Prospective planning comparison of magnetic resonance-guided vs. internal target volume-based stereotactic body radiotherapy of hepatic metastases - Which patients do really benefit from an MR-linac?

Purpose/objective: To compare online MR-guided SBRT (MRgRT) of liver metastases with state-of-the-art ITV-based SBRT (ITV-SBRT) and assess which patients benefit most from MRgRT.

Materials And Methods: In a prospective randomized trial (MAESTRO study, NCT05027711), patients were randomized to either gated and online adaptive MRgRT or ITV-SBRT if a biologically effective dose (BED) of 100 Gy was feasible with ITV-SBRT. Otherwise, patients were treated with MRgRT. In this subgroup analysis of 20 patients, a dosimetric comparison of MRgRT and ITV-SBRT plans was performed. Tumor control and normal tissue complication probabilities were calculated.

Results: In 40 % of all patients, MRgRT enabled SBRT with less fractions and/or higher prescription BED. Almost all target volume metrics were improved with MRgRT. MRgRT PTV D95% was significantly higher in the overall cohort (91.0 ± 22.9 Gy vs. 79.5 ± 27.2 Gy, p = 0.001), in uncritical (111.3 ± 6.2 Gy vs. 104.7 ± 4.1 Gy, p = 0.022) and in critical cases with limited healthy liver volume or nearby gastrointestinal organs at risk (74.1 ± 16.9 Gy vs. 58.5 ± 18.5 Gy, p = 0.041). Target volume V100% was also superior with MRgRT. Calculated 2-year tumor control probability was significantly superior with MRgRT overall (73.0 ± 6.2 % vs. 69.7 ± 7.9 %, p = 0.002), in uncritical cases (78.3 ± 1.4 % vs. 76.8 ± 1.0 %, p = 0.022) and in critical cases (68.5 ± 4.8 % vs. 63.8 ± 5.8 %, p = 0.041), without elevated normal tissue complication probability.

Conclusion: Dosimetrically, gated MRgRT was beneficial for virtually all the hepatic metastases analyzed in this study. Patients with metastases located critically near gastrointestinal OAR or with limited healthy liver volume should be allocated to centers providing MRgRT.