Background
The incidence rate of thyroid nodules has an annual increasing trend worldwide. The overall prevalence rate was 49 %-68 % in general population of China, Europe and America by using ultrasound [
1‐
3]. As thyroid nodules are usually the first sign of cancer, the primary goal of treatment is to distinguish between malignant and benign lesions.
In practice, more and more physicians took thyroid ultrasound as the preferred examination because of its noninvasive and inexpensive. However, recent researches reported that screening for thyroid cancer has led to a significant increase in the global diagnosis rate of the disease, but no change in mortality [
4,
5]. Therefore, the American Association of Clinical Endocrinologists (AACE) does not recommend ultrasound as a screening test for the general population or patients with a normal thyroid on palpation and a low clinical risk of thyroid disease [
6]. Similarly, The United States Preventive Services Task Force recommends against screening for thyroid cancer in asymptomatic adults [
7]. Nevertheless, the AACE prescription standard for ultrasound screening is only at a level 4 evidence and GRADE C recommendation, which means it is based on expert experience with no conclusive risks or benefits [
6]. We believe that it is necessary to evaluate the nodules that can only be detected by ultrasound. Therefore we can decide whether it is appropriate to carry out ultrasound examination for patients who do not have nodules detected by palpation.
Up to now, no studies have directly compared the risk of ultrasound-found and palpation-found nodules. More researches evidence is needed to carry out for accumulating evidence to help us balance the over diagnosis and missed diagnosis of malignant lesions.
The purpose of this retrospective study was to investigate the incidence of malignant lesions in ultrasound-found nodules.
Discussion
Our study indicated that ultrasound-found nodules presented a greater malignancy risk than palpation-found ones. There were several explanations for the results. First, only 10 % of ultrasound-found nodules were purely cystic, which are highly likely to be benign [
6,
11]. In contrast, purely cystic nodules were nearly 40 % in palpation-found ones. Second, ultrasound-found nodules were smaller in size and they had higher TI-RADS score than palpation-found ones.
In our study, the evaluation of ultrasound features of nodules was based on TI-RADS published by ACR in 2017 [
8]. In ACR TI-RADS, the characteristics of thyroid nodules were evaluated from five features. Each feature has a score, and a total score was obtained by adding the five scores. The total score determined the TI-RADS category of nodules, ranging from 1 to 5. In the research of Kwak et al. [
12], the counting method was used rather than weighting method. The counting method is to summarize the number of suspicious ultrasound features, regardless of the weight of malignant possibility of the features. Recently, Zhou et al. [
13] published a multicenter study on Chinese population, which verified the diagnostic efficacy of the newly established Chinese TI-RADS (C-TIRADS). C-TIRADS also adopts counting method, which has good sensitivity and specificity. In our study, taking into account the familiarity with the criteria of medical and ultrasound staff, we used ACR TI-RADS to evaluate the nodules. Statistical analysis shows that there is no difference in diagnostic performance between weighted method and counting method [
13,
14]. If we take other TI-RADS for statistical analysis, the results may not be different from the current ones.
Unlike palpation-found nodules, ultrasound-found nodules were smaller and often located deep in the thyroid tissue. Hence the relation of the nodule size between malignancy risk and prognosis as recommended by AACE became controversial [
2].
Though some studies found that malignant risk is associated with nodule size, for example, a series of observational studies have found that thyroid cancers over 4 cm were associated with more aggressive behavior whereas tumors smaller than 1.5 cm had a good overall prognosis [
15‐
17]. However, some other researches indicated no correlation between size and risk [
18,
19]. A recent study found that the impact of nodule size on the malignancy risk differed according to the ultrasound pattern. A large nodule size (≥ 3 cm) showed a higher malignancy risk than smaller nodules in intermediate- and low-suspicion nodules [
20].
We are convinced that our study findings as demonstrated above, highlight a conflict between AACE recommendations and clinical practice in the following aspects: AACE’s recommends not to conduct ultrasound screening for thyroid nodules, while the recommendation of FNA was based on nodule size [
6]. American Thyroid Association (ATA) also recommends FNA based on nodule size [
11]. In our study, the average diameter of nodules detected by ultrasound was 1.75 cm, which was within both of the recommended range of FNA.
Another conflict between AACE recommendations and recent reality highlighted in this study is that AACE recommends against ultrasound screening based on a significant increase in global thyroid cancer prevalence but a constant mortality rate [
5,
6]. However, latest published study found that in America, incidence-based thyroid cancer mortality rose from 0.40 to 100,000 person-years in 1994–1997 to 0.46 per 100,000 person-years in 2010–2013 [
21]. In China, the mortality increased from 0.30 to 100,000 in 2005 to 0.35 per 100,000 in 2015 [
22].
Under current guidelines, only a small portion of patients could be under active surveillance for microcarcinoma [
23]. Furthermore, in our study of ultrasound-found nodules, there were as high as two thirds that were non-microcarcinoma. For these nodules larger than 1 cm, it is generally believed that the benefits of surgery outweigh the risks [
6].
The limitation of this study is that ultrasound evaluations were performed by different operators and machines, so bias may exist.
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