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Purpose/objective(s): MR-guided radiotherapy is a potential game changer in the field of radiation oncology, given the optimal visualization of the pelvic anatomy and the possibility to daily recalculate the plan based on real-time anatomy conditions. We report preliminary dosimetric data concerning the use of 1.5-T MR-guided daily-adaptive radiotherapy for abdomino-pelvic lymph-nodal oligometastases. We aimed to assess the impact of this technology on mitigating daily variations for both target coverage and organs-at-risk (OARs) sparing.Materials/methods: A total of 150 sessions for 30 oligometastases in 23 patients were analyzed. All patients were treated with MR-guided stereotactic body radiotherapy (SBRT) for a total dose of 35 Gy in 5 fractions. For each fraction, a quantitative analysis was performed for PTV volume, V35 Gy and Dmean. Similarly, for OARs we assessed daily variations of volume, Dmean, Dmax. Any potential statistically significant change between baseline planning and daily-adaptive sessions was assessed using the Mann-Whitney test, assuming a P-value < 0.05 as significant.Results: Median baseline PTV, bowel, bladder and single intestinal loop (in the case of targets very close to intestine) volumes were respectively 6.2cc (range, 0.7-41.2cc), 993cc (119-3654cc), 75cc (39.7-202.9cc), 15.7cc (9.1-37.7cc). No significant volume variations were detected for PTV (P = 0.17) bowel (P = 0.12), bladder (P = 0.14) and single intestinal loops (P = 0.21). Median baseline V35 Gy and Dmean for PTV were respectively 83.75% (72-98.8%) and 35.6 Gy (34.6-36.1 Gy). We recorded a positive trend in favor of daily-adaptive strategy vs baseline planning for improved target coverage (P = 0.06 and P = 0.07 for PTV-V35 Gy and PTV-Dmean). Concerning OARs, a significant difference was observed in favor of daily-adapted treatments in terms of single intestinal loop Dmax [26.9 Gy (13.2-26.9 Gy) at baseline vs 24 Gy (12.1-24 Gy); P = 0.014] and Dmean [16 Gy (6.518 Gy) at baseline vs 13.7 Gy (6.7-17.6 Gy); P = 0.0016]. For both bladder and bowel no significant differences were observed for Dmax and Dmean: for bladder, median Dmax was 15.3 Gy (0.4-34.3 Gy) at baseline vs 14.6 Gy (0.7-34.3 Gy), P = 0.24; median Dmean was 2.2 Gy (0.2-16.6 Gy) at baseline vs 2.2 Gy (0.2-16.4 Gy), P = 0.30. Similarly for bowel, no differences in terms of Dmax [28.7 Gy (7.7-34 Gy) vs 27.9 Gy (7.8-33.1 Gy); P = 0.06] and Dmean [4.3 Gy (1.3-10.9 Gy) vs 3.9 Gy (1.4-10.5 Gy); P = 0.25] were observed.Conclusion: Daily-adaptive MR-guided SBRT reported a significantly improved single intestinal loop sparing for lymph-nodal oligometastases. A minor advantage was also reported in terms of PTV coverage, although not statistically significant. The potentially improved safety profile of MR-guided SBRT may lead the way to propose ultra-hypofractionated schedules or single fraction treatments.Author Disclosure: F. Cuccia: None. M. Rigo: None. D. Gurrera: None. L. Nicosia: None. V. Figlia: None. N. Giaj-Levra: None. R. Mazzola: None. F. Ricchetti: None. G. Attinà: None. A. De Simone: None. S. Naccarato: None. G. Sicignano: None. R. Ruggieri: Honoraria; Elekta, Stockholm, Sweden. Consultant; Elekta, Stockholm, Sweden. F. Alongi: Honoraria; Elekta, Stockholm, Sweden. Consultant; Elekta, Stockholm, Sweden. [ABSTRACT FROM AUTHOR]