CBs were observed over a large energy range from 117.8 to 334 μJ. From 10 spots, a maximal ratio between the last non-MB pulse and the first CB pulse of 1.76 ± 0.24 was calculated. Because there is no information about which pulse caused the CB, it is assumed that the last pulse of each pulse train needs to be at least the first pulse causing CB. Consequently, the calculated ratio could actually be smaller.
This ratio could be useful to determine a therapeutic window. The ratio could be used as a guideline to safely increase the energy after MB detection to ensure a sufficient therapeutic effect without CB formation. Because the desired endpoint of SLT referring to the mechanism of action is not completely understood, it might be sufficient so cease the laser close above detection of MB and keep the energy as low as possible to avoid side effects as transient IOP spike or peripheral anterior synechia as suggested by Tang et al.
14 Alternatively, in case a larger total energy might lead to an improved IOP reduction,
15 the MB threshold could be used to add a certain percentage of energy above the MB threshold.
A therapeutic window as the ratio between CB and MBF of 1.76 is smaller than expected. Lin
16 reported an energy factor of 10 between MB and CB formation, however, on single bovine melanosomes irradiated with a single nanosecond pulse at 532 nm. They also found a threshold for MBF of 55 mJ/cm
2. However, for single porcine RPE melanosomes, thresholds of 90 and 290 mJ/cm
2 were reported by Brinkmann et al.
17 for pulse durations of 8 ns and 1 μs, respectively. Thresholds increase with the pulse duration owing to heat diffusion during longer laser pulses. Also, they reported a decreased radiant exposure threshold for the damage in RPE tissue as well as decreased threshold when multiple pulses are applied because of accumulating effects.
17 Thus, despite the completely different model, the different pulse duration and the fact that a pulse train is used in this study could explain the discrepancy in the ratio between micro and macro bubble formation. Furthermore, in a cluster of melanosomes, a large amount of small MB can grow together into a CB. The mean MB threshold of 310 mJ/cm
2 in the whole data set is in the same regime as the threshold reported by Brinkmann et al.
17 with microsecond pulses. The radiant exposure typically applied in SLT ranges from 477 to 955 mJ/cm
2, which is in fact at least a factor 10 above the radiant exposure threshold reported for MBF using nanosecond pulses on a single melanin particle. However, the comparison of radiant exposure is difficult due to the uncertainty in determining the correct spot size, and it should be noted again that these experiments were performed in porcine eyes and therapeutic windows found here might not directly apply to human eyes. Also, the number of spots from which the ratio between MB and CB is calculated here might be too small to directly draw conclusions of an optimal therapeutic window.