## 4 Discussion  
We have demonstrated the combination of two complementary modalities, OCT and RS are highly suitable to perform staging and grading for the tissue. OCT can be used as a red-flag technology to differentiate between healthy and pathologic tissue, as well as for tumor-staging. Figure 2 shows a typical healthy bladder wall, where the urothel, lamina propria, and musculars layers are well discriminable. Blood vessels can also be identified in the healthy bladder wall. A pathological degeneration of the tissue can be specifically identified by the changes in the formation of these layers and lack of discriminatory boundary features between urothel and lamina propria. Figure 3 shows two different types of bladder cancer, stage T1 (Figure 3a) and stage T2 (Figure 3b). T1 in Figure 3a is pronounced through loss of structural discrimination between urothel and lamina propria but still conserving some structural features, which cannot be visualized anymore in Figure 3b. T2 in Figure 3b, on the other hand, lacks any structural information. The borders between different stages cannot be exactly identified as the loss of structural features is fading gradually and is not indicated by sharp boundaries. Hence, further investigation and more data are required to define these classification borders. As presented elsewhere [15], we validated that OCT is capable of staging until T1. A kNN-classifier was trained to detect structural features and provide the correct corresponding staging. The classifier was only trained on pathologic data and achieved a sensitivity of 80%, specificity of 60% and an overall accuracy of 71%. Sensitivity explains the correct identification of pTa and specificity the likelihood of correctly classifying pT2. We tested the stability of the classifier by adding a false data set. The performance of the kNN dropped by 9% for accuracy, 10% for sensitivity and 7% for specificity. In this proof-of-principle study, the amount of data is limited, and more biopsies need to be examined to build a more robust classifier. However, the potential of OCT for bladder cancer detection with high-speed scanning of 3D volumes of the bladder wall followed by identification of suspicious lesions and texture analysis for improved staging could be demonstrated. In future, OCT could potentially assist a surgeon in staging a suspicious lesion by providing information about structural features and the depth of the lesion into the urothel up to the invasion of the musculars in real-time. Upon identification of a suspicious lesion by OCT, RS provided molecular information comparable to histology in vivo and in situ within seconds as pathologic changes of the bladder wall are linked to molecular changes. The main Raman bands that could be attributed to low-grade and high-grade bladder cancer were in good agreement with previously identified Raman bands [20; 31; 45; 46], i.e., 750 cm\({}^{-1}\) (Symmetric breathing of tryptophan), 853 cm\({}^{-1}\) (ring breathing of tyrosine), 940 cm\({}^{-1}\) (C-C stretching in protein), 1003 cm\({}^{-1}\) (symmetric ring breathing of phenylalanine), 1081 cm\({}^{-1}\) (C-C and C-O in lipids and glucose, C-N stretching of proteins), 1253 cm\({}^{-1}\) (amide III, C-N stretching mode of proteins, collagen), 1315 cm\({}^{-1}\) (CH\({}_{3}\)CH\({}_{2}\) twisting mode of lipids and collagen), 1342 cm\({}^{-1}\) (DNA/RNA and C-H deformation of proteins and tryptophan), 1450 cm\({}^{-1}\) (CH\({}_{2}\) bending of lipids and proteins in general), 1660 cm\({}^{-1}\) (amide I, proteins, C=C stretching of lipids). PCA provided a first indication of the significance of Raman bands and capability of low-grade to high-grade tumors being differentiated. Figure 5a shows the comparison of low-grade to high-grade Raman spectra. The loadings of the first three PCs demonstrate the Raman bands significant for discrimination of low-grade and high-grade Raman spectra, see Figure 5b.