Target motion mitigation promotes high-precision treatment planning and delivery of extreme hypofractionated prostate cancer radiotherapy: Results from a phase II study

Background and purpose While favourable long-term outcomes have been reported in organ-confined prostate cancer treated with 5 × 7–8 Gy extreme hypofractionation, dose escalation to 5 × 9–10 Gy improved local control but was associated with unacceptable rates of late rectal and urinary toxicities. The purpose of this study was to explore the feasibility of intra-fractional prostate immobilization in reducing toxicity, to promote dose escalation with extreme hypofractionated radiotherapy in prostate cancer. Material and methods 207 patients received 5 consecutive fractions of 9 Gy. An air-inflated (150 cm3) endorectal balloon and an intraurethral Foley catheter with 3 beacon transponders were used to immobilize the prostate and monitor intra-fractional target motion. VMAT-IGRT with inverse dose-painting was employed in delivering the PTV dose and in sculpting exposure of normal organs at risk to fulfil dose-volume constraints. Results Introduction of air-filled balloon induced repeatable rectum/prostate complex migration from its resting position to a specific retropubic niche, affording the same 3D anatomical configuration daily. Intra-fractional target deviations ≤1 mm occurred in 95% of sessions, while target realignment in ≥2 mm deviations enabled treatment completion as scheduled. Nadir PSA at median 54 months follow-up was 0.19 ng/mL, and bRFS was 100%, 92.4% and 71.4% in low-, intermediate- and high-risk categories, respectively. Late Grade 2 GU and GI toxicities were 2.9% and 2.4%, respectively. No adverse changes in patient-reported quality of life scores were observed. Conclusion The unique spatial configuration of this prostate motion mitigation protocol enabled precise treatment planning and delivery that optimized outcomes of ultra-high 5 × 9 Gy hypofractionated radiotherapy of organ-confined prostate cancer.