SU‐F‐T‐02: Estimation of Radiobiological Doses (BED and EQD2) of Single Fraction Electronic Brachytherapy That Equivalent to I‐125 Eye Plaque: By Using Linear‐Quadratic and Universal Survival Curve Models

Abstract

Purpose:To test the radiobiological impact of hypofractionated choroidal melanoma brachytherapy, we calculated single fraction equivalent doses (SFED) of the tumor that equivalent to 85 Gy of I125‐BT for 20 patients. Corresponding organs‐at‐risks (OARs) doses were estimated.Methods:Twenty patients treated with I125‐BT were retrospectively examined. The tumor SFED values were calculated from tumor BED using a conventional linear‐quadratic (L‐Q) model and an universal survival curve (USC). The opposite retina (α/β = 2.58), macula (2.58), optic disc (1.75), and lens (1.2) were examined. The % doses of OARs over tumor doses were assumed to be the same as for a single fraction delivery. The OAR SFED values were converted into BED and equivalent dose in 2 Gy fraction (EQD2) by using both L‐Q and USC models, then compared to I125‐BT.Results:The USC‐based BED and EQD2 doses of the macula, optic disc, and the lens were on average 118 ± 46% (p < 0.0527), 126 ± 43% (p < 0.0354), and 112 ± 32% (p < 0.0265) higher than those of I125‐BT, respectively. The BED and EQD2 doses of the opposite retina were 52 ± 9% lower than I125‐BT. The tumor SFED values were 25.2 ± 3.3 Gy and 29.1 ± 2.5 Gy when using USC and LQ models which can be delivered within 1 hour. All BED and EQD2 values using L‐Q model were significantly larger when compared to the USC model (p < 0.0274) due to its large single fraction size (> 14 Gy).Conclusion:The estimated single fraction doses were feasible to be delivered within 1 hour using a high dose rate source such as electronic brachytherapy (eBT). However, the estimated OAR doses using eBT were 112 ∼ 118% higher than when using the I125‐BT technique. Continued exploration of alternative dose rate or fractionation schedules should be followed.