1715 Assessment of tumor aggressiveness using intravoxel incoherent motion MRI in patients with papillary thyroid carcinoma

Target audience: Researchers investigating imaging biomarkers in cancer. Purpose: Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer, representing about 80 percent of all thyroid cancer cases. Thyroid surgery which includes partial thyroid lobectomy or total thyroidectomy is routinely recommended for all patients. However, recent studies have shown that even without any treatment, the vast majority of these still remain stable in size and confined to the thyroid gland when followed with active surveillance for up to 10 years. There remains an urgent need to non-invasively differentiate between patients into two groups before treatment: one group harboring indolent disease that can be followed with active surveillance, and a second group with PTC that is likely to progress and therefore requires immediate surgery. Intravoxel incoherent motion imaging (IVIM) MRI allows for non-invasive measurement of water molecular diffusion and blood perfusion simultaneously, which may provide unique imaging biomarkers to assess tumor aggressiveness. The purpose of this study was to assess tumor aggressiveness using IVIM-MRI in patients with PTC. Methods: IVIM-MRI data acquisition: Our institutional review board approved this retrospective study and issued a waiver of informed consent. 15 patients (age: 28-66 years, M/F: 4/11, and tumor location (left/right/bilateral lobe) 4/6/5) were referred for IVIM-MRI study by physicians at our institution. All patients underwent pretreatment IVIM-MRI on a GE 3T Signa scanner with an 8-channel neurovascular phased-array coil prior to surgery. IVIM-MRI acquisitions were performed using a single-shot echo planar imaging (SS-EPI) spin echo sequence (TR = 4000 ms, TE = 98-104 ms, NEX = 4, ASSET is on, and shim FOV=14 mm) with 18 b values of b = 13, 17, 23, 30, 40, 53, 70, 92, 122, 161, 212, 280, 369, 488, 644, 850, 1121, and 1448 s/mm, respectively. The IVIM-MRI scans focused on the thyroid gland: with 4-8 slices of thickness 4-6 mm, FOV of 20~24 cm, and acquisition matrix of 128 × 128. IVIM-MRI data analysis: The regions of interest (ROIs) on the tumor and normal thyroid tissues were placed by an experienced neuro-radiologist. For both tissue types, the apparent diffusion coefficients (ADC) as well as f (vascular fraction), D (pure diffusion coefficient), D (pseudo-diffusion coefficient) were calculated by using monoexponential and biexponential functions with a scheme of noise correction respectively. Histopathologic examination: All patients underwent surgery after the MRI. The surgical specimen was reviewed by an experienced pathologist. Tumor aggressiveness was evaluated for each surgical specimen using following histopathologic features: presence or absence of tall cell variants, necrosis, vascular and/or capsular invasion, extrathyroidal thyroid extension, and regional or distant metastases. The tumor was