Dosimetric impact of daily MR-guided adaptive radiotherapy across different anatomical sites

MR-guided adaptive radiotherapy (MRgART) enables daily plan adaptation to account for anatomical variations; however, its resource-intensive workflow necessitates careful patient selection, and the relative benefit across anatomical sites remains to be clearly defined.

To systematically evaluate the site-dependent dosimetric impact of daily MRgART and identify anatomical regions deriving the greatest benefit from on-table adaptation.

Sixty-six patients treated with five-fraction SBRT on a 0.35 T MR-Linac were retrospectively analyzed: lung (n = 15), pancreas (n = 12), liver (n = 12), abdominal lymph node (n = 12), and prostate (n = 15). For each fraction, predicted and reoptimized plans were generated. Dose-volume parameters were normalized and cumulatively assessed using DVH summation. Target coverage (PTV D95, GTV/CTV D98), CI, GI, and OAR metrics were evaluated using paired Wilcoxon tests.

The largest median improvement in PTV D95 was observed in abdominal lymph node (5.3%), lung (4.4%), and prostate (4.1%) cohorts. Significant improvements in GTV/CTV D98 were observed in all sites except pancreas. CI improved significantly in all cohorts except prostate, whereas GI improvements were limited to abdominal lymph node and prostate cases. Although improvements in target coverage were limited in pancreatic cases, this subgroup demonstrated the most consistent reductions in gastrointestinal OAR doses. Lung cases showed significant improvement only in lung V20, while liver cases exhibited modest and heterogeneous OAR changes.

Daily MRgART provides site-dependent benefit, with the greatest clinical impact in pancreatic and abdominal lymph node cases. In high-volume settings, prioritization for daily adaptation may focus on these groups, followed by lung and prostate, while liver cases derive more limited benefit.