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

Abstract

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. 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. 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. 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.