Changes in the clinicopathological characteristics and genetic alterations of follicular thyroid cancer

in European Journal of Endocrinology

Objective

Changes in the clinicopathological characteristics and genetic alterations of follicular thyroid cancer (FTC) over time have not been reported. Moreover, the prognostic effects of RAS and TERT promoter mutations in FTC have not been clearly elucidated. We investigated changes in the clinicopathological characteristics of patients with FTC over four decades, as well as the clinical significance of genetic mutations of FTC.

Design and methods

This retrospective study included 690 patients with FTC who underwent thyroidectomy between 1973 and 2015 at the Seoul National University Hospital. In 134 samples, genetic tests for N/H/KRAS and TERT promoter mutations and PAX8/PPARγ rearrangement were performed.

Results

The age at diagnosis has increased (P < 0.001) in recent decades and extrathyroidal extension of the tumor has become less common (P = 0.033). Other clinicopathological characteristics and prognosis of FTC have not significantly changed. The prevalence of RAS mutations decreased (P = 0.042) over time, whereas that of TERT promoter mutations remained stable. RAS mutations were associated with distant metastasis and persistent disease, and TERT promoter mutations were associated with distant metastasis, advanced TNM stage, recurrence and disease-specific mortality. FTC patients with coexistent RAS and TERT promoter mutations showed a higher recurrence risk than those with only one mutation.

Conclusions

The age at diagnosis of FTC and the frequency of extrathyroidal extension have changed over four decades. Moreover, the prevalence of RAS mutations decreased. RAS and TERT promoter mutations may be associated with poor clinical outcomes in FTC, especially when the two mutations coexist.

Abstract

Objective

Changes in the clinicopathological characteristics and genetic alterations of follicular thyroid cancer (FTC) over time have not been reported. Moreover, the prognostic effects of RAS and TERT promoter mutations in FTC have not been clearly elucidated. We investigated changes in the clinicopathological characteristics of patients with FTC over four decades, as well as the clinical significance of genetic mutations of FTC.

Design and methods

This retrospective study included 690 patients with FTC who underwent thyroidectomy between 1973 and 2015 at the Seoul National University Hospital. In 134 samples, genetic tests for N/H/KRAS and TERT promoter mutations and PAX8/PPARγ rearrangement were performed.

Results

The age at diagnosis has increased (P < 0.001) in recent decades and extrathyroidal extension of the tumor has become less common (P = 0.033). Other clinicopathological characteristics and prognosis of FTC have not significantly changed. The prevalence of RAS mutations decreased (P = 0.042) over time, whereas that of TERT promoter mutations remained stable. RAS mutations were associated with distant metastasis and persistent disease, and TERT promoter mutations were associated with distant metastasis, advanced TNM stage, recurrence and disease-specific mortality. FTC patients with coexistent RAS and TERT promoter mutations showed a higher recurrence risk than those with only one mutation.

Conclusions

The age at diagnosis of FTC and the frequency of extrathyroidal extension have changed over four decades. Moreover, the prevalence of RAS mutations decreased. RAS and TERT promoter mutations may be associated with poor clinical outcomes in FTC, especially when the two mutations coexist.

Introduction

Follicular thyroid cancer (FTC) is the second most common type of thyroid malignancy following papillary thyroid cancer (PTC), which accounts for 10%–32% of cases of differentiated thyroid cancer (DTC) (1, 2). In the last decades, the incidence of thyroid cancer has sharply increased worldwide. This trend was especially predominant for small PTC, which shows less aggressive features and favorable outcomes (3, 4). However, long-term trends in the characteristics and outcomes of FTC, distinct from other types of thyroid cancer, have not been reported.

Several previous studies have presented temporal changes in the mutational frequencies associated with PTC. In the United States, the overall prevalence of BRAF mutations was stable, but increased from 50.0% to 76.9% in conventional PTC over the last four decades (5). Moreover, RAS mutations increased from 2.7% to 24.9% due to an increase in follicular-variant PTC (FVPTC). In Europe, the frequency of BRAF mutations increased gradually from 28.0% to 58.1% over the last 15 years (6). The incidence of the RET/PTC rearrangement, in contrast, decreased from 33.0% to 9.8% over the same period. In Korea, which is a BRAF mutation-prevalent country, BRAF-mutated PTCs increased from 62.2% to 73.7% over the last two decades (7). However, no study has evaluated changes in the mutational frequencies of FTC over time.

