Papillary thyroid microcarcinoma (PTMC) is defined by the World Health Organization as a papillary thyroid carcinoma (PTC) measuring ≤ 1 cm in diameter (1). The clinical significance of PTMC remains variable and controversial, because these tumors generally have a clinically indolent course with excellent clinical prognosis. However, some cases of PTMC have aggressive clinicopathological characteristics and poor clinical outcomes (2). Therefore, determination of markers capable of identifying these aggressive tumors, especially in the phase before surgery, would be very useful in guiding appropriate clinical management of PTMCs. The revised American Thyroid Association (ATA) guidelines indicate that thyroid cancer should be treated based on risk stratification and genetic testing, and should be assessed on the basis of the disease stage (3). Therefore, correct risk stratification is important in clinical decision-making.

The activating somatic point mutation, BRAF results in substitution of valine to glutamate (^V600E), leading to the constitutive activation of the BRAF kinase and consequent uncontrolled activation of the mitogen-activated protein kinases signaling pathway (4). It is the most common genetic alteration observed in PTC. BRAF^V600E mutation occurs in approximately 45%–80% of PTC cases and 25% of anaplastic thyroid cancer, depending on the population examined and the geographical region (5). However, it does not occur in other thyroid cancers and benign tumors (6). Many studies have indicated that BRAF^V600E is significantly associated with the aggressive clinicopathological characteristics of PTC, such as capsular invasion, multifocality, lymph node metastases (LNMs), and advanced clinical stage (678). However, the role of BRAF^V600E as a prognostic marker in PTMC is not clear, although PTMC also belongs to the well-differentiated PTC group. Some individual and meta-analysis studies have revealed that BRAF^V600E is significantly correlated with more aggressive characteristics of PTMCs; however, other studies do not show such a relationship (910111213).

Thus, in this study, we examined the percentage of BRAF^V600E mutation and the relationship between BRAF^V600E mutation and the clinicopathological features of PTMC in a group of single centers.

The clinicopathological characteristics according to BRAF^V600E mutation status are shown in Table 1. Of the 911 enrolled patients, 717 (78.8%) patients were BRAF^V600E-positive and 194 (21.3%) were BRAF^V600E-negative. There were more male patients in the BRAF^V600E-positive status group than in the negative status group. In the BRAF^V600E-negative status group, LNMs were higher. The mean age, family history, tumor size, extent of surgery, capsular invasion, multicentricity, bilaterality, central LNMs, distant metastases, and recurrence rate were the same in both groups. Subgrouping with a number of risk factors (capsular invasion, multifocality, central node metastasis, LMNs, distant metastasis) revealed that 2 risk factors were more prevalent in the BRAF^V600E-positive group and three or more risk factors were more prevalent in the negative group. Disease-free survival (DFS) was not significantly different between the 2 groups (P=0.350) (Fig. 1). There was no mortality during the study period.

We analyzed the clinicopathological characteristics according to the surgical extent (less than total thyroidectomy and bilateral total thyroidectomy) and BRAF^V600E mutation status. A total of 507 patients received less than total thyroidectomy with CCND and 404 patients received bilateral total thyroidectomy with CCND. In patients that underwent less than total thyroidectomy, there were more male patients in the BRAF^V600E-positive status group in than the negative status group (P=0.014). Other factors, such as family history, tumor size, capsular invasion, multiplicity, bilaterality, node metastases, and recurrence rate were the same in both groups. In patients who received bilateral total thyroidectomy, only LNMs were higher in BRAF^V600E-negative group (P=0.004) and the other factors were not different between the 2 groups (Table 2).

We performed univariate and multivariate analyses for evaluating the correlation between BRAF^V600E mutation and clinicopathological factors. Capsular invasion showed a positive correlation and LNMs showed negative correlation with BRAF^V600E mutation (Table 3). Both factors were significantly correlated with BRAF^V600E mutation.

According to the 2016 annual report of cancer statistics in Korea, thyroid cancer is the third most common cancer in all populations and the second most common cancer in women (14). Although the incidence decreased after the controversy regarding the overdiagnosis of thyroid cancer, it is still a cancer with high prevalence in Korea. The prevalence of thyroid cancer has been increasing at a quite rate in recent decades, especially PTC. It is still debated as to whether improved detection is the only reason for the increase in the diagnosis of PTCs or whether the tumorigenesis of thyroid cancer has changed (15). Additionally, the rate of PTMCs is consistently diagnosed and treated at a level of over 50% in all PTCs. PTC, especially PTMC has low mortality and comparably good prognosis and clinically favorable results. The BRAF^V600E mutation is a common mutation in PTMC and active research has been conducted since the early 2000s on the prognosis of the mutation. In recent years, BRAF^V600E has emerged as a highly specific biomarker and useful prognostic factor associated with high-risk clinicopathological factors in PTMCs. Some studies have reported that the BRAF^V600E mutation is associated with poor clinicopathological outcomes in PTMC such as the high incidence of capsular invasion, metastasis of the tumor, advanced clinical stage, and high recurrence rate. Other studies have shown that the BRAF^V600E mutation is not associated with age, sex, or multicentricity (11121617).

