often not visible under visual inspection.

A promising technique under development for epithelial pre-cancer detection is ultraviolet-visible (UV-VIS) optical spectroscopy. Optical spectroscopy offers several benefits over traditional diagnostic methods, which include visual inspection (through a microscope/endoscope), followed by biopsy. First and foremost, optical spectroscopy non-invasively probes the endogenous absorption, scattering and fluorescence of a large number of biological molecules already present in the tissue, thus providing a wealth of biochemical information related to disease progression without the need for tissue removal. Additionally, advances in sensitive detectors and optical fibers make it possible to measure optical spectra rapidly and remotely from human tissues in vivo. This diagnostic tool can potentially improve the accuracy of current diagnostic procedures, reduce unnecessary biopsies, and permit early detection of pre-cancerous lesions. A large number of clinical investigations carried out on a variety of tissue sites including the cervix, skin and oral cavity demonstrate that optical spectroscopy provides sensitive and specific detection of epithelial pre-cancers and early cancers [2].

Squamous epithelial tissues consist of a surface, stratified squamous epithelium, which is several cell layers thick, a basement membrane, which is a single cell layer thick and an underlying stroma, which contains structural proteins and blood vessels. The neoplastic cells originate near the basement membrane and proliferate upward within the epithelium. In invasive cancer, these cells ultimately break through the basement membrane and invade into the stroma. Thus, it is important for optical techniques to be able to localize the sampling volume to the epithelium where the earliest pre-cancerous changes arise and to be able to separate the optical signals collected from the epithelium versus the stroma.

Illustration: Cross-section of squamous epithelial tissue

Our goal is to develop novel optical sensing techniques based on UV-VIS optical spectroscopy for the earliest detection of squamous epithelial pre-cancers. An important component of tissue optical spectroscopy is the geometry of the illumination and collection system, for which fiber-optic probes are most commonly used. The fiber-optic probe contains illumination and collection optical fibers, the tips of which are placed near, or in contact with the tissue surface. The illumination fiber delivers light from the source to the tissue surface. The light propagates through the tissue, and a fraction of the light propagating through the tissue exits the tissue surface. The collection fiber collects a portion of the emitted light from the tissue surface and couples it to the detector. The geometry of the illumination and collection fibers and the optical properties (absorption and scattering) and fluorescence efficiency of the tissue through which the light propagates, define the optical sensing depth. Current probe geometries for tissue optical spectroscopy have a fixed illumination and collection geometry and thus, provide a fixed optical sensing depth, in a tissue with a given set of optical properties. The optical signals measured with this type of probe geometry tend to volume-average the endogenous absorption, scattering and/or fluorescence from the epithelial and stromal sub-layers. Thus, currently used probe geometries do not exploit the depth-dependent endogenous optical contrast present in epithelial pre-cancers and early cancers.

Our strategy is to develop fiber optic probes that can exploit the depth-dependent endogenous optical contrast present in these early pre-cancerous lesions. We have examined the effect of various fiber-optic probe parameters on the sensitivity of detected optical signals to specific layers [3], [4] with Monte Carlo modeling[5]. Specifically, our group is designing novel fiber probes [6], [7] with angled illumination and collection fibers to sample optical signals specifically from the epithelial layer of the tissue where the earliest pre-cancerous changes arise and flat-tip illumination and collection fibers to collect optical signals from the underlying stromal layer. We are also developing model-based algorithms [8] to extract the underlying physiological, biochemical, and structural properties of the different tissue sub-layers from these depth-dependent optical measurements. Clinical trials on squamous epithelia of the cervix are planned to test the diagnostic efficacy of our technology.

Illustration of angled fiber optic probe design: angled excitation: light coming out of the probe tip enters a cuvette containing Rhodamine standard at an angle of 45° relative to the normal axis of the probe tip surface.

Illustration of angled fiber optic probe design: angled illumination and collection: the source and detector fibers both placed at an angle of 45° relative to the normal axis of a tissue surface restrict the propagation of detected light to the top layer of a theoretical epithelial tissue model, based on the light distribution of detected photons from a Monte Carlo simulation.

Back to Top

References

1. "Cancer Facts and Figures," (American Cancer Society, 2003).
2. N. Ramanujam, "Fluorescence spectroscopy of neoplastic and non-neoplastic tissues," Neoplasia 2, 89-117 (2000).
3. C. Zhu, Q. Liu, and N. Ramanujam, " Effect of fiber optic probe geometry on depth-resolved fluorescence measurements from epithelial tissues: a Monte Carlo simulation," J Biomed Opt 8, 237-247 (2003).
4. Q. Liu, and N. Ramanujam, " Relationship between depth of a target in a turbid medium and fluorescence measured by a variable-aperture method," Optics Letters 27, 104 (2002).
5. Q. Liu, C. Zhu, and N. Ramanujam, " Experimental validation of Monte Carlo modeling of fluorescence in tissues in the UV-visible spectrum," J Biomed Opt 8, 223-236 (2003).
6. M. C. Skala, G. M. Palmer, C. Zhu, Q. Liu, K. M. Vrotsos, C. L. Marshek-Stone, A. Gendron-Fitzpatrick, and N. Ramanujam, " Investigation of fiber-optic probe designs for optical spectroscopic diagnosis of epithelial pre-cancers," Lasers Surg Med 34, 25-38 (2004).
7. Q. Liu, and N. Ramanujam, " Experimental proof of the feasibility of using an angled fiber-optic probe for depth-sensitive fluorescence spectroscopy of turbid media," Opt Lett 29, 2034-2036 (2004).
8. Q. Liu, and N. Ramanujam, "Sequential Estimation Of Optical Properties Of A Two-Layered Epithelial Tissue Model From Depth-Resolved Ultraviolet-visible Diffuse Reflectance Spectra," Applied Optics 45, (2006).