title: Longitudinal diffusion MRI for treatment response assessment: Preliminary experience using an MRI-guided tri-cobalt 60 radiotherapy system.
abstract: OBJECTIVE
  To demonstrate the preliminary feasibility of a longitudinal diffusion magnetic resonance imaging (MRI) strategy for assessing patient response to radiotherapy at 0.35 T using an MRI-guided radiotherapy system (ViewRay).
  
  
  METHODS
  Six patients (three head and neck cancer, three sarcoma) who underwent fractionated radiotherapy were enrolled in this study. A 2D multislice spin echo single-shot echo planar imaging diffusion pulse sequence was implemented on the ViewRay system and tested in phantom studies. The same pulse sequence was used to acquire longitudinal diffusion data (every 2-5 fractions) on the six patients throughout the entire course of radiotherapy. The reproducibility of the apparent diffusion coefficient (ADC) measurements was assessed using reference regions and the temporal variations of the tumor ADC values were evaluated.
  
  
  RESULTS
  In diffusion phantom studies, the ADC values measured on the ViewRay system matched well with reference ADC values with <5% error for a range of ground truth diffusion coefficients of 0.4-1.1 × 10(-3) mm(2)/s. The remote reference regions (i.e., brainstem in head and neck patients) had consistent ADC values throughout the therapy for all three head and neck patients, indicating acceptable reproducibility of the diffusion imaging sequence. The tumor ADC values changed throughout therapy, with the change differing between patients, ranging from a 40% drop in ADC within the first week of therapy to gradually increasing throughout therapy. For larger tumors, intratumoral heterogeneity was observed. For one sarcoma patient, postradiotherapy biopsy showed less than 10% necrosis score, which correlated with the observed 40% decrease in ADC from the fifth fraction to the eighth treatment fraction.
  
  
  CONCLUSIONS
  This pilot study demonstrated that longitudinal diffusion MRI is feasible using the 0.35 T ViewRay MRI. Larger patient cohort studies are warranted to correlate the longitudinal diffusion measurements to patient outcomes. Such an approach may enable response-guided adaptive radiotherapy.
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Yingli
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Minsong
authors.initials: M
authors.lastname: Cao
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Ke
authors.initials: K
authors.lastname: Sheng
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Yu
authors.initials: Y
authors.lastname: Gao
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Allen
authors.initials: A
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Mitch
authors.initials: M
authors.lastname: Kamrava
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Percy
authors.initials: P
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Nzhde
authors.initials: N
authors.lastname: Agazaryan
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: James
authors.initials: J
authors.lastname: Lamb
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: David
authors.initials: D
authors.lastname: Thomas
authors.affiliation: Department of Radiation Oncology, University of California, Los Angeles, California 90095.
authors.forename: Daniel
authors.initials: D
authors.lastname: Low
authors.affiliation: Department of Radiological Sciences, University of California, Los Angeles, California 90095.
authors.forename: Peng
authors.initials: P
authors.lastname: Hu
mesh_terms: D000328:Adult; D003037:Cobalt Radioisotopes / Q000627:therapeutic use*; D038524:Diffusion Magnetic Resonance Imaging; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D006801:Humans; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided; D012509:Sarcoma / Q000000981:diagnostic imaging / Q000532:radiotherapy; D016896:Treatment Outcome
chemical_list: D003037:Cobalt Radioisotopes