E Doraisamy*, T Baig, S Koerner, C Houser, H Kim, UPMC Hillman Cancer Center, Pittsburgh, PA

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PO-GePV-T-59 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate the effect of total reference air kerma (TRAK) on reduced ureter dose in MRI based high dose rate (HDR) brachytherapy for cervical cancer, and to determine whether ring-to-tandem TRAK ratio may serve as a quality metric in plan evaluation.

Methods: 20 fractions of HDR brachytherapy delivered to 4 patients were retrospectively analyzed. All fractions had T2 weighted MRI image acquired for planning, with ureters contoured as an OAR and with maximum ureter dose from the original plan recorded. Earlier research from our institution demonstrated that development of ureteral stenosis was associated with ureter EQD2 D0.1cc greater than 77 Gy. All patients in this study exceeded this ureter dose, and we replanned each fraction to meet this plan constraint without compromising HRCTV coverage and without increasing dose to other OARs when compared to the original plan. Ring to tandem TRAK ratio was recorded for the original plan and re-plan. We also recorded patient external beam radiotherapy plan and use of parametrial boost.

Results: All 4 patients in this study were treated with IMRT, 45 Gy in 25 fractions; 2 of them received additional parametrial boost of 5.4 Gy. Reduction in ureter dose was achieved for all patients, with per-patient decreases of 8.1% to 27.4% in the mean ring to tandem TRAK ratio.

Conclusion: We demonstrate that it is possible to achieve maximum ureter D0.1cc < 77 Gy while maintaining HRCTV coverage. Replan decrease in TRAK ratio suggests that TRAK ratio may be useful as a quality metric in reducing the ureteral dose, regardless of availability of MRI-based ureter OAR delineation. Further study needs to be done to determine the effectiveness of TRAK ratio as a quality metric when ureters are not visualized or contoured.

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