## 4 Discussion  
We tested the hypothesis that SD-OCT has the potential to predict RPE cell damage after microsecond laser exposure. The Spectralis Centaurus system and an experimental SRT laser [30] were used to investigate OCT M-scan fringe washouts during microsecond laser irradiation by using laser pulses in the range of 2 to 20 \(\upmu\)s.  
As reported by [PERSON] et al. [38] and [PERSON] et al. [39], we confirmed that RPE radiant exposure damage thresholds increase with increasing pulse duration (Figure 6). Furthermore, our results for porcine RPE damage exposure thresholds were in good agreement with data of 1 and 3 \(\upmu\)s laser pulses reported by [PERSON] et al. and 5 and 50 \(\upmu\)s laser pulses found by [PERSON] et al. Recently published mean RPE damage exposure thresholds on lightly pigmented porcine eyes reported by [PERSON] et al. for laser pulses of 2 to 50 \(\upmu\)s duration are also consistent with our findings for 2 \(\upmu\)s pulses. However, for longer pulses, up to 20 \(\upmu\)s pulse duration, the damage exposure thresholds reported by [PERSON] et al. were significantly higher than those we observed. This is consistent with the theory that for short pulses, particle absorption by single melanosomes dominates, while for longer pulses, the lower bulk absorption by melanosomes plays a major role [40]. In principle, absolute values should always be considered as rough guide values only, as they can vary greatly and the whole concept of a laser system that compensates for differences in melanin concentration across individuals or across species variations argues against the use of an optimum fixed absolute value. Regarding evidence for the selectivity of longer pulse durations up to 20 \(\upmu\)s, the results of [PERSON] et al. are particularly relevant. These authors showed that MBF is the dominant origin of RPE cell damage for pulses up to 50 \(\upmu\)s duration [39]. [PERSON] et al. reached a similar conclusion [18]. Consequently, the transition for MBF-based to pure thermal RPE cell damage can thus be assumed to be in the range of 20 to 50 \(\upmu\)s. Our results substantiate that thresholds for both RPE cell damage and OCT fringe washouts are approximately the same (Figures 6 and 10). Therefore, it can be inferred that RPE cell damage up to a pulse duration of 20 \(\upmu\)s is predominantly caused by MBF, and thus reliable dosimetry can be performed by monitoring localized MBF to enable SRT. In addition, OCT also detects vibrations in the proximity of the MBF and is sensitive enough to detect subliminal changes. The results of the calcein-AM assay underline the necessity of dosimetry for ophthalm-mospecically sub-visible laser therapies, such as SRT. In accordance with [PERSON] et al., we observed a large difference between the ED\({}_{15}\) and ED\({}_{85}\) values (e.g., 10 \(\upmu\)s: \(ED_{15}^{RPE}=25\)\(\upmu\), \(ED_{85}^{RPE}=62\)\(\upmu\), \(\Delta Q=37\)\(\upmu\)\(334\)\(\mathrm{mJ/cm^{2}}\)), which is due in part to the high inter- and intra-individual melanin density variations in the different RPE samples [39]. Regarding the evaluation of our porcine RPE-choroid-sclera explants, practically all samples showed local radiant exposure damage threshold variability (Figure 8). This variation may be related to local differences in the melanin distribution of the RPE. While in some regions, a certain amount of energy caused clear-cut RPE damage; in adjacent regions, the same energy caused little or no RPE cell damage (e.g., Figure 8,c lesions applied with 44 \(\upmu\)J 397 \(\mathrm{mJ/cm^{2}}\) and pulses of 10 \(\upmu\)s duration). This variability is important with regard to possible over- and under-treatment of patients. In the absence of laser dosimetry control (or a laser treatment system with marked inhomogeneity due to a high IMF or strongly varying laser pulse energy), overtreatment may occur even with pulse durations that are considered as selective. Our findings demonstrate that OCT-based dosimetry can prevent overtreatment and collateral retinal damage.