The high distribution (78.8%) of BRAF^V600E among patients with PTMC observed in our study, was similar to that observed in other studies (incidence rate, 27.3%–87.1%) (5918). These findings are consistent with the significantly higher prevalence of PTMC in Asia, especially in Korea relative to Western countries (51920). However, the mechanisms underlying the difference in PTMC prevalence among various countries are not well understood. A recent theory suggests that the difference originates from the intake of iodine, Korea and other Asian countries have a higher intake of iodine, and the geographic differences are made apparent by the change in prevalence over time (18192122). There has been an increase in BRAF^V600E-associated thyroid cancers from 62.2% to 73.7% over the last two decades in Korea (23). Similarly, the overall prevalence of PTC had remained stable for an extended period of time, but has increased rapidly from 50.0% to 76.9% over the last four decades in the United States (24). More in-depth research on changes in mutational variation and their clinical outcomes are needed.

When the clinicopathological characteristics were compared to the BRAF^V600E mutation status, the aggressive factors, like tumor size, capsular invasion, multicentricity, bilaterality, central node metastases, distant metastasis were found to be similar in both BRAF^V600E-positive and -negative status patient groups. However, there were more male patients in the BRAF^V600E-positive group than in the BRAF^V600E-negative group (19.1% vs. 13.4%). These results were more prominent in the less than total thyroidectomy group compared to the patient group according to the surgical range (less than total vs. bilateral total thyroidectomy). Many studies have been published regarding whether male thyroid cancer patients have a poor prognosis or not. The results from a meta-analysis by Liu et al. revealed that the male sex is a poor, independent prognostic factor for all PTMCs. However, sex is not an independent prognostic factor for cause-specific survival in PTMC (25), and Kim et al. (26) reported that sex was not an independent prognostic factor for tumor recurrence. Although other factors involved in mediating the aggressiveness of PTMC and recurrence showed no significant difference, it is difficult to find an explanation for a higher BRAF^V600E-positive rate in male patients. A long-term follow-up study would be needed to investigate this further.

Most PTMCs have an indolent clinical course and an excellent prognosis, but early spread to the lymph nodes is not rare, and so, there is a high incidence of disease recurrence in the lymph nodes. The highest LNM rate is up to 50% (27), and lateral LNMs are found in 3.7%–45% of the patients with PTMC. Many studies have reported that patients with nodal metastasis have an increased rate of disease recurrence (2829). In this study, there was no difference in the central lymph node metastasis between the 2 groups. However, lateral neck node metastases were detected more in the BRAF^V600E-negative status group. The overall lateral lymph node metastasis rate was only 2.9%; 6.7% in the BRAF^V600E-negative group and 1.8% in the positive group. In the subgroup analysis based on the surgical range of bilateral total thyroidectomy patients, the overall lateral lymph node metastasis rate was 6.4%; 4.3% in the BRAF-positive group and 12.9% in the negative group. Multivariate analysis to evaluate the factors significantly correlated with the BRAF^V600E mutation revealed that lateral neck node metastasis was negatively correlated with the BRAF^V600E mutation. Same in our result, other studies reported that higher lateral neck node metastasis rates were detected in the BRAF^V600E-negative group in PTMC. However, the lateral lymph node metastasis was not significantly associated with BRAF^V600E mutation in PTMC (1130). Though we couldn't investigate the exact reason that the correlation of lymph node metastasis rate with the BRAF^V600E mutation status in our study, there was no difference in the recurrence rate and DFS based on the BRAF^V600E mutation status. Therefore, this suggests that BRAF^V600E mutation is not correlated with the poor prognostic factor.

The BRAF^V600E mutation analysis was performed in our institution in June 2015, on all patients who underwent surgery for differentiated thyroid cancer. However, due to the short duration of the follow-up, it is difficult to draw an accurate conclusion about the effect of the BRAF^V600E mutation on PTMCs. Long-term follow-up of BRAF^V600E mutations, disease recurrence, and mortality is therefore necessary.

There were some limitations in this study. Due to the retrospective nature of the study and single-center analysis, the registration information, patient volume, and inspection items could not be designed beforehand. As we mentioned, BRAF^V600E mutation analysis was performed on FNAB or post-surgery specimens from selected patients. This strategy may have created discrepancies in the results obtained from BRAF^V600E mutation testing. However numerous studies have demonstrated that a preoperative BRAF mutation analysis can be performed readily and reliably by the use of FNAB specimen (313233). In addition, the indolent clinical course of PTMC implies that the follow-up duration would not be enough to verify the prognosis.

In conclusion, BRAF^V600E mutations in PTMC was associated with presence of capsular invasion and absence of lateral neck node metastasis. However, BRAF^V600E was not found to be a prognostic factor that affected the recurrence of PTMC in our study. It would be of great importance to identify the BRAF^V600E mutation status in PTMC patients who were treated in a large-scale institution. In addition, further investigation is needed to define the factors associated with BRAF^V600E and to correlate them as prognostic factors.