To interpret the acquired DRS spectra, a widely accepted analytical model, introduced by Farrell et al. [35], was used to estimate the various DRS absorption and scattering coefficients. The absorption coefficients represent Epacadostat cost the concentration of physiologically relevant absorbers in the tissue, such as hemoglobin, water, and fat, as well as functional parameters like tissue oxygenation. The main scattering parameters are the reduced scattering coefficient (at 800 nm), the reduced scattering slope of the Mie scatterer (Mie-scattering slope), and the Mie-to-total scattering fraction. The Mie-scattering slope is related

to the average particle size [36]. In the Mie-to-total scattering fraction, the total scattering of tissue is assumed to be composed of Mie and Rayleigh scattering. In tissue, Mie scattering represents scattering caused by biologic cells and cellular components, whereas Rayleigh scattering is elastic scattering of light by particles that are much smaller than the wavelength of light (e.g., macromolecular aggregates such as collagen fibrils). The validation of the DRS analytic method has been described

previously by our group [34] and [37]. Intrinsic fluorescence from the tissue was calculated by correcting the acquired fluorescence spectra for absorption and scattering using the short SDS DRS spectra. For the latter, a modified photon migration Proteasome purification method [38] was used on the basis of the work by Müller et al. [39] and Zhang et al. [40]. The corrected spectra were fitted using the fluorescence spectra (excitation at 377 nm) of endogenous tissue fluorophores [collagen, elastin, nicotinamide adenine dinucleotide (NADH), and flavin adenine dinucleotid

(FAD)] as a priori knowledge. The optical oxidation-reduction (redox) ratio, which is linked to the metabolic state of the tissue, was defined as NADH/(NADH+FAD) [41] and [42]. Because collagen and elastin have almost identical fluorescence spectra, estimated amounts of collagen and elastin were combined as collagen + elastin. In case the tissue contained diagnostic levels of endogenous fluorophores other than the ones included in the standard fit model, the area underneath the fitted curve (known Thymidine kinase fluorophores) was subtracted from the total area under the original curve (measured fluorescence). Samples were stained with both standard hematoxylin and eosin (Merck, Darmstadt, Germany) (HE) and Masson trichrome (MT) (Sigma-Aldrich, St. Louis, MO) dyes. The HE-stained sections were used to quantify vital, necrotic, and fibrotic tissue fractions. The necrotic and fibrotic fractions were calculated as a percentage of the overall tissue area across each section. For this purpose, at least 10 different fields were investigated at a 400 × magnification. Immunohistochemical analysis of tumors was performed using anti-γH2AX [rabbit polyclonal; Cell Signaling Technology (Beverly, MA), No.