Impact of daily plan adaptation on accumulated doses in ultra-hypofractionated magnetic resonance-guided radiation therapy of prostate cancer.

BACKGROUND AND PURPOSE: Ultra-hypofractionated online adaptive magnetic resonance-guided radiotherapy (MRgRT) is promising for prostate cancer. However, the impact of online adaptation on target coverage and organ-at-risk (OAR) sparing at the level of accumulated dose has not yet been reported. Using deformable image registration (DIR)-based accumulation, we compared the delivered adapted dose with the simulated non-adapted dose.

MATERIALS AND METHODS: Twenty-three prostate cancer patients treated at two clinics with 0.35 T magnetic resonance-guided linear accelerator (MR-linac) following the same treatment protocol (5 × 7.5 Gy with urethral sparing and daily adaptation) were included. The fraction MR images were deformably registered to the planning MR image. Both non-adapted and adapted fraction doses were accumulated with the corresponding vector fields. Two DIR approaches were implemented. PTV* (planning target volume minus urethra+2mm ) D 95%, CTV* (clinical target volume minus urethra) D 98%, and OARs (urethra+2mm , bladder, and rectum) D 0.2cc, were evaluated. Statistical significance was inferred from a two-tailed Wilcoxon signed-rank test ( p  < 0.05).

RESULTS: Normalized to the baseline, the accumulated PTV* D 95% increased significantly by 2.7 % ([1.5, 4.3]%) through adaptation, and the CTV* D 98% by 1.2 % ([0.1, 1.7]%). For the OARs after adaptation, accumulated bladder D 0.2cc decreased by 0.4 % ([-1.2, 0.4]%), urethra+2mm D 0.2cc by 0.8 % ([-1.6, -0.1]%), while rectum D 0.2cc increased by 2.6 % ([1.2, 4.9]%). For all patients, rectum D 0.2cc was still below the clinical constraint. Results of both DIR approaches differed on average by less than 0.2 %.

CONCLUSIONS: Online adaptation in MRgRT improved target coverage and OARs sparing at the level of accumulated dose.