## 2 Literature Review  
Optical coherence tomography (OCT) is one the most extensively studied technologies for the identification of tumor margins. This approach facilitates the extraction of information about tumor margins during surgery in real-time mode, enabling inspection/scanning of foci of the cancerous area or tumor cells that have spread to a different region than their origin (metastasized cancer). One of the most notable works in this research field addresses image-guided surgery (IGS) of breast cancer using OCT [15]. The specific methodology utilizes optical \"ranging\" of near InfraRed (IR) wavelengths to extract structural information in the spatial domain and increased imaging resolution at the microscopic/cellular level (at the order of microns). This is achieved via constructing cross-sectional maps in the two-dimensional image space based on the physical phenomenon of the interaction of light with an illuminated surface (optical backscattering). Many other studies demonstrate OCT as an efficient framework for the delineation of boundaries between normal and diseased breast tissue, utilizing techniques for the evaluation of human breast tissue through processing and analysis in frequency (Fourier transform) and spatial domain [16]. Another direction for assessments lies in the 3D volumetric data acquisition of human breast cancer tumor margins and axillary lymph nodes by rotating and retracting an OCT needle-probe below the tissue surface during imaging [17]. Despite the high resolution and efficiency in discriminating normal from cancerous tissue, the limited image penetration depth achieved (2-3 mm) has restricted the utilization of OCT as a real-time imaging modality. In addition, quantitative and objective assessments are based on the subjective interpretation of the acquired image by a specialized and well-trained medical expert. Similarly, optical radiomic signatures, extracted from corresponding optical data (optical coherence tomography images), led to enhanced identification of melanoma, with unique contrast and specificity [18]. The concept of elastic scattering spectroscopy (ESS) was used in the ultra-violet (UV)-visible spectral bands [19] and constitutes one of the earliest studies attempting to evaluate and characterize the boundaries of cancerous breast tissue samples through the application of optical spectroscopy. An analogous attempt was performed in [20] to examine the extent of cancer removal through surgical operations applying the technology of inelastic scattering of monochromatic light (Raman spectroscopy). Although these techniques are quite prone to morphological alterations at both sub-cellular and cellular plane (microscopic level) and enable efficient discrimination of different tissues states via their light fingerprints, their small effective illumination-source-to-detector separation limits the ability to sense the presence of malignant tissue up to 1-2 mm from the surface. A group of researchers from MIT, in particular, a team from the GRH Spectroscopy Laboratory, have developed a mobile scanning system (spectroscopic device incorporating a fiber-optic probe) and properly formulated to evaluate tumor margins intraoperatively; the portable scanner is based on two principles, namely intrinsic fluorescence spectroscopy (IFS) and diffuse reflectance spectroscopy (DRS) [21]. Fluorescence-based image acquisition and analysis methods have facilitated both the accurate demarcation of the infiltrating contour of tumors in real-time and in vivo mode and the evaluation of their histological characteristics. For instance, confocal laser endomicroscopy (CLE) enables the in vivo extraction of brain tissue optical data (fluorescent-based images) with highly increased resolution (at the cellular level), as accomplished in optical biopsies. On the other hand, the principle of fluorescence-guided surgery (FGS) is adopted in applications where improved visualization quality of tumor margins is required so that the extent of cancerous tissue removal in surgical operation increases.