Impact of Daily Unity MR-Guided Adaptive Replanning on Prostate SBRT Plan Quality Compared with Reference-Plan Recalculation on Daily Anatomy
Abstract
Purpose
To quantify the impact of daily MR-guided adaptive replanning for prostate SBRT on protocol dose-constraint fulfillment, compared with non-adapted reference SBRT plans recalculated on daily anatomy, using both constraint-level pass/fail classification and the radiobiological P+ metric.
Methods
Patients with localized prostate cancer treated on a Unity MR-Linac (36.25 Gy in 5 fractions) were retrospectively analyzed. For each fraction, the reference SBRT plan was recalculated on the daily MR image set and compared with the clinically delivered daily-optimized adaptive plan. Structure-specific constraints were evaluated for PTV (D98, D99) and organs at risk including bladder (V1810, V3700), rectum (D0.5 cm³, V1810–3600), urethra (D0.03 cm³), sigmoid (D0.035 cm³), penile bulb (Dmean), and femoral heads (D1 cm³, V1450). Each constraint was categorized as: both pass, both fail, improved (adaptive-only pass), or worsened (non-adapt-only pass). P+ was calculated for each fraction, and paired t-tests were used to compare adaptive versus non-adaptive plans.
Results
Across all prostate SBRT fractions, most structure-level constraints fell into the “both pass” category, indicating similar baseline dosimetric adequacy between recalculated and adaptive plans. However, a meaningful subset of constraints, particularly PTV D98/D99 and selected rectum and bladder endpoints, were improved, where only the adaptive plan met protocol criteria. Worsened constraints, where only the non-adapted recalculation passed, were relatively uncommon and primarily involved rectum and urethra metrics. P+ values were more favorable for adapted plans, and all paired t-tests yielded p-values < 0.05, confirming a systematic dosimetric and biological advantage for daily adaptation.
Conclusion
Unity-based daily adaptive replanning for prostate SBRT to 36.25 Gy increases the proportion of fractions in which key PTV and pelvic OAR constraints are satisfied compared with simple reference-plan recalculation on daily anatomy. These findings support routine daily adaptation in prostate MRgRT to safeguard PTV coverage and rectum/bladder dose endpoints in the setting of day-to-day anatomical variation.