## 4 Discussion
Their ex vivo results suggested approximately equal thresholds for MBF and RPE cell damage at 2 \(\upmu\)s, whereas for laser pulses of 5.2 \(\upmu\)s and 20 \(\upmu\)s duration, the MBF threshold was ~7% and ~25%, respectively, which was higher than for cell damage [40]. By contrast, our investigator-based evaluation of OCT M-scans (Appendix A: Figure A1 and Table A2) suggested that OCT signal loss occurs even at exposures ~6% lower than that at which RPE cell damage occurs, and that this holds true across the whole pulse duration range from 2 to 20 \(\upmu\)s. These results show the exciting potential for OCT-based assessment of RPE cell damage and may have the following causes: (1) MBF is the primary damage mechanism up to 20 \(\upmu\)s. OCT is extremely sensitive to such damage and even detects very small microbubbles that do not actually lead to cell damage. (2) The OCT fringe washout technique is more sensitive for longer pulse durations up to 20 \(\upmu\)s than the optoacoustic and light reflection methods [22; 40]. (3) OCT fringe washout detects fast thermal tissue expansion and subsequent thermal cell damage, rather than primarily responding to MBF. A current weakness of OCT dosimetry in terms of patient application is the limited axial SD-OCT scan depth of 1.8 mm. Therefore, axial movements of the patient as well as drift, tremor and saccades of the eye are a challenge. Such artifacts can lead to false-positive indications of RPE damage or complete signal loss. Accordingly, we are currently evaluating to what extent eye-tracking and other features of the system (e.g., positioning the patient in a supine position or an optimized energy ramp mode) can be modified to counteract these instabilities. Furthermore, one could also take advantage of small axial SD-OCT scan depth by checking the focal plane of the treatment laser via this measuring range to thereby further increase the safety of the SRT delivery. The OCT dosing control algorithm was developed for a treatment device with a different positive detection criterion (at least 50% dead RPE cells within the exposure spot, not a cluster of three dead RPE cells) [37]. This criterion leads to fine fringe washouts being discarded by the algorithm, which results in a low number of falsely identified events (overall FP = 24). Since the algorithm focuses on detecting large RPE lesions, many small lesions were missed, leading to a higher number of missed events (overall FN = 93). The overall moderate sensitivity of 85% and high specificity of 93% of the algorithmic evaluation can therefore be attributed to the assumption made during the algorithm development. Therefore, the dosing algorithm for fringe washouts in SD-OCT M-scans should be compared again to RPE damage using a calcein-AM viability assay and an area-based detection criterion (i.e., at least 50% dead RPE cells). Furthermore, the lesions should be applied with a laser burst or ramp application, as currently advocated for SRT [41]. For clinical application, the OCT algorithm should also be compared to RPE barrier leakage found via fluorescence angiography. The above discussion shows that the decision regarding whether a lesion is considered successful or unsuccessful can have a major influence on the assessment of a dosimeter's performance. The question therefore arises, of what proportion of the RPE in the area of the applied laser beam needs to be injured for a treatment to be considered successful. It is questionable whether lesioning a cluster of three RPE cells is sufficient to rejuvenate the RPE and stimulate their metabolism sufficiently to have a positive effect on retinal disease. To our knowledge, no in vivo study with patients has gauged the minimum number of lesioned RPE cells necessary for a successful treatment. However, experimental results for ex vivo porcine RPE-choroid explants revealed increased pigment epithelium derived factor (PEDF) secretion in 200 \(\upmu\)m SRT spots compared to 100 \(\upmu\)m spots [11]. This indicatesthat laser spot size in SRT plays an important role.