Comparison of threshold irradiances and online dosimetry for selective retina treatment (SRT) in patients treated with 200 nanoseconds and 1.7 microseconds laser pulses

Background Selective retina therapy (SRT) solely affecting the RPE while sparing of the photoreceptors is usually performed with a train of repetitive laser pulses of 1.7 microseconds in duration. It was our purpose to evaluate the principle feasibility of SRT with shorter 200 nanoseconds laser pulses in patients. Methods Nineteen patients with macular disorders [diabetic maculopathy (DMP), geographic atrophy (GA), drusen maculopathy and central serous chorioretinopathy (CSR)] were treated with a prototype of a SRT laser (Nd:YLF laser; 527 nm; 1.7 microseconds and 200 nanoseconds pulse duration; 30 pulses at 100 Hz; spot size: 200 µm). Test lesions (n = 175) with increasing energy were applied at the lower arcade to determine the individual angiographic and ophthalmoscopic threshold radiant exposures (therapeutic window) before applying the central treatment lesions within these ranges additionally guided by online optoacoustic measurements. Postoperatively RPE damage was visualized and confirmed by fluorescein angiographic leakage and correlated with optoacoustic results. Additionally ED50 damage thresholds were calculated by probit analysis. Results None of the short repetitive 200 nanoseconds laser pulses led to retinal hemorrhages or retinal ruptures. Nearly all of the test- and treatment lesions could be visualized by angiography indicating desired RPE damage but were ophthalmoscopically invisible suggesting intact neurosensory retinal structures. ED50 cell damage threshold energies were significantly lower using 200 nanoseconds (99.6 µJ; n = 122) instead of 1.7 microseconds (196.3 µJ; n = 53) laser pulses. Optoacoustic and angiographic visibility correlated in 83.7% (200 nanoseconds) and 87.5% (1.7 microseconds). Conclusions Selective RPE effects can safely be achieved using shorter 200 nanoseconds laser pulses in patients without adverse effects to the neurosensory retina. The required pulse energy compared to the standard 1.7 microseconds regime was reduced by about a factor of 2 suggesting a reduced heat generation and flow into adjacent tissues during the shorter laser impact and thus possibly enhancing selectivity. Optoacoustics also seem to be a viable alternative in 200 nanoseconds treatment for a non-invasive online dosimetry control system. Lesers Surg. Med. 40:616–624, 2008. © 2008 Wiley-Liss, Inc.