## 4 Discussion
When calculating the light dosimetry for the treatment of atherosclerotic plaque by PDT, it is important to consider edge effects. These effects are significant, in particular if the diffuser length is in the order of \(\upmu_{\text{eff}}\)\({}^{-1}\). They are due to the lateral spreading of light within the tissues. This spreading is balanced and canceled at the center of the illuminated area, whereas it is not the case close to the edges where lateral diffusion is unbalanced. As described by [PERSON] [41], edge losses at the periphery of a longitudinally uniform beam extend about 3\(\upmu_{\text{eff}}\)\({}^{-1}\) from the beam edge. This corresponds to about 4 mm when using the optical coefficients reported by [PERSON] [37,44] at 630 nm for the aorta. Figure 4: (**a**) Monte-Carlo simulation of the evolution of the normalized fluence with depth in a semi-infinite tissue presenting the optical properties reported by [PERSON] [44] at 630 nm for the aorta (\(\upmu_{\text{a}}\) = 0.29 mm\({}^{-1}\); \(\upmu_{\text{s}}\)\({}^{\prime}\) = 2.40 mm\({}^{-1}\); \(\upmu_{\text{eff}}\) = 1.53 mm\({}^{-1}\)). The air–tissue interface is illuminated at 90 degrees with a broad and collimated light beam. The dashed line corresponds to a fit of the simulation with Equation (3). (**b**) Same condition, excepting that the illumination of the air–tissue interface is Lambertian. Only when the extent of the illuminated area exceeds 6\(\mathrm{\SIUnitSymbolMicro eff}^{-1}\) is the fluence rate in the central zone (where the plaque is supposed to be located) longitudinally uniform. Finally, it should be noted that the plaque itself frequently presents optical properties that are not only heterogeneous but also significantly different compared to those of the normal surrounding tissues [46]. Since these optical properties cannot realistically be determined before each treatment, one reasonable strategy is to adjust the light dosimetry on the basis of the type of plaque to be treated. Intra-arterial applications require that the diffuser be inserted into a catheter to reach the desired position. The measurements reported here for a modified commercially available catheter designed for drug delivery demonstrated that the plastic sheathing produced by this specific catheter has virtually no impact on the light emission profile of the diffuser, making it a suitable system to illuminate optimally intra-arterial lesions and avoid absorption, with a possible resulting heating, or a patchy or irregular illumination pattern, which could possibly have negative consequences. Overall, these results show that this system displays optimal illumination and dosimetry properties to achieve the sought-after results. Conceptualization, M.Z., M.-N.G., A.P., E.B., S.C. and G.W. ; Data curation, M.Z., Y.X., A.P. and G.W. ; Formal analysis, M.Z., Y.X., A.P. and G.W. ; Project administration, M.-N.G., S.C. and G.W. ; Resources, A.P., Y.X., M.d.K., E.B., H.v.d.B. and G.W. ; Validation, M.Z., Y.X., M.J., M.-N.G., A.P., M.d.K., E.B., H.v.d.B., S.C. and G.W. ; Writing--original draft, M.Z., A.P. and G.W. ; Writing--review & editing, M.Z., A.P. and G.W. All authors have read and agreed to the published version of the manuscript. This work was supported by the Swiss National Science Foundation Grants CR3213_150271/1 and 315230_185262. The authors are grateful to [PERSON] for editorial assistance. [PERSON] is a shareholder of Medlight SA. [PERSON] and [PERSON] were employed by Acrostak SA at the time of this study. The other authors have no relevant financial interests in this article and no potential conflicts of interest to disclose.