Multi-Institutional Experience of MR-Guided Stereotactic Body Radiation Therapy for Adrenal Gland Metastases

Abstract

<h3>Purpose/Objective(s)</h3> While dose-escalation is associated with improved local control (LC) for adrenal gland metastases (AGM), the proximity of gastrointestinal (GI) structures limits the dose that can be safely prescribed via CT-based stereotactic body radiation therapy (SBRT). The relative advantages of magnetic resonance guided SBRT (MRgSBRT), including tumor tracking, treatment under breath hold, and online plan adaptation, create the potential for safe dose escalation. The present study investigated the efficacy and safety of MRgSBRT for treatment of AGMs across multiple institutions. <h3>Materials/Methods</h3> This is a multi-institutional retrospective review of 57 consecutive patients who underwent MRgSBRT on a 0.35-T MR Linac to 61 AGMs from April 2019 to September 2021. Univariate analysis (UVA) with the Kaplan-Meier (KM) method was used to estimate time-to-event outcomes, including overall survival (OS), progression-free survival (PFS), and LC, with group differences compared by log-rank testing. Median follow up was calculated with the reverse KM method. <h3>Results</h3> Median follow up was 12.2 months (95% CI 8.1-17.2 months). Median age at the time of SBRT was 67 years (range: 28-84 years). Primary histologies included non-small cell lung cancer (N=38), renal cell carcinoma (N=6), and melanoma (N=5), amongst others. Most AGMs were left-sided (N=32) with a median maximum diameter of 2.7cm (range: 0.6-7.6cm) treated to a median total dose of 50 Gy (range: 30-60 Gy) in 5-10 fractions with a median BED₁₀ 100 Gy (range: 48-132 Gy). Forty-five cases (74%) required adaptation, more frequently in left-sided lesions due to the proximity of the stomach and small bowel (88% vs 59%). There were 3 cases of reirradiation, including 60 Gy in 10 fractions (N=1) and 40 Gy in 5 fractions (N=2). One-year LC, PFS, and OS were 90%, 40%, and 76%, respectively. On UVA, only melanoma histology predicted for inferior 1-year LC (53% vs 92%, <i>p</i>=0.022). There was a nonsignificant improvement in LC for BED₁₀ of 132 Gy (1-year LC 100% vs 87%, <i>p</i>=0.148). No patients who received BED₁₀ 132 Gy (N=13) experienced local failure. There were no instances of grade 3+ acute or late toxicity. <h3>Conclusion</h3> Our early outcomes demonstrate MRgSBRT achieves favorable LC and no grade 3+ toxicity despite prescribing a median BED₁₀ of 100 Gy to targets in proximity of GI organs at risk. The unique advantages of MR guidance and online adaptive replanning may be especially beneficial for achieving safe dose escalation of radioresistant histologies such as melanoma.