Stereotactic Magnetic Resonance Guided Adaptive Radiation Therapy for Infradiaphragmatic Oligometastatic Disease:...

Although recent studies have demonstrated the benefits of stereotactic ablative radiotherapy (SABR) in the treatment of oligometastatic (OM) disease, toxicity remains a concern in the infradiaphragmatic region. This is largely because of challenges in soft tissue visualization, motion management, and the proximity of gastrointestinal organs at risk. Stereotactic magnetic resonance guided adaptive radiation therapy (SMART) may enhance the safety and efficacy of SABR in abdominopelvic targets.

Patients with infradiaphragmatic OM disease, including up to 5 lesions treated with SMART between May 2018 and September 2023, were evaluated. Progression-free survival, overall survival, and local control (LC) were analyzed using Kaplan-Meier and Fine and Gray proportional subhazards models, whereas treatment-related toxicities were assessed using the National Cancer Institute Common Terminology Criteria for Adverse Events v4.0 criteria.

One hundred eighty-three targets in 138 patients with primarily lung, colorectal, and noncolorectal gastrointestinal cancers were prescribed a median dose of 50 Gy in 5 fractions on a 0.35-T magnetic resonance-Linac. Overall, 62.8% of treated metastases were within 5 mm of a dose-limiting organs at risk, necessitating online adaptive planning in 670 of 875 (76.6%) delivered SABR fractions. The 1-year progression-free survival rate was 35.8% for the entire population and significantly differed between OM and oligoprogressive patients (42.4% vs 25.4%, P = .03). There was a trend toward lower LC in colorectal versus noncolorectal histology in the definitive dose group (biologically effective dose₁₀ ≥ 75 Gy) with 2-year LC rates of 74.0% versus 86.0%, respectively, P = .08. Acute and late grade ≥3 toxicities were 0% and 2.2%, respectively.

SMART is feasible and effective for treating OM disease lesions in proximity to dose-limited organs at risk. Safe dose escalation is facilitated by online adaptive radiation therapy and is associated with long-term LC. Patient selection is key to identifying which patients with OM or oligoprogressive disease should be considered most appropriate for SMART.