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Top Line: When delivering simultaneous integrated boost for prostate cancer, what urethral dose is associated with increased toxicity?

The Study: The FLAME trial showed that MR-guided simultaneous integrated boost (SIB) of 95 Gy in 35 fractions improved biochemical control without increasing toxicity in men with intermediate or high risk prostate cancer. An important reason it didn’t increase toxicity was that OAR constraints were prioritized over high-dose PTV coverage. There was no urethral dose constraint, though, which begs the question of whether GU toxicity is worse in those who have a SIB volume near the urethra. FLAME bladder constraints included D1cc ≤80Gy and V72 ≤10%. In this secondary analysis, the urethra was delineated on MRI using a 6mm diameter cylindrical structure. They found that both an increasing bladder D2cc and urethra D0.1cc were associated with a continuous increased risk of grade 2+ GU toxicity. While there was no clear threshold dose for GU toxicity, the authors recommended shooting for a urethra constraint of D0.1cc ≤80Gy and a bladder constraint of D2cc ≤80 Gy as these corresponded to a ~10% cumulative risk of grade 2+ GU toxicity.

TBL: Urethral dose should be respected when delivering the FLAME simultaneous integrated boost for prostate cancer to limit grade 2+ GU toxicity. | Groen, Radiother Oncol 2021

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