Methodology and Feasibility of Acquiring Pancreatic Diffusion Weighted Imaging (DWI) Using A Linear Accelerator Equipped with a 1.5 Magnetic Resonance Imaging (MRI) System

Abstract

Changes in diffusion weighted image (DWI) parameters during chemotherapy (chemo) for pancreatic adenocarcinoma have been shown to correlate with clinical outcomes. It remains poorly understood if similar changes occur during a treatment course with radiation therapy (RT). The growing prevalence of integrated magnetic resonance imaging (MRI) and linear accelerators (MR-Linac) offers an opportunity to acquire advanced MRI sequences easily and routinely during a course of RT. We sought to determine the feasibility of acquisition and subsequent analysis of DWI using a 1.5 MR-Linac for patients undergoing RT for pancreatic cancer and investigate changes of mean tumor apparent diffusion coefficient (ADC) values during concurrent chemo-RT. Patients undergoing RT for pancreatic adenocarcinoma on conventional RT linear accelerators were enrolled in a prospective imaging trial and had fully quantitative MRI imaging with weekly DWI acquired using a 1.5 MR Linac. DWI was acquired using a free breathing single-shot spin echo EPI sequence (FOV: 380 mm2, matrix: 128x128, TE: 76 ms, TR: 3000 ms, ten b-values ranging from 0 to 800 s/mm). Fat suppression type, acceleration, and number of averages per b-value were optimized to maximize image quality and minimize geometric distortion. Tumors were contoured using imaging processing software and mean ADC values for each tumors were calculated. A total of 6 patients enrolled on this prospective imaging clinical trial (NCT03500081). The median patient age was 65 and the median tumor size was 37mm. Five of six patients were males and one was female. The resectability status included two resectable, one borderline resectable, one locally advanced type A, and two locally advanced type B patients. Each patient had a minimum of two MR scans acquired during their course of treatment, with a total of 20 MRI’s with DWI images available. Multiple optimizations were made in the DWI scan parameters including SPAIR fat suppression, SENSE factor of 2.33, and b-value averages ranging between 2 and 7. These changes resulted in a mean DWI scan time of 312 seconds. The range of measured ADC values was 0.69 to 1.76. Measured changes in ADC values were variable; 3 patients demonstrated a measurable increase in mean ADC while 3 patients had stable ADC values with increasing radiation dose. We present the first series to demonstrate the feasibility of acquiring DWI MR images using a commercially available 1.5 MR Linac in patients with pancreatic adenocarcinoma. Our results reveal measurable changes in mean tumor ADC that occur in some patients during a course of RT for pancreatic adenocarcinoma. Such data presents a highly promising methodology for ADC-based biologically adaptive RT and future correlation with pathologic treatment response.