Multicenter analysis of MRI-guided, stereotactic body radiotherapy as ablative treatment for lymph nodal metastases of the abdomen and pelvis

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BACKGROUND: Purpose of this study was to assess safety and efficacy of magnetic resonance-imaging (MRI)-guided, stereotactic body radiotherapy (SBRT) for lymph nodal metastases (LNM) of the abdomen and pelvis. METHODS: This multi-center retrospective study was conducted at four institutions practicing online-adaptive, MRI-guided radiotherapy (MRIdian, ViewRay© or Unity MRL, Elekta©). Patients treated with MRI-guided SBRT for pelvic and abdominal LNM were enrolled. Descriptive statistics were performed. Oncological endpoints were calculated and toxicity was assessed. RESULTS: 161 patients treated for 210 lymph node metastases between May 2019 and October 2023 were included. Prostate cancer (45.3 %) was the most frequent histology. The locations of lymph node metastases were pelvic (42.2 %), retro-peritoneal (39.8 %), and intra-peritoneal (18 %) Target volume sizes varied significantly between these different anatomical sites treated, with intra-peritoneal targets being significantly larger (p < 0.001) compared to those retroperitoneal or pelvic. Patients were treated with a median dose of 35 Gy (range 25-50) in a median of five fractions (range 3-10), without differences between localizations. Acute adverse effects of grade I, II and III occurred in 23.6 %, 11.8 %, and 0.6 % of the patients, respectively. Two patients (1.2 % overall and 5.3 % of intra-abdominal cases) experienced late adverse events, both grade IV gastro-intestinal ulceration four and twelve months after irradiation of bulky intra-peritoneal LNM. The 2-years local control-rate was 85.8 %. In univariate analyses, smaller target-volumes, prostate histology, pelvic localization, and higher BED were statistically significantly associated with improved overall survival, distant control rates and freedom from progression but not with local control rates. CONCLUSION: MRI-guided SBRT for abdominal and pelvic LNM achieved durable local tumor control rates of over 85% after two years and was associated with a favorable toxicity profile. MR-guided SBRT is a valid option for ablative treatment of nodal oligometastases, even for the difficult to treat intra-abdominal targets.

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