Session Item

Stereotactic body radiotherapy (SBRT) is an effective treatment for oligometastatic disease. However, target proximity to organs at risk (OARs) within the pelvis may limit safe delivery of an ablative dose. Magnetic resonance (MR)- guided adaptive radiotherapy (MRgART) may improve the therapeutic ratio. This study assessed the feasibility of MRgART for pelvic lymph node oligometastases.

Nine patients with pelvic lymph node oligometastases were treated with MRgART. Eight patients had single pelvic lymph node metastases, and one patient had three pelvic lymph node metastases. Plans were prescribed to 30-40 Gy in 3-5 fractions with the goal of 95% planning target volume (PTV) to receive 100% of the prescribed dose, subject to strict OAR constraints. Daily real-time adaptive plans were created. Treatment times, dosimetric comparisons and acute toxicity were prospectively recorded. Acute toxicity was reported according to Common Terminology Criteria for Adverse Events v.5 acute toxicity (within 3 months after the end of treatment).

A total of 37 fractions (adapted plans) were delivered to 9 patients. Pre-treatment plans met the all the OAR criteria for all patients, while PTV dose criteria were met for 7 patients. 34/37 adapted plans met all OARS criteria while 29/37 adapted plans met PTV dose criteria. Violations were primarily caused by surrounding OARs overlapping or adjacent to the PTV. Mean session duration (patient setup, adaptive plan creation and treatment delivery) was 43.5 minutes and well tolerated by all patients. Eight patients had complete data on acute toxicity at 3-month follow-up, only 1 patient experienced grade 1 acute toxicity related treatment. 

SBRT for pelvic lymph node oligometastases using MRgART is feasible based on dose criteria, plan quality metrics, treatment session duration and acute toxicity. Follow-up is awaited to allow evaluation of late toxicity and patient reported outcomes.