Magnetization transfer ratio: a potential biomarker for the assessment of postradiation fibrosis in patients with rectal cancer.

OBJECTIVES: Magnetization transfer-magnetic resonance imaging (MT-MRI) uses differences in the magnetization interaction of the free "unbound" water protons and the macromolecular-bound protons. The aim of this study was to evaluate whether the magnetization transfer ratio (MTR) may be used to identify fibrosis in patients with rectal cancer treated with chemoradiotherapy. MATERIALS AND METHODS: This study was part of a rectal cancer imaging study, which was approved by the local institutional review board. Twenty-six patients, treated with neoadjuvant chemoradiotherapy, underwent a standard MRI including T2-weighted sequences and a diffusion-weighted sequence. An axially oriented MT sequence was performed at the center of the (former) tumor location. Regions of interest were manually drawn on the MT-MRI (with reference to the T2-weighted and diffusion-weighted images), covering areas of residual tumor, fibrosis, or the normal or edematous rectal wall. The results were compared with that of the histopathological examination. Differences in MTR between the 4 tissue types were analyzed, and a receiver operating characteristic (ROC) curve was generated to assess the diagnostic potential. RESULTS: Thirty-eight regions of interest were analyzed on the MT-MRI. The mean (SD) MTR of the fibrosis was 37.7% (2.7%), which was significantly higher than that of the residual tumor (29.6% [4.2%]; P < 0.001), the normal rectal wall (30.3% [4.7%]; P = 0.003), and the edematous rectal wall (18.2% [4.0%]; P < 0.001). The use of MTR resulted in an area under the ROC-curve of 0.96, a sensitivity of 88%, and a specificity of 90% for the diagnosis of fibrosis. CONCLUSIONS: Magnetization transfer ratio can be used to discriminate postradiation fibrosis from residual tumor and the normal rectal wall after chemoradiotherapy. Magnetization transfer imaging can thus be a promising tool for the unsolved dilemma of interpreting postradiation fibrosis in rectal cancer.