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Michael Lawrence, Seema Garg, Daniel Higginson, Sarah Moyer, Lawrence Marks, Bhisham Chera; A Novel Method for Evaluating Physiological Changes in Retinal Oxygenation Following Radiation Therapy. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2897.
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© ARVO (1962-2015); The Authors (2016-present)
Incidental radiation of the retina can be an unavoidable consequence of radiation therapy (RT) for tumors near the eye which may lead to retinopathy. Radiation induced physiological changes in the retina has not been well studied. We herein present a novel method of assessing retinal oxygenation and correlating this physiological measurement to RT dose.
The computed tomography (CT) RT treatment planning scan of 8 cancer patient receiving high dose RT for tumors near the retina were retrieved. The retina was divided into the following 4 quadrants using the optic disc as a central reference: superior nasal (SN), inferior nasal (IN), superior temporal (ST), and inferior temporal (IT). The median RT dose to each quadrant was calculated. Retinal Oximetry (RO, Retinal Oximeter, Oxymap, Iceland) was performed in all 8 patients using a standardized imaging protocol. RO images were first taken centered on the optic disc (OD) and then superior, inferior, medial, lateral, superior-lateral, superior-medial, inferior-lateral, and inferior-medial to the retina. A portion of the OD was included in all images for reference. The 9 RO images were patched together, using the OD as a reference point. The composite RO image was divided into 4 quadrants corresponding to the 4 major retinal arteriole, venule pairs (i.e. SN, IN, ST, IT). Using a vessel threshold of 8 pixels we then determined the retinal oxygen extraction, defined as the difference between the arteriole oxygen saturation (SaO2) and the venule oxygen saturation (SvO2), for each of the corresponding retinal quadrants.
A primary arteriole-venule pair could be identified for each quadrant and SaO2 and SvO2 by quadrant could be analyzed in all 8 patients (64 retinal quadrants). The SN, IN, ST, and IT median retina doses were 41 Gy (25-54 Gy), 51 Gy (45-70 Gy), 24 Gy (12-55 Gy), and 42 Gy (30-72 Gy), respectively. The corresponding median oxygen extraction in the SN, IN, ST, and IT retinal quadrants were 26% (16-43%), 32% (18-41%), 30% (19-35%), and 36% (6-61%), respectively.
Irradiation of tumors near the eye leads to significant heterogeneity in RT dose to areas of the retina. Regional oxygenation also varied across the irradiated retina.. This methodology will be used in a clinical protocol to examine the relationship between changes in retinal physiology (i.e. retinal oxygen extraction) and RT dose.
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