Feasibility and Initial Results of Daily Diffusion Weighted Imaging (DWI) Metrics Obtained during Stereotactic Body Radiation Therapy (SBRT) delivered using a 1.5 MR Linac

Abstract

Changes in DWI metrics, specifically the apparent diffusion coefficient (ADC), of tumors following concurrent chemotherapy and radiation therapy (chemo-RT) were shown to correlate with clinical outcomes. Such changes have been primarily described in patients undergoing conventionally fractionated chemo-RT. DWI changes that occur following treatment with SBRT remain poorly understood. We sought to characterize changes in daily DWI metrics collected during the short course of SBRT, using a 1.5 MR Linac. Patients of various tumor sites undergoing SBRT on a 1.5 MR Linac were included. On each treatment day, patients had a fully quantitative DWI that was acquired during the time for daily treatment adaption. DWI was acquired using a single-shot spin echo EPI sequence (FOV: 380 mm2, matrix: 128x128, TE: 69 ms, TR: 5500 ms, four b-values ranging from 0 to 550 s/mm). Fat suppression, acceleration, and number of averages per b-value were optimized to maximize image quality and minimize geometric distortion. Gross tumor volumes (GTVs) were contoured daily by the attending physician using an image processing software. Distributions of ADC values for voxels contained in the GTV were used to determine imaging features including mean, median, kurtosis, skewness and standard deviation. Student’s t-test was used to compare tumor ADC values. Of 38 patients screened, 27 patients (17 males, 10 females) had daily DWI scans available for analysis. The median age was 68. Patients received total doses ranging from 24Gy to 60Gy and fraction number ranged from 3 to 6. 128 DWI scans were analyzed. Mean tumor ADCs ranged from 0.79 to 2.08, median ADCs ranged from 0.75 to 1.99, kurtosis ranged from -1.11 to 3.61, standard deviations ranged from 0.23 to 0.93 and skewness ranged from -0.86 to 1.36. The comparison of mean tumor ADC values measured on the first day of treatment to those measured on the last day of treatment failed to reveal a statistically significant difference (p = 0.087). 59% of patients had an increase in mean ADC at the time of delivery of the second fraction. That percentage was 63% at the third fraction, 80% at the fourth fraction and 75% at the fifth fraction. We characterized daily DWI metrics for patients undergoing SBRT using a 1.5 MR Linac. We observe measurable changes in certain image features (mean, median, kurtosis, standard deviation and skewness) of tumor ADC maps. Although there was no statistical difference between the mean ADC values measured at the first and last fractions, we noted that an increasing proportion of patients exhibits a favorable rise in mean or median tumor ADC throughout the course of SBRT. The ability to longitudinally track MRI metrics represents a novel potential biomarker of response to SBRT that warrants further investigation and correlation with clinical endpoints.