Impact of tumour proximity to organs-at-risk in adaptive MR-guided SBRT for central lung tumours and metastases.

BACKGROUND

Centrally located lung tumours present challenges for SBRT due to elevated toxicity risk. Online adaptive MR-guided radiotherapy (oMRgRT) offers improved target coverage and Organ at risk (OAR) sparing by accounting for interfractional anatomical changes. This study evaluated the dosimetric impact of oMRgRT, with emphasis on tumour location relative to OARs and the clinical benefit of adaptation.

METHODS

We retrospectively analysed 36 PTVs across 294 treatment sessions using a 0.35 T MR-Linac. Tumours were categorised by proximity to six critical OARs: proximal bronchial tree (PBT), trachea, heart, great vessels, brachial plexus, and oesophagus. Predicted/reoptimised plans from all fractions were compared to assess improvements in target coverage and OAR sparing. All dosimetric parameters were presented as a percentage of baseline plan metrics. Statistical tests included the Wilcoxon signed-rank test and the Mann-Whitney U test.

RESULTS

Adaptive planning significantly improved target volume dosimetry. PTV D98% increased from 92.8 ± 8.7% to 99.9 ± 1% (p < 0.01); Vprescription dose (PD) improved from 92.7 ± 5.4% to 97.7 ± 1.1% (p < 0.01). GTV D98% rose from 98.5 ± 5.5% to 100.4 ± 4.3% (p < 0.01), with VPD increasing from 97.7 ± 3.9% to 98.1 ± 3.5% (p < 0.01). Improvements in PTV coverage were observed across all subgroups, with the greatest gains in GTV coverage, most notable in tumours adjacent to the PBT and heart. Tumours near the trachea, great vessels, and brachial plexus showed minimal change. The most significant reductions in OAR doses were also seen in the heart and PBT groups, while proximity to the trachea resulted in minimal benefit.

CONCLUSION

Daily oMRgRT significantly improves target coverage and OAR sparing in centrally located tumours, especially in anatomically complex regions. The adaptive approach enables clinically meaningful trade-offs between tumour coverage and OAR sparing. Further studies are needed to refine adaptation protocols based on tumour sublocation.