Dosimetric Impact and Robustness of Daily Online Adaptive Radiotherapy for Focal-Boosted Prostate Stereotactic Body Radiation Therapy
Abstract
Purpose
To evaluate dosimetric trends and robustness of online adaptive radiation therapy for focal-boosted prostate SBRT. Materials/
Methods
Sixty-two patients treated for prostate cancer were analyzed on a prospective phase II clinical trial. Patients received 38 Gy to the CTV (prostate and proximal seminal vesicle), 36.25 Gy to the PTV (CTV + 2 mm), and 50 Gy simultaneous integrated boost to the mpMRI/PSMA PET-defined dominant intraprostatic lesion (DIL/GTV). Treatments were delivered on ViewRay MRIdian (n=59) or Varian Ethos (n=3). Patients optionally underwent insertion of rectal hydrogel spacer (n=56; 90%). Plans were optimized to meet clinical trial dosimetric constraints, prioritizing OAR sparing, followed by PTV/CTV coverage, then GTV coverage. Before daily treatment delivery, on-board MRI or CBCT was acquired, and targets/OARs were recontoured. The initial plan was recalculated on daily anatomy (predicted plan), and online reoptimization (adaptive plan) was performed at physician discretion. Plan adaptation frequency, dosimetric impact, and robustness with respect to bladder volume and spacer use were evaluated.
Results
Plan adaptation was selected for 96% of fractions, demonstrating improved PTV/CTV coverage while reducing OAR hotspots and low dose wash, with stable GTV coverage. Among 310 predicted fractions, 77% failed at least one planning
Objective
41% failed PTV/CTV coverage criteria, 62% failed OAR constraint, and 26% failed both. Hydrogel spacer was associated with significantly improved rectal sparing (p<0.05) for initial and adaptive planning, however under daily anatomical variation, predicted rectal dosimetry was similarly higher between spacer and non-spacer cohorts. Predicted fractions strongly exhibited bladder volume dependence for bladder V20Gy compliance, whereas adapted fractions showed uniformly high bladder compliance across all bladder-volume quartiles.
Conclusion
Online adaptive radiotherapy improved target coverage and OAR sparing on a per-fraction basis for SIB dose-painted SBRT. Adaptive planning is robust to bladder-volume variation and provides additional dosimetric benefit when combined with rectal hydrogel spacer use.