Role of apparent diffusion coefficient values in differentiation between malignant and benign solitary thyroid nodules

BACKGROUND AND PURPOSE: Accurate imaging characterization of a solitary thyroid nodule has been clearly problematic. The purpose of this study was to evaluate the role of the apparent diffusion coefficient (ADC) values in the differentiation between malignant and benign solitary thyroid nodules.MATERIALS AND METHODS: A prospective study was conducted in 67 consecutive patients with solitary thyroid nodules who underwent diffusion MR imaging of the thyroid gland. Diffusion-weighted MR images were acquired with b factors of 0, 250, and 500 s/mm² by using single-shot echo-planar imaging. ADC maps were reconstructed. The ADC values of the solitary thyroid nodules were calculated and correlated with the results of histopathologic examination. Statistical analysis was performed.RESULTS: The mean ADC value of malignant solitary thyroid nodules was 0.73 ± 0.19 × 10^?3 mm²/s and of benign nodules was 1.8 ± 0.27 × 10^?3 mm²/s. The mean ADC values of malignant nodules were significantly lower than those of benign ones (P = .0001). There were no significant differences between the mean ADC values of various malignant thyroid nodules, but there were significant differences between the subtypes of benign thyroid nodules (P = .0001). An ADC value of 0.98 × 10^?3 mm²/s was proved as a cutoff value differentiating between benign and malignant nodules, with 97.5%, 91.7%, and 98.9% sensitivity, specificity, and accuracy, respectively.CONCLUSION: The ADC value is a new promising noninvasive imaging approach used for differentiating malignant from benign solitary thyroid nodules.

Nodular thyroid is commonly detected on palpation in 4%–7% of the population,^1,2 on sonographic examination in 10%–40%, and by pathologic examination at autopsy in 50%.^3,4 In contrast, compared with the high prevalence of nodular thyroid disease, thyroid cancer is rare. The challenge of imaging thyroid nodules is to reassure most patients who have benign disease and to diagnose the minority of patients who will prove to have a malignancy.^5,6Ultrasonography has been used in the assessment of the thyroid nodules as a primary imaging technique.^2,7,8 Currently, there is no single sonographic criterion that can reliably distinguish benign from malignant thyroid nodules.^4,9 The results of predicting thyroid cancer with color Doppler sonography are controversial, with some reporting that Doppler sonography is helpful and others reporting that it did not improve diagnostic accuracy.^4,5,10,11 The hazards of radiation exposure are unavoidable in nuclear scintigraphy,² and not all functioning nodules on scintigraphy are benign.^6,9 The risk of cancer in a cold nodule is 4 times more common than in a hot nodule.^3,6 Fine-needle aspiration biopsy (FNAB) with cytologic evaluation is commonly used, but it is inconclusive in 15%–20% of patients, in addition to the possible, but less likely, associated hemorrhage.^2,4 The incidence of cancer in patients with thyroid nodules selected for FNAB is approximately 9.2%–13%.⁵ FNAB is considered an effective method for differentiating between benign and malignant thyroid nodules.^4,7,12–14Routine T1- and T2-weighted MR imaging has a limited role in the evaluation of thyroid nodules. It cannot distinguish benign from malignant nodules or assess the functional status of thyroid nodules.^12–15 Diffusion-weighted MR imaging has been used to characterize head and neck tumors, in which there are significant differences in the apparent diffusion coefficient (ADC) values of malignant tumors and benign lesions.^16–19 Tezuka et al,²⁰ in their study using diffusion-weighted MR imaging to assess the thyroid function, reported that the ADC values of patients with Grave''s disease exceeded those of patients with subacute thyroiditis, with a sensitivity and specificity of 75% and 80%, respectively, in differentiating between both disease entities. They concluded that diffusion-weighted MR imaging could be clinically important in evaluating the thyroid function. To our knowledge, there have been no articles about diffusion-weighted MR imaging evaluating thyroid nodules.The aim of our study was to evaluate the role of ADC values in differentiating between malignant and benign solitary thyroid nodules.