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Di Zhou, Ekaterina Semenova, Ping Wong, Nina Kalach, Walter Choi, Paul Finger; 103Pd versus 125I Plaque Radiation Dose to Normal Ocular Structures in the Treatment of 319 Uveal Melanomas. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4238.
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To calculate and compare the pre-operative radiation doses to normal intraocular structures in a large series of patients scheduled for plaque brachytherapy using Pallidum-103 (103Pd) versus Iodine-125 (125I).
We performed a retrospective chart review of 319 patients treated for uveal melanoma in 2006-2012. IRB approval was obtained and the study followed the guidelines of the Declaration of Helsinki. The radiation dose to tumor and normal ocular structures (outside the targeted zone) were calculated prior to surgery for both 103Pd and 125I Specific intraocular locations included: a 5mm depth, opposite retina, macula, optic disc, lens and the tumor apex (prescription point). Dose measurements were then related to tumor characteristics such as location (anterior versus posterior location), size, and T-stage according to the American Joint Committee on Cancer (AJCC) System.
Three hundred nineteen patients were diagnosed with T1-T4 uveal melanomas ranging in maximum (apical) height from 1.8 to 14.2 mm (mean 4.0 mm,CI 2.4-2.8). The range of the maximum basal diameter is 1.8 to 21 mm (mean 9.9 mm, CI 9.5-10.3). 21.6% of the tumors are in the anterior location and 78.4% of the tumors are in the posterior location. Table I shows the comparative dosimetry of 103Pd versus that of 125I to intraocular locations for all tumors. By design, the dose to the prescription point (tumor apex) for 103Pd and 125I plaques was equivalent. However, 103Pd shows a reduced mean doses to all the normal intraocular structures outside the targeted zone when compared to that of 125I in all tumors (p < 0.05).
Radioactive 103Pd seeds emit lower energy photons (21 KeV) versus 125I (28 KeV). This shift in photon energy leads to shift in radiation exposure within the eye. That is, for an equivalent tumor apex dose the less energetic photons are more quickly absorbed within the melanoma and continue to be more quickly absorbed before they reach most normal intraocular structures. Thus, in a large cohort of patients with uveal melanoma, this study demonstrates that 103Pd seeds in standard gold eye plaques offer a method to deliver relatively more radiation within the tumor (mean base to apex) and less dose to most essential intraocular structures.
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