Clinical Radiology 69 (2014) 1198e1199

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Correspondence

RE: Ultrasonographic elastography of thyroid nodules: Is adding strain ratio to colour mapping better? Sir d We read with great interest the recently published article entitled “Ultrasonographic elastography of thyroid nodules: is adding strain ratio to colour mapping better?”.1 The authors concluded that sonoelastography is helpful to predict malignant thyroid nodules, but adding strain ratio (SR) analysis to colour mapping does not improve the diagnostic performance compared to colour mapping alone. According to their study, the best cut-off value was found to be 1.21 to predict malignancy for the SR analysis. We would like to add a few comments about SR evaluation. In the literature, various cut-off values for SR have been suggested for the discrimination of malignancy in thyroid nodules, which might question the use of SR analysis in daily practice. The sources of variability in real-time sonoelastographic SR analysis using external compression can occur during selection of an imaging plane and applying different compression levels, which creates different colour patterns. We would like to share our experience related to this variability in two different studies, which highlights the importance of selection of sample size. In a preliminary study in which we aimed to evaluate sono-elastography to investigate 40 solitary thyroid nodules (34 benign and six malignant), the best cut-off point to differentiate benign from malignant nodules was found to be 3.25 for the SR.2 In another study comprising 150 thyroid nodules (141 benign and nine malignant), the best cut-off point for SR was calculated to be 1.935 to discriminate malignancy (p ¼ 0.000), with 100% sensitivity, 76% specificity, 100% negative predictive value, 78.5% positive predictive value, and 78% accuracy rate.3 The variability in the cut-off points used in different studies in the literature is mainly attributed to the selection of the study sample. Due to selection bias, many studies also include a higher proportion of malignant nodules (w25%, range 5e63%) compared to the general population (w5%) as indicated in the literature.4 Therefore, there is a need for further studies with larger

DOI of original article: http://dx.doi.org/10.1016/j.crad.2014.06.008.

sample sizes and a proportion of malignant nodules similar to that observed in the general population to determine the best cut-off value in SR analysis to discriminate malignancy in thyroid nodules.

References 1. Chong Y, Shin JH, KO ES, et al. Ultrasonographic elastography of thyroid nodules: is adding strain ratio to colour mapping better? Clin Radiol 2013;68:1241e6. 2. Tatar IG, Kurt A, Yilmaz KB, et al. The learning curve of real time elastosonography: a preliminary study conducted for the assessment of malignancy risk in thyroid nodules. Med Ultrason 2013;15:278e84. 3. Tatar IG, Kurt A, Yilmaz KB, et al. The role of elastosonography, gray-scale and colour flow Doppler sonography in prediction of malignancy in thyroid nodules. Radiol Oncol 2014. http://dx.doi.org/10.2478/raon-2014-0007 [E-pub ahead of print]. 4. Bhatia KS, Lee YY, Yuen EH, et al. Ultrasound elastography in the head and neck. Part II. Accuracy for malignancy. Cancer Imaging 2013;13:260e76.

lu I.G. Tatar*, O. Ergun, A. Kurt, B. Hekimog Diskapi Training and Research Hospital, Diskapi-Altındag, Ankara, Turkey * Guarantor and correspondent: I.G. Tatar, Ankara Diskapi Training and Research Hospital, Department of Radiology, 06110 Diskapi-Altındag, Ankara, Turkey. Tel.: þ90 312 5962616; fax: þ90 3122307649. E-mail addresses: [email protected], [email protected] (I.G. Tatar) http://dx.doi.org/10.1016/j.crad.2014.06.021 Ó 2014 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

RE: Ultrasonographic elastography of thyroid nodules: Is adding strain ratio to colour mapping better? A reply Sir d We thank Dr Tatar and colleagues for their comments on our article.1 They pointed out that the variability in the cut-off points of different studies are caused by selection of the study sample.2,3 In our opinion, another

Correspondence / Clinical Radiology 69 (2014) 1198e1199

important reason for different cut-off levels is likely to depend on the ultrasonography (US) devices used. Currently, there are many elastographic US devices from various companies. The cut-off values can be different according to the manufacturer, although similar elastographic types are used. We attempted to obtain the cut-off value of the strain ratio (SR) for our US device, the B-mode of which is ranked as the highest resolution in the thyroid field. Unfortunately, adding SR to colour mapping did not improve performance compared to colour mapping alone. A higher proportion (about 25%) of malignant thyroid nodules in many studies has been included compared to the general population (approximately 5%),4 which may cause selection bias. Malignant nodules compromised of 51.4% of our 142 cases. We are not sure that the percentage of malignant nodules influences the selection bias. However, it is avoidable because tissue obtained from surgery should be used as the reference standard. The definition of benign nodules is crucial and a sufficient follow-up period is required if patients with benign nodules are not going to undergo surgery. Our study included benign nodules that had undergone fine-needle aspiration (FNA), but not thyroid surgery, and that were monitored using US follow-up examinations for at least 24 months to minimize falsenegative diagnoses. Diagnosis of thyroid nodules using FNA has several limitations. Slow-growing papillary carcinomas or follicular neoplasms can occasionally be misdiagnosed as benign using FNA. Data that frequently included these lesions in which a surgical specimen was not obtained would have more confounding factors. It is doubtful whether data comprising only 5% malignant nodules would be accurate as most of the remaining benign nodules would not undergo surgical confirmation

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and it is unclear how long benign nodules should be followed-up. Therefore, we agree that there is a need for further research with larger sample sizes and a sufficient follow-up period to determine current elastographic values.

References 1. Chong Y, Shin JH, KO ES, et al. Ultrasonographic elastography of thyroid nodules: is adding strain ratio to colour mapping better? Clin Radiol 2013 Dec;68(12):1241e6. 2. Tatar IG, Kurt A, Yilmaz KB, et al. The learning curve of real time elastosonography: a preliminary study conducted for the assessment of malignancy risk in thyroid nodules. Med Ultrason 2013;15:278e84. 3. Tatar IG, Kurt A, Yilmaz KB, et al. The role of elastosonography, gray-scale and colour flow Doppler sonography in prediction of malignancy in thyroid nodules. Radiol Oncol 2014. http://dx.doi.org/10.2478/raon-20140007 [E-pub ahead of print]. 4. Bhatia KS, Lee YY, Yuen EH, et al. Ultrasound elastography in the head and neck. Part II. Accuracy for malignancy. Cancer Imaging 2013;13:260e76.

J.H. Shin* Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea * Samsung Medical Center, Sungkyunkwan University School of Medicine, Department of Radiology and Center for Imaging Science, 81 Irwon-ro, Gangnam-gu, Seoul 135-710, Republic of Korea. Tel.: þ82 2 3410 2518; fax: þ82 2 3410 2559. E-mail addresses: [email protected], [email protected] (J.H. Shin) http://dx.doi.org/10.1016/j.crad.2014.06.008 Ó 2014 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Re: Ultrasonographic elastography of thyroid nodules: Is adding strain ratio to colour mapping better? A reply.

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