PURPOSE: To investigate the effect of shorter axial length on the laser spot size and laser energy during photodynamic therapy (PDT) in monkeys. DESIGN: Experimental study with four rhesus monkeys. METHODS: PDT was performed on the normal retina of monkeys whose ocular axial lengths are shorter (19.55 to 20.25 mm) than that of humans. After the PDT, the eyes were enucleated, and the diameter of the irradiated laser spot was measured with a microcaliper. RESULTS: The area of actual laser spot was only 0.56 to 0.61 times of the planned area, which indicated that the laser energy/area was 1.64 to 1.78 times more intense than planned initially. CONCLUSION: These results are the in vivo demonstration that the diameter of PDT laser spot is smaller for eyes with shorter axial lengths.
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