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T. M. Ranchod, A. J. Brucker, C. Liu, G.-S. Ying, C. A. Cukras, T. B. Hopkins; Evaluation of Actual versus Expected Pdt Spot Size. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1800.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the accuracy of the photodynamic therapy (PDT) laser spot size at the retina and its association with spot size, phakic status, axial length, refractive error and various PDT contact lenses for two FDA-approved PDT lasers.
Fundus photographs were taken from 11 eyes (6 phakic, 4 pseudophakic and 1 aphakic; axial length range 22.8 to 25.8 mm; refractive error range -1.88 to +3.75) of 11 patients (6 female; mean age 66 years, range 36 to 85 years) with the Ophthalmic Imaging Systems (OIS) WinStation 4000TM. Disc size was calculated using OIS software. Digital slit lamp photographs were taken of the PDT laser spot focused on the retina, using various spot sizes in combination with three different contact lenses. Spot size at the retina was determined by measuring the ratio of disc diameter to spot diameter in Adobe Photoshop and applying this ratio to the disc measurements taken from OIS WinStation 4000TM. Two masked graders and one unmasked grader took measurements of laser spot size independently. The accuracy of PDT laser spot size was evaluated using both the percentage of desired size and the bias from the desired size, and its association with ocular characteristics was examined using generalized linear regression.
Spot size at the retina averaged 86.0% (+/-1%) of desired spot size (81% for the Area centralis lens, 88% for the Quadra aspheric lens, 91% for the Volk lens, p=0.009) for the Coherent Opal Photoactivator laser and 103% (+/- 2%) for the Zeiss VISULAS 690s laser (97% for the Area centralis lens, 105% for both the Quadra aspheric and Volk lenses, p=0.03). Multivariate analysis demonstrated that the difference between actual and expected spot size increased with larger spot size for both the Coherent Opal Photoactivator laser (p=0.006) and Zeiss VISULAS 690s laser (p=0.01) independent of other variables. Phakic status, axial length and refractive error were not associated with accuracy of laser spot size for either type of laser.
PDT spot size at the retina appears to be consistently smaller than desired for the Coherent laser compared to consistently within 5% of expected size for the Zeiss laser, with a greater discrepancy as spot size increases. This effect is independent of contact lens magnification or refractive error. The relative inaccuracy of treatment spot size using the Coherent laser may help explain why, in clinical trial results, PDT treatment appears less successful for larger CNV lesions, as they may be consistently under-treated compared to smaller lesions.
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