Reappraising the role of thyroid scintigraphy in the era of TIRADS: A clinically-oriented viewpoint

Reappraising the role of thyroid scintigraphy in the era of TIRADS: A clinically-oriented viewpoint

Abstract

Thyroid nodules (TNs) are a common entity, with the majority being benign. Therefore, employing an accurate rule-out strategy in clinical practice is essential. In the thyroid field, the current era is significantly marked by the worldwide diffusion of ultrasound (US)-based malignancy risk stratification systems of TN, usually reported as Thyroid Imaging Reporting And Data System (TIRADS). With the advent of US (and later TIRADS), the role of thyroid scintigraphy (TS) in clinical practice has gradually diminished. The authors of the present paper believe that the role of TS should be reappraised, also considering its essential role in detecting autonomously functioning thyroid nodules and its limited contribution to detecting thyroid cancers. Thus, this document aims to furnish endocrinologists, radiologists, surgeons, and nuclear medicine physicians with practical information to appropriately use TS.

Background

The thyroid nodule (TN) is a common pathological entity occurring in up to 60–70% of adults [1]. The majority of TNs are benign, with a cancer prevalence per patient estimated lower than 2% [2]. The huge number of benign TNs encompasses hyperplasia, several types of adenomas, and inflammatory lesions within autoimmune thyroid disorders [3]. Among thyroid malignancies there are differentiated carcinomas, medullary carcinoma, and other rare cancers such as undifferentiated/anaplastic, lymphoma, and metastases from other organs [3]. Thus, using an accurate rule-out strategy in clinical practice is essential. In the thyroid field, the current era is marked by the worldwide diffusion of ultrasound (US)-based malignancy risk stratification systems of TN, developed by most important international societies and usually reported as Thyroid Imaging Reporting And Data System (TIRADS) [4,5,6]. These systems were conceived to standardize the lexicon of thyroid US and reduce as much as possible the indication for fine needle aspiration cytology (FNAC). Indeed, the literature has confirmed that TIRADSs are reliable in detecting thyroid cancer, and papillary carcinoma (PTC) in particular, and saving on unnecessary FNACs (i.e., biopsies performed with benign cytological report). Regarding the latter, some discrepancies have been observed among TIRADSs, being the rate of unnecessary FNAC lower according to ACR-TIRADS as compared to the other ones. However, evidence-based studies have primarily tested TIRADS performance against PTC [7], while the other cancers and benign lesions may present differently at US with consequent heterogeneous assessment according to TIRADS. An important project endorsed by international societies is currently ongoing to develop the international TIRADS which should solve the weaknesses of the TIRADSs and will replace them in the future [8]. Integrating patients’ clinical features and results of other imaging procedures into the interpretation of TIRADS assessments should be optimal for clinicians.

Among the other thyroid imaging procedures, thyroid scintigraphy (TS) was traditionally used as the initial TN evaluation until 1990’s when US become pivotal in this diagnostic work-up. With the advent of US (and later TIRADS), the role of TS in clinical practice was progressively debunked, and its indication is currently limited to some specific conditions [9]. However, some benign pathological entities of TN cannot be identified by TIRADS, and TS may contribute to improving the performance of TIRADSs in reducing the rate of unnecessary FNACs. In addition, patients referred with inconclusive cytological report may be addressed to treatment alternative to surgery depending on the TS pattern. From the functional point of view, while thyroid cancer generally presents with low or absent uptake at TS, functioning nodules can be encountered among benign lesions; in particular, autonomously functioning thyroid nodule (AFTN) is the term conventionally used to define those TNs that overproduce hormones as to exceed the body tissues requirement. In the context of the need to improve the performance of US and TIRADSs, whether we should exclude AFTNs before FNAC with the aim of further reducing unnecessary FNACs is currently a matter of debate. AFTNs are virtually benign but have heterogeneous US presentation and often large size, being non negligible the likelihood to indicate (unnecessary) FNAC in these nodules [10,11,12]. However, anecdotal cases of hyperfunctioning/toxic cancers have been described in the literature [1314]. Furthermore, considering that TIRADS was based on the US presentation of PTC and can overlook follicular and medullary carcinoma, whether TS can still hold a role in cancer risk stratification should be re-discussed. Thus, a reappraisal of the role of TS in the era of TIRADS diffusion is needed to furnish endocrinologists, radiologists, surgeons, and nuclear medicine physicians with practical information.

Conclusions for clinical practice

The current era of medicine is significantly marked by the need of limiting as much as possible unnecessary diagnostic tests and their related costs. In the thyroid field, clinicians and US operators are asked to save on unnecessary FNACs with the intent to reduce, and ideally avoid, further inconvenient implications. Therefore, thyroidologists should also aim to avoid other unnecessary diagnostic tests and imaging procedures, especially the low cost-effective ones. Over the last two decades, thyroid US has achieved such a high clinical performance that TN patient management is primarily guided by US/TIRADS. As a result, the role of TS has significantly evolved. Indeed, its use to stratify the risk of malignancy is substantially discharged and no longer recommended by guidelines. Even if TSn remains the gold standard for detecting AFTN, considering the high reliability of TIRADS in risk stratification of TNs and the possibility to find a cancer in toxic TN, TSn can be appropriately recommended before FNAC in patients with suspected solitary toxic AFTN (i.e., low/suppressed TSH), also to determine their eligibility for RAI. In addition, TSn can be useful to detect both toxic and non-toxic AFTN among TNs with indeterminate FNAC, also in the attempt to reduce the resection rate of these cases. Anyway, FNAC can be performed according to TIRADS without harms, ad TSn can be used as second-line imaging, when clinically appropriate. In any case, FNAC must be performed in TNs assessed as high-risk according to TIRADS, independently of their TS pattern.

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