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What is the optimal fractionation for image-based high-dose-rate brachytherapy boost in medically inoperable
Abigail Groszkiewicz1, Jamie L Lesnock2, Paniti Sukumvanich2
1Department of Radiation Oncology, University of Pittsburgh Medical Center, Pittsburgh, PA.
Background And Purpose:
For the treatment of medically inoperable endometrial cancer, there is currently no standard recommended fractionation regimen for brachytherapy boost after completion of external beam radiotherapy (EBRT). This study compares disease control and toxicity with 4 fractions (Fx) versus 5 Fx of brachytherapy.
Materials And Methods:
Single institution retrospective review of medically inoperable patients treated with EBRT followed by high-dose-rate (HDR) brachytherapy boost. Brachytherapy was performed using three-dimensional image-based planning with magnetic resonance (MR) imaging each fraction when able and was delivered as either 25 Gy in 5 Fx or 24 Gy in 4 Fx based on a transition in the institutional standard. Primary endpoints were locoregional failure (LRF) and late toxicity.
Results:
Ninety-four patients were included; 5 Fx n = 64, 4 Fx n = 30. Overall median follow-up was 39.8 months; 5 Fx 50.6 months, 4 Fx 25.8 months. Both groups received similar doses in equivalent 2 Gy fractions (EQD2) to the high-risk clinical target volume (HRCTV) (p = 0.26), and the 4 Fx group received significantly less dose to critical organs at risk (OARs) including the rectum (p = 0.02), sigmoid (p < 0.01), and bladder (p = 0.02). Comparing the 5 Fx group to the 4 Fx group, there was no significant difference in 5-year LRF (11.2% vs. 0.0%, p = 0.24) or in 5-year overall grade 3+ late toxicity (7.7% vs. 5.9%, p = 0.81).
Conclusion:
Treatment of inoperable endometrial cancer with 4 Fx of brachytherapy boost compared to 5 Fx after EBRT does not compromise cancer control or toxicity and was associated with less dose to OARs; this de-escalation suggests potential to widen the therapeutic ratio for HDR brachytherapy boost.
