Target Coverage, Organ at Risk Metrics, and Tumor Control for Metastases to the Pancreas Treated With Adaptive MR-Guided Stereotactic Body Radiation Therapy

Abstract

Metastases to the pancreas (MPs) are difficult to treat with radiation therapy (RT) due to respiratory and bowel motion and proximity to critical organs at risk (OARs). Magnetic resonance-guided stereotactic body radiation therapy (MRgSBRT) may improve tumor coverage and OAR sparing through motion control with breath hold gating and continuous MR imaging during treatment, and the option for daily online adaptive replanning (ARP) to account for changes in tumor and/or OAR position. We evaluated the need for online adaptive MRgSBRT and its effect on target coverage, OAR metrics, and tumor control in patients with MPs.In this IRB-approved retrospective study, we reviewed the records of patients with MPs treated with MRgSBRT between July 2019 and January 2021. Patients were identified from a prospectively maintained database. Data were collected regarding dose to targets and OARs, use and rationale for ARP, toxicity, and outcomes. Treatment response was defined as a decrease in size of the treated MP. Local failures were defined as recurrences within the RT field. Descriptive statistics were used.We identified 8 MPs in 6 patients. Median age was 70 years (range, 61-88). Patients had oligoprogressive or oligometastatic renal cell carcinoma (n = 3), carcinoid (n = 1), hepatocellular carcinoma (n = 1), and lung adenocarcinoma (n = 1). The median time from diagnosis to development of metastatic disease was 17 months (range, 0-105 months) and to treatment of the MP was 36 months (range, 7-230 months). All patients received 40 Gy in 5 fractions with breath hold technique. The median GTV was 8.5 cc (range, 2.2-132.8 cc) and PTV was 27.7 cc (range, 8.3-175.7 cc). 35/40 (87%) fractions required ARP, necessary to meet stomach and duodenal metrics in 11 (27%) and 16 (40%) fractions, respectively. The mean stomach, duodenal, and PTV metrics before and after ARP are reported in Table 1. On average, ARP increased the PTV coverage by 4.9% and decreased the stomach V33, stomach V35, and duodenum V33 by 0.61 cc, 0.31 cc, and 1.10 cc, respectively. At a median follow-up of 5.0 months, 6/8 (75%) lesions had a partial response. There were no local failures. There was one distant failure 27 days after treatment. One patient developed a grade 2 pyloric ulcer which resolved with medical management. There were no other ≥ grade 2 toxicities.In this study, ARP was necessary in most MRgSBRT treatments to MPs. Adaptive MRgSBRT improved PTV coverage and OAR metrics, and it provided high rates of tumor control with minimal toxicity. Further study in larger patient populations and in peripancreatic metastases is needed.