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• W4386986204 abstract "Purpose High-dose-rate brachytherapy (HDR) is delivered by sequentially moving (“stepping”) a single radioactive source through a series of dwell positions. For any given point, this delivery method produces a series of uniquely variable dose-rate functions based on how the source is sequenced through dwell positions. While the sequencing in the dose-rate function does not affect physical dose for a fixed point, prior studies with explicit modeling of the radiobiological intrafraction sublethal damage repair (iSLDR) process have suggested non-trivial changes in biologically effective doses are theoretically possible. This work serves as a preliminary investigation into the impact upon biologically effective dose from intentional resequencing of dwell positions in modern HDR brachytherapy. Materials and Methods A series of dose-rate functions were constructed to deliver the same the total dose of 19 Gy over a delivery duration of 972 s. In each of the functions, a “two-pulse” dose-rate function was created which contained two high rate pulses (HRP) and the remainder of dose delivered with low rate pulse (LRP). Each HRP had a duration of 97.2 s with a dose rate of 7.72 cGy/s. And the remainder of delivery time (777.6 s) was a LRP with a dose rate of 0.51 cGy/s. For a reference, a dose-rate function with no HRP was also created with time-averaged uniform dose-rate. The biologically effective dose (BED) for each 2-pulse dose-rate function and the reference function was calculated with inclusion of a closed-form modification of the standard Lea-Catcheside dose protraction factor for a sequence-specific series of discrete dose-rates, gss. The gss was incorporated into the standard linear-quadratic calculation as BED=nd*(1+gss*d/(α/β)) such that gss was implicitly associated with only the “repairable” proportion of iSLDR to the β component of BED. BED calculations were performed using a repair half-time of 0.27 h and α/β=3 Gy, as drawn from AAPM Task Group 137 for prostate brachytherapy. Calculations were repeated using α/β=10 and 1.5 Gy. Results The total delivery time was evenly split into ten time-bins. Nine two-pulse HRP dose-rate sequences were created as shown in Table I. In each, the first HRP was sequenced into one of the first nine time-bins; and the second HRP was always sequenced in the last time-bin. The reference time-averaged uniform dose rate was created with a time-averaged uniform dose rate of 1.95 cGy/s. Among the two-pulse sequences, gss ranged from 0.7553 to 0.8709; gss=0.8040 for the reference time-averaged uniform dose rate sequence. Sequences with the most elapsed time between HRPs (i.e. time bins 1 and 10) had lower BED than the reference, by as much as -3.7%, -5.1%, -5.5% for α/β=10, 3, and 1.5 Gy, respectively. For HRP delivered immediately in succession (i.e. time bins 9 and 10), the BED was higher than reference, by as much as +5.0%, 7.0%, and 7.6% for α/β=10, 3, and 1.5 Gy, respectively. Among the sequences, α/β=10 Gy produced the narrowest range of relative BED (-3.7% to +5.0%), while α/β=1.5 Gy produced the broadest range of relative BED (-5.5% to 7.6%). Conclusions Our results demonstrate that, in presence of iSLDR, different dwell position sequencing can produce different biologically effective dose for a point with a fixed physical dose value, that are different from the time-averaged uniform dose-rate approximation. Optimizing the source dwell position sequencing can be a clinically viable approach to further modify the biologically effective dose in HDR delivery for enhanced target coverage and reduced effect on sensitive organs. High-dose-rate brachytherapy (HDR) is delivered by sequentially moving (“stepping”) a single radioactive source through a series of dwell positions. For any given point, this delivery method produces a series of uniquely variable dose-rate functions based on how the source is sequenced through dwell positions. While the sequencing in the dose-rate function does not affect physical dose for a fixed point, prior studies with explicit modeling of the radiobiological intrafraction sublethal damage repair (iSLDR) process have suggested non-trivial changes in biologically effective doses are theoretically possible. This work serves as a preliminary investigation into the impact upon biologically effective dose from intentional resequencing of dwell positions in modern HDR brachytherapy. A series of dose-rate functions were constructed to deliver the same the total dose of 19 Gy over a delivery duration of 972 s. In each of the functions, a “two-pulse” dose-rate function was created which contained two high rate pulses (HRP) and the remainder of dose delivered with low rate pulse (LRP). Each HRP had a duration of 97.2 s with a dose rate of 7.72 cGy/s. And the remainder of delivery time (777.6 s) was a LRP with a dose rate of 0.51 cGy/s. For a reference, a dose-rate function with no HRP was also created with time-averaged uniform dose-rate. The biologically effective dose (BED) for each 2-pulse dose-rate function and the reference function was calculated with inclusion of a closed-form modification of the standard Lea-Catcheside dose protraction factor for a sequence-specific series of discrete dose-rates, gss. The gss was incorporated into the standard linear-quadratic calculation as BED=nd*(1+gss*d/(α/β)) such that gss was implicitly associated with only the “repairable” proportion of iSLDR to the β component of BED. BED calculations were performed using a repair half-time of 0.27 h and α/β=3 Gy, as drawn from AAPM Task Group 137 for prostate brachytherapy. Calculations were repeated using α/β=10 and 1.5 Gy. The total delivery time was evenly split into ten time-bins. Nine two-pulse HRP dose-rate sequences were created as shown in Table I. In each, the first HRP was sequenced into one of the first nine time-bins; and the second HRP was always sequenced in the last time-bin. The reference time-averaged uniform dose rate was created with a time-averaged uniform dose rate of 1.95 cGy/s. Among the two-pulse sequences, gss ranged from 0.7553 to 0.8709; gss=0.8040 for the reference time-averaged uniform dose rate sequence. Sequences with the most elapsed time between HRPs (i.e. time bins 1 and 10) had lower BED than the reference, by as much as -3.7%, -5.1%, -5.5% for α/β=10, 3, and 1.5 Gy, respectively. For HRP delivered immediately in succession (i.e. time bins 9 and 10), the BED was higher than reference, by as much as +5.0%, 7.0%, and 7.6% for α/β=10, 3, and 1.5 Gy, respectively. Among the sequences, α/β=10 Gy produced the narrowest range of relative BED (-3.7% to +5.0%), while α/β=1.5 Gy produced the broadest range of relative BED (-5.5% to 7.6%). Our results demonstrate that, in presence of iSLDR, different dwell position sequencing can produce different biologically effective dose for a point with a fixed physical dose value, that are different from the time-averaged uniform dose-rate approximation. Optimizing the source dwell position sequencing can be a clinically viable approach to further modify the biologically effective dose in HDR delivery for enhanced target coverage and reduced effect on sensitive organs." @default.
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• W4386986204 title "PO77" @default.
• W4386986204 doi "https://doi.org/10.1016/j.brachy.2023.06.178" @default.
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