ESTRO 2021 Abstract Book

S699

ESTRO 2021

Purpose or Objective 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 and 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 35Gy in 5 fractions. For each fraction, a quantitative analysis was performed for PTV volume, V35Gy 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 intestinum) 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 V35Gy and Dmean for PTV were respectively 83.75% (72-98.8%) and 35.6Gy (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-V35Gy and PTV-Dmean). Concerning OARs, a significant difference was observed in favor of daily-adapted treatments in terms of single intestinal loop Dmax [26.9Gy (13.2-26.9Gy) at baseline vs 24Gy (12.1-24Gy); p=0.014] and Dmean [16Gy (6.518 Gy) at baseline vs 13.7Gy (6.7-17.6Gy); 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 (0.4-34.3Gy) at baseline vs 14.6Gy (0.7-34.3Gy), p=0.24; median Dmean was 2.2Gy (0.2-16.6Gy) at baseline vs 2.2Gy (0.2-16.4Gy), p=0.30. Similarly for bowel, no differences in terms of Dmax [28.7Gy (7.7-34Gy) vs 27.9Gy (7.8-33.1Gy); p=0.06] and Dmean [4.3Gy (1.3-10.9Gy) vs 3.9Gy (1.4-10.5Gy); p=0.25] were observed. ( Tables 1 and 2 )

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. PD-0864 Case reports demonstrating the clinical rationale for in vivo portal dosimetry on the Unity MR- linac B. Vivas Maiques 1 , I. Olaciregui Ruiz 2 , A. Mans 2 , T. Janssen 2 , J. Kass 2 1 The Netherlands Cancer Institute (NKI-AVL), Radiotherapy, Amsteram, The Netherlands; 2 The Netherlands Cancer Institute (NKI-AVL), Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Portal dosimetry allows for dosimetric verification of online adapted plans on the Unity MR-linac. A unique advantage is the possibility to detect workflow incidents and patient related deviations, i.e. discrepancies