The most frequent genetic alterations in FTC are RAS mutations and the PAX8/PPARγ rearrangement (8). The prognostic value of RAS mutations is still unclear, although some evidence suggests that RAS-mutated FTCs may be at risk for a poor prognosis (9, 10) or distant metastasis (11, 12). Moreover, no evidence exists linking the PAX8/PPARγ rearrangement with clinical outcomes. However, recent studies have demonstrated that TERT promoter mutations, especially when they coexist with BRAF mutations, are associated with a poor prognosis in PTC (13). Regarding the prognostic effect of the coexistence of TERT promoter and RAS mutations, we recently demonstrated for the first time that their coexistence may increase the risk of disease-specific mortality and recurrence in DTC, including both PTC and FTC (14). However, the corresponding effect in FTC only has not been analyzed.

In this study, we investigated changes in the clinicopathological characteristics, prognosis and mutational profiles of FTC over four decades, as well as associations between genetic alterations and clinicopathological outcomes.

Patients and methods

Patients and tissue samples

This study included 690 consecutive patients with FTC (544 females and 146 males) who underwent thyroidectomy at Seoul National University Hospital (Seoul, Korea) between 1973 and 2015, and analyzed long-term trends in clinicopathological characteristics and prognoses. The median follow-up duration was 6.9 years (interquartile range, 3.6–11.6 years). The diagnosis of FTC was based on the postoperative pathology report, and clinicopathological data at baseline and during follow-up were collected retrospectively from their electronic medical records.

Among the FTC patients who underwent surgery in 1997–2003 and 2009–2012, genetic analyses were performed of 134 patients (67 patients in each period) whose formalin-fixed paraffin-embedded (FFPE) DNA samples were available, including 119 patients from our previous report (14). For the mutational analysis, all tissue samples were reviewed by an expert pathologist who specializes in thyroid pathology, and the FTC tumor region of the samples was microdissected for subsequent DNA extraction. Pathological diagnoses were made according to the latest World Health Organization classifications for thyroid cancer (15). This study was conducted according to the guidelines of the Declaration of Helsinki. The research protocol was approved by the institutional review board committee of the Seoul National University Hospital (H-1207-124-420).

Mutational analyses

All samples of the FFPE tumor block were digested with proteinase K (Sigma) for more than 24 h at 56°C, and DNA was then isolated from the digested tissue using a Tissue SV Mini kit (General Biosystem Inc., Seoul, Korea). NRAS (codon 12/13 and codon 61), HRAS (codon 12/13 and codon 61), KRAS (codon 12/13 and codon 61) and TERT promoter (C228T and C250T) mutations were examined using polymerase chain reaction (PCR) and amplified using appropriate primers (Supplementary Table 1, see section on supplementary data given at the end of this article). PCR was performed using a BioMix kit (Bioline, Taunton, MA, USA). Purified PCR products obtained using a QIAquick Gel Extraction kit (Qiagen) were used for sequencing with a Big Dye Terminator Cycle Sequencing Ready Reaction kit (Applied Biosystems). The sequences were analyzed using ABI Prism 3730 Genetic Analyzer (Applied Biosystems). Each DNA sample was assayed at least twice in order to confirm its RAS and TERT promoter mutation status by sequencing with both forward and reverse primers. RAS mutation status was assessed in all 134 samples, but TERT promoter mutation status was only assessed in 120 samples because of PCR failure in 14 samples. To evaluate the PAX8/PPARγ rearrangement, we performed fluorescence in situ hybridization (FISH) analysis in FFPE tumor tissues using the PPARγ (3p25) break probe (Kreatech Diagnostics, Amsterdam, The Netherlands). These commercially available probes are designed as a dual-color probe where the two regions cross the break-point (Supplementary Fig. 1). The FISH analysis could only be done in 48 cases (2010–2012), depending on the staining and preservation status of FFPE.

Statistical analyses

Data are presented as frequencies and percentages, as means and standard deviations or as medians and interquartile range. Categorical variables were compared with the Pearson chi-square test or the Fisher exact test (if the number was <5). The independent t test or the Wilcoxon–Mann–Whitney test was used for continuous variables. In order to compare trends of the clinicopathological characteristics of FTC over three periods, the linear-by-linear association test was used for categorical variables and analysis of variance for continuous variables. The log-rank test was used to compare variables based on the time of events. Cox proportional hazards regression was used to assess the risk of recurrence. Statistical significance was defined as two-sided P values <0.05. All statistical analyses were performed with SPSS 20.0 (IBM Corp., Armonk, NY, USA).

