3324 - Evaluation of Organ at Risk Dose Variation under ATP only Workflow Compared with ATS in MRI Guided Prostate SBRT

Purpose/Objective(s):

Magnetic resonance-guided adaptive radiotherapy on MR-Linac, facilitates daily plan modification through two different strategies, adapt-to-position (ATP), which involves re-optimization based on isocenter shift, or adapt-to-shape (ATS), which entail recontouring and re-optimization. We conducted a comparative analysis of the dosimetric performance of MR-guided online adaptive radiotherapy using ATS and ATP for prostate cancer treated with MR guided SBRT on MR-Linac, with a focus on target coverage and organ-at-risk dose.

Materials/Methods:

In this study, nine patients diagnosed with prostate cancer underwent treatment using MR-Guided Stereotactic Body Radiation Therapy (SBRT) on MR-Linac, receiving a dose of 36-37.5 Gy across five fractions. The treatment protocol employed a daily online re-planning ATS workflow, which necessitated daily re-contouring of anatomical structures and the target areas. A Planning Target Volume (PTV) margin of 3 mm around and 2 mm at rectum, was applied. For each fraction, a daily T2-weighted MRI facilitated the re-contouring of all structures, and an ATS plan was subsequently generated. Retrospectively, an ATP plan was created for each fraction using a rigid shift between the daily and reference images, with a focus on optimizing the match at the prostate-rectum interface. Dosimetric parameters were averaged on a per-fraction basis to calculate mean values for each patient across both workflows. Statistical comparisons were performed using paired t-tests, and effect size was assessed through Cohen’s Dz.

Results:

ATS demonstrated a significant enhancement in near-minimum target dose coverage compared to ATP, as evidenced by improvements in PTV D95 (+2.82 Gy, p=0.0034, dz=1.37) and PTV D98 (+3.80 Gy, p=0.0040, dz=1.33). Additionally, CTV D95 and D98 values were notably higher with ATS (p<0.01). The rectal dose–volume exposure was reduced with ATS, including reductions in rectum V29 (-9.34%, p=0.020), rectum V36 (-2.35 cc, p=0.0396), and rectum V18.1 (-10.78%, p=0.041). Although improvements in V95% were observed, they did not reach statistical significance. The high-dose spill (PTV V110) and urethral dose metrics were comparable between the two workflows. In terms of bladder metrics, V18.1 favored ATP, while parameters such as V37 and V38.1 showed no significant differences between the groups.

Conclusion:

ATS markedly enhances target coverage and rectal sparing in comparison to ATP, without an increase in urethral dose. These results advocate for the implementation of daily contour-based adaptation to improve dosimetric robustness and the therapeutic ratio in magnetic resonance-guided prostate stereotactic body radiotherapy on MR-Linac.