Determination of prescription dose for Cs-131 permanent implants using the BED formalism including resensitization...

The current widely used biological equivalent dose (BED) formalism for permanent implants is based on the linear-quadratic model that includes cell repair and repopulation but not resensitization (redistribution and reoxygenation). The authors propose a BED formalism that includes all the four biological effects (4Rs), and the authors propose how it can be used to calculate appropriate prescription doses for permanent implants with Cs-131.

A resensitization correction was added to the BED calculation for permanent implants to account for 4Rs. Using the same BED, the prescription doses with Au-198, I-125, and Pd-103 were converted to the isoeffective Cs-131 prescription doses. The conversion factor F, ratio of the Cs-131 dose to the equivalent dose with the other reference isotope (Fr: with resensitization, Fn: without resensitization), was thus derived and used for actual prescription. Different values of biological parameters such as α, β, and relative biological effectiveness for different types of tumors were used for the calculation.

Prescription doses with I-125, Pd-103, and Au-198 ranging from 10 to 160 Gy were converted into prescription doses with Cs-131. The difference in dose conversion factors with (Fr) and without (Fn) resensitization was significant but varied with different isotopes and different types of tumors. The conversion factors also varied with different doses. For I-125, the average values of Fr/Fn were 0.51/0.46, for fast growing tumors, and 0.88/0.77 for slow growing tumors. For Pd-103, the average values of Fr/Fn were 1.25/1.15 for fast growing tumors, and 1.28/1.22 for slow growing tumors. For Au-198, the average values of Fr/Fn were 1.08/1.25 for fast growing tumors, and 1.00/1.06 for slow growing tumors. Using the biological parameters for the HeLa/C4-I cells, the averaged value of Fr was 1.07/1.11 (rounded to 1.1), and the averaged value of Fn was 1.75/1.18. Fr of 1.1 has been applied to gynecological cancer implants with expected acute reactions and outcomes as expected based on extensive experience with permanent implants. The calculation also gave the average Cs-131 dose of 126 Gy converted from the I-125 dose of 144 Gy for prostate implants.

Inclusion of an allowance for resensitization led to significant dose corrections for Cs-131 permanent implants, and should be applied to prescription dose calculation. The adjustment of the Cs-131 prescription doses with resensitization correction for gynecological permanent implants was consistent with clinical experience and observations. However, the Cs-131 prescription doses converted from other implant doses can be further adjusted based on new experimental results, clinical observations, and clinical outcomes.