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Dose Modification for Cs-131 Permanent Implants Using Resensitization-Corrected Normal Tissue BED
W Luo*, M Randall , J Molloy , P Aryal , J Feddock , C Wooten , Univ Kentucky, Lexington, KY
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SU-E-T-114 Sunday 3:00PM - 6:00PM Room: Exhibit HallμμPurpose: To apply resensitization (redistribution and reoxygenation) correction to normal tissue BED calculation and have it verified with clinical outcomes. Method and Materials: The BED formalism without resensensitization for permanent implants was BED = D*RE – BF, where D is the prescribed dose, RE = 1 + (β/α)R0/(μr+λ), BF = K*Teff, K = ln(2)/(αTp), and Teff = Taveln(αDTp/T1/2). α and β are LQ parameters, R0 the initial dose rate, μr the repair constant, λ the source decay constant, and Tp the repopulation time. Resensitization can be included in the extended LQ equation (LQR) S = exp[-αD - βG(Tr)D² + 1/2σ² G(Ts)D² + Teff/Tp], where G(Tr) and G(Ts) describe repair and resensitization, and 1/2σ² represents cell-to-cell diversity. Combining Dale’s formalism with LQR led to RE = 1+(β/α)R0/(μr+λ)-(1/2σ²/α)R0/(μs+λ), where μs is the resensitization constant. We used this formula to calculate the BED for normal tissue based on the prescribed dose for Au-198 GYN permanent implants from which we have gained extensive clinical experience. Then, we calculated the dose with Cs-131 which has the equal BED as Au-198. Results: The prescribed doses for Au-198 ranged from 10 to 120 Gy. The converted doses for Cs-131 implants ranged from 9 to 161 Gy (without resensitization correction) and 8.9 to 156 Gy (with resensitization correction), which resulted in the average value of dose conversion factor, Fn (no resensitization correction) = 1.14, and 1.10 for Fr (with resensitization correction) which agreed with the results from the calculation for tumor. The doses derived with 1.10 reduced the complications such as brisk moist desquamation in actual clinical cases. Conclusion: Resensitization correction in BED for normal tissues led to significant reduction in prescription dose and thus in toxicity. The results further show that resensitization correction is needed for permanent implant dose calculation.
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