Results

Changes in clinicopathological characteristics and outcomes of FTC patients between 1973 and 2015

The 690 patients with FTC were classified into three groups by the calendar year of diagnosis: 1973–1995 (n = 68), 1996–2005 (n = 231), and 2006–2015 (n = 391; Table 1). The mean age of patients in 1973–1995 was significantly younger than those in other periods (P < 0.001), but there was no difference in age after 1995. Most of histological characteristics, including the size and proportion of widely invasive FTC, vascular invasion, multiplicity and lymph node metastasis, did not change over time. However, extrathyroidal extension, especially gross invasion, has decreased recently (P = 0.033). Although the overall incidence of distant metastasis decreased from 14.7% to 5.1%, no significant difference was found in the survival analysis using the time from the beginning to the occurrence of an event (log-rank P = 0.346). Moreover, the percentage of patients with distant metastasis at the initial presentation did not significantly vary over time. The most frequent sites of distant metastasis were lung and bone throughout the period. Unilateral lobectomy alone was common in 1973–1995, after which total thyroidectomy or lobectomy with completion thyroidectomy became more frequent, although there was no significant temporal trend (P = 0.179). The proportion of patients who received radioactive iodine (RAI) therapy, especially as a remnant ablation, increased over time (P < 0.001), but the total dose of RAI significantly decreased (P < 0.001). During the median follow-up of 6.9 years, 9.0% of the patients experienced persistent disease or tumor recurrence, and 2.2% died from FTC. No significant differences were found among time periods in the proportion of patients with no evidence of disease, persistence, recurrence or disease-specific mortality.

Table 1

Clinicopathological characteristics of patients with follicular thyroid cancer, 1973–2015.

Year
1973−19951996−20052006−2015Pa
Cases68231391
Follow-up duration, yearsb21.4 (13.2−26.4)11.7 (6.6−14.0)4.9 (2.7−7.2)
Age at diagnosis, years39.0 ± 11.145.2 ± 14.2c46.3 ± 14.3c<0.001
Male sex, n (%)16 (23.5)41 (17.7)89 (22.8)0.516
Body mass index, kg/m223.9 ± 5.123.8 ± 3.623.7 ± 6.20.972
Tumor size, cm3.4 ± 2.23.4 ± 1.83.2 ± 1.80.467
Widely invasive FTC, n (%)7 (10.3)37 (16.0)48 (12.3)0.729
Vascular invasion, n (%)14 (20.6)39 (16.9)63 (16.1)0.419
Multiplicity, n (%)11 (16.2)41 (17.7)52 (13.3)0.221
Extrathyroidal extension, n (%)5/29 (17.2)32/179 (17.9)42/382 (11.0)0.033
 Microscopic/gross1/4 (3.4/13.8)1/31 (0.6/17.3)33/9 (8.6/2.4)<0.001
Resection margin, n (%)1 (1.5)6 (2.6)7 (1.8)0.823
Lymph node metastasis, n (%)0 (0.0)8 (3.5)10 (2.6)0.574
Distant metastasis, n (%)10 (14.7)26 (11.3)20 (5.1)0.346d
 At initial presentation4 (5.9)12 (5.2)16 (4.1)0.420
 During follow-up6 (8.8)14 (6.1)4 (1.0)
 5 years/10 years4/5 (5.9/7.4)18/22 (7.8/9.5)18/NA (4.6/NA)
Site of distant metastasis, n (%)
 Lung9 (90.0)17 (65.4)12 (60.0)
 Bone5 (50.0)19 (73.1)15 (75.0)
 Mediastinum0 (0.0)3 (11.5)2 (10.0)
 Brain1 (10.0)2 (7.7)0 (0.0)
 Others2 (20.0)4 (15.4)4 (20.0)
Type of thyroidectomy, n (%)0.179
 Total thyroidectomy6 (8.8)90 (39.0)152 (38.9)
 Subtotal thyroidectomy10 (14.7)27 (11.7)11 (2.8)
 Lobectomy46 (67.6)48 (20.8)100 (25.6)
 Lobectomy and completion6 (8.8)66 (28.6)128 (32.7)
RAI treatment, n (%)22 (32.4)111 (48.1)233 (59.6)<0.001
 Remnant ablation14 (20.6)86 (37.2)213 (54.5)
 For distant metastasis8 (11.8)25 (10.8)20 (5.1)
Total dose of RAI, mCib200 (110−858)120 (60−390)c60 (60−130)c<0.001
No evidence of disease, n (%)57 (83.8)202 (87.4)369 (94.4)0.317d
Persistence, n (%)3 (4.4)12 (5.2)15 (3.8)0.568
Recurrence, n (%)
 Overall8 (11.8)17 (7.4)7 (1.8)0.399d
 5 years/10 years0/1 (0.0/1.5)7/14 (3.0/6.1)4/NA (1.0/NA)
Disease-specific mortality, n (%)
 Overall5 (7.4)9 (3.9)1 (0.3)0.177d
 5 years/10 years0/0 (0.0/0.0)5/8 (2.2/3.5)1/NA (0.3/NA)

P values from chi-square test for trend (categorical variables) and from analysis of variance (continuous variables); bValues presented as median (interquartile range); cSignificantly different from subjects in 1973−1995; dLog-rank P values.

