PHSOR04 Presentation Time: 12:45 PM: Investigating the Implications of a Newly-Available Higher Initial Source Strength for Ultrahypofractionated High-Dose-Rate 192Ir Brachytherapy in Prostate Monotherapy

Christopher Tien,Zhe (Jay) Chen

doi:10.1016/j.brachy.2022.09.022

Abstract

<h3>Purpose</h3> Recently, ultrahypofractionated (UHF) regimens for high-dose-rate (HDR) Ir-192 prostate monotherapy have been becoming more popular. However, as fewer fractions are being used, the dose per fraction and, correspondingly, the delivery time per fraction increase. For example, the delivery time for a single-fraction regimen (∼19 Gy) is more than triple the delivery time per fraction in a 9-fraction regimen (∼6 Gy/fraction). As the source strength decreases exponentially over time, delivery times can be longer than 30 minutes for single-fraction regimens which is on the same magnitude as plausible values for prostate repair half-time. As radiobiological models show a supra-linear relationship between biological effective dose (BED) and delivery time, the aim of this investigation was to quantify the impact of adopting a newly-available 61050 U source strength to reduce delivery times in three common prescription regimens: two UHF regimens (1-fx and 2-fx) and a moderate hypofractionation (9-fx). <h3>Materials and Methods</h3> The simplified BED was estimated using the linear quadratic (LQ) model in its traditional form BED<sub>T</sub>=nd(1+d/α/β). To account for the sublethal damage repair during protracted delivery, the Lea-Catcheside factor, g<sub>0</sub>, was used to modify the two-track term of the LQ model, β, such that BED=nd(1+g<sub>0</sub>d/α/β) with radiobiological parameters of α/β=3.0 Gy and repair half-time of 0.27 h drawn from AAPM TG-137 recommendations. Calculations were initially performed assuming 40700 U (10 Ci) of Ir-192 with a 90-day exchange cycle for 9 × 6 Gy, 2 × 13.5 Gy, and 1 × 19Gy regimens. This was repeated using a 61050 U (15 Ci) source Delivery time was linearly scaled based on 19 Gy prescription dose delivered in 15 minutes. <h3>Results</h3> Compared with the BED<sub>T</sub>, the inclusion of g<sub>0</sub> decreased the BED<sub>T</sub> by up to 30%. Of the three prescriptions studied, both BED<sub>T</sub> and relative BED (BED/BED<sub>T</sub>) were lowest for the 1-fx regimen. The relative BED is shown in Figure 1. For the 9-fx regimen, BED<sub>T</sub>=162.0 Gy and the relative BED using 40700 U and 61050 U sources ranged from 0.906-0.957 and 0.935-0.971, respectively. For the 2-fx regimen, BED<sub>T</sub>=148.5 Gy and the relative BED using 40700 U and 61050 U sources ranged from 0.815-0.909 and 0.867-0.937, respectively. For the 1-fx regimen, BED<sub>T</sub>=139.3 Gy and the relative BED using 40700 U and 61050 U sources ranged from 0.692-0.841 and 0.773-0.889, respectively. As seen in Figure 1, the resultant change in relative BED is similar going from 1 to 2 fraction schemes as with going from 2 to 9 fraction schemes, mainly due to the exponential nature of g<sub>0</sub> against delivery time. The 61050 U source has an increased dose rate for nearly half (∼43 days) of a quarterly (90-day) source exchange cycle before decaying to 40700 U. <h3>Conclusions</h3> A BED calculation which incorporated sublethal damage repair was compared against the traditional BED<sub>T</sub>. Relative BED was reduced by up to 30% in the 1-fx regimen for prostate monotherapy due to the fast intrafractional sublethal damage repair time in prostate paired with increasingly longer delivery times. The newly-available 61050 U source may provide one practical method for an institution to recoup BED for nearly half (∼43 days) of a typical 90-day source exchange cycle before decaying to 40700 U. Future work will study the impact on other common HDR brachytherapy sites such as cervix, vagina, breast, and skin.

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