Multiparametric Ultrasound of Thyroid Nodules: Where Do We Stand?

Thyroid nodules are being increasingly detected during thyroid evaluations or incidentally during ultrasound examinations for other clinical purposes [1] or screening programs, thus leading to potential overdiagnosis and overtreatment [2]. Therefore, the present thyroid nodule epidemic represents a challenge for clinical management, since the vast majority of identified nodules end up being diagnosed as benign [3]. The clinical practice guidelines [4] [5] limit the number of nodules to be submitted to fine-needle aspiration cytology, and selection no longer relies upon size and growth pattern. The recent consensus statements recommend that a combination of ultrasonographic features [4] [5] should be used to select nodules to aspirate or re-aspirate. Therefore, the role of ultrasonography is expanding to include not only initial diagnosis but also risk stratification, selection of nodules to be submitted to FNAC, management and follow-up of nodules with indeterminate cytology [6], and even provision of prognostic information. The Thyroid Imaging Reporting and Data System (TIRADS) was proposed to facilitate understanding and communication of US reports among specialists [7]. This classification modeled on the Breast Imaging Reporting and Data System and based on the likelihood of malignancy [8] was further modified by Kwak et al. [9].

The TIRADS reporting system for conventional ultrasound was recommended in the WFUMB Elastography Guidelines [10] but different TIRADS reporting systems are currently available and they are not widely used. Interobserver variability is also reported as a possible limitation [11]. Furthermore, technology developments including CEUS and US-elastography have resulted in new prospects in ultrasound, leading to the possibility of performing multiparametric ultrasound as advocated [12]. CEUS provides visualization of the macro- and microvascularization, but the diagnostic performance of CEUS regarding thyroid nodule characterization shows high variability with respect to sensitivity and specificity. As a consequence, the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) guidelines on non-hepatic CEUS applications report that CEUS for the characterization of thyroid nodules is an active research field but currently cannot be recommended for clinical use [13].

Using either SE (strain quasistatic elastography) or SWE (shear wave elastography) to assess stiffness as an indicator of malignancy is recognized as a valid and useful additional tool to grayscale and color Doppler US in thyroid evaluation [10] [14] [15] [16]. It may be performed with no patient preparation and dedicated equipment that supports elastography is required. Elastography is especially recommended for use as an additional tool to conventional ultrasound and for guiding the follow-up of lesions previously diagnosed as benign at FNA biopsy [13] and can be used in combination with conventional US to improve specificity [14]. To date, the majority of papers and meta-analyses in the literature have shown US-elastography to be more accurate than ultrasound with only a few exceptions [13] [14] [17] [18]. However, it was clearly debated and reported [17] that adequate training, suitable cut-off values for both strain and SWE, adequate equipment and clinically appropriate examinations are necessary to achieve reliable SE. Expert suggestions are to minimize pre-compression, carotid pulsation artifacts, to check the ROI size and positions, avoid areas with artifacts or with gross calcifications or cystic areas and instruct patients to cooperate properly [19]. Future technical developments, such as quality-indicator tools or possibly 3 D US, should prove to be helpful for reducing interobserver and intraobserver variability.

There are still open and debated issues such as: what is the most accurate and reproducible elastographic technique; what is the role of US-elastography for thyroid nodules that are indeterminate at FNAC; what is the reproducibility of these techniques; and to what extent does thyroiditis influence the accuracy of US-elastography in thyroid nodule characterization.

In conclusion, there is a demand to reduce the number of unnecessary FNAC procedures and treatments or follow-up studies. To date, multiparametric systematic ultrasound evaluation including US-elastography seems to be the recommended imaging modality. Future technological developments and multicenter studies on large non-selected populations are warranted to better define the role and limitations of thyroid nodule assessment.

• References

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