FTC, follicular thyroid cancer; NA, not applicable; RAI, radioactive iodine.

Prevalence of RAS and TERT promoter mutations and the PAX8/PPARγ rearrangement in FTC

None of the characteristics of the 134 patients who underwent genetic analysis were significantly different from the overall samples of the corresponding period (Supplementary Table 2). Among them, 43 (32.1%) and 7 (5.8%) patients had tumors with RAS and TERT promoter mutations respectively (Table 2). The prevalence of RAS mutations decreased from 40.3% to 23.9% (P = 0.042), and the HRAS codon 61 mutation mainly contributed to this decrease. The NRAS codon 61 mutation was most common in both periods. The codon 12/13 mutations of NRAS, HRAS and KRAS were not found. TERT promoter mutations in each period were found in three (4.5%) and four (7.4%) cases respectively (P = 0.699). The major type of TERT promoter mutation was TERT C228T in both periods, although TERT C250T was observed in one case in 2009–2012. The PAX8/PPARγ rearrangement was found in one (2.1%) case in 2009–2012.

Table 2

Changes of mutational frequencies in follicular thyroid cancer between 1997–2003 and 2009–2012.

Year
1997−20032009−2012P
Cases6767
RAS mutation, n (%)27 (40.3)16 (23.9)0.042
 NRAS codon 6119 (28.4)14 (20.9)
 HRAS codon 618 (11.9)1 (1.5)
 KRAS codon 610 (0.0)1 (1.5)
 N/H/KRAS codon 12, 130 (0.0)0 (0.0)
TERT promoter mutation, n (%)3/66 (4.5)4/54 (7.4)0.699
C228T3/66 (4.5)3/54 (5.6)
C250T0/66 (0.0)1/54 (1.9)
PPARγ rearrangement, n (%)NA1/48 (2.1)NA

NA, not applicable.

Clinicopathological characteristics and outcomes of FTC according to mutational status

Comparing the clinicopathological features of FTC according to RAS mutational status (Table 3), distant metastasis occurred more frequently in RAS-mutated FTCs (log-rank P = 0.030), even though other features did not differ between RAS wild-type and RAS-mutated FTCs. RAS mutations were significantly associated with a lower frequency of no evidence of disease (log-rank P = 0.011) and a higher proportion of persistent disease (P = 0.037). However, no differences in recurrence or disease-specific mortality were found.

Table 3

Clinicopathological characteristics and outcomes of follicular thyroid cancer according to the mutational status of RAS.

RAS
Wild typeMutantP
Cases, n (%)91 (67.9)43 (32.1)
Follow-up duration, yearsa6.5 (4.5−13.3)6.1 (4.5−13.4)
RAS mutation type, n (%)
 N/H/KRAS33/9/1 (24.6/6.7/0.7)0.002
Age at diagnosis, years45.9 ± 14.442.7 ± 13.00.223
Male sex, n (%)21 (23.1)9 (20.9)0.781
Body mass index, kg/m224.4 ± 3.623.9 ± 3.90.470
Tumor size (cm)3.6 ± 2.03.2 ± 1.30.267
Widely invasive FTC, n (%)16 (17.6)7 (16.3)0.852
Vascular invasion, n (%)20 (22.0)7 (16.3)0.443
Multiplicity, n (%)11 (12.1)5 (11.6)0.939
Extrathyroidal extension, n (%)15 (17.9)6 (15.8)0.779
 Microscopic/gross6/9 (7.1/10.7)1/5 (2.6/13.2)
Resection margin, n (%)2 (2.2)2 (4.7)0.593
Lymph node metastasis, n (%)1 (1.1)2 (4.7)0.241
Distant metastasis, n (%)3 (3.3)6 (14.0)0.030b
 At initial presentation0 (0.0)3 (7.0)0.031
 During follow-up3 (3.3)3 (7.0)
 5 years/10 years2/2 (2.2/2.2)3/4 (7.0/9.3)
Site of distant metastasis, n (%)
 Lung3 (3.3)3 (7.0)0.385
 Bone1 (1.1)3 (7.0)0.097
 Other sites1 (1.1)1 (2.6)
TNM stage, n (%)0.644
 I−II/ III−IV73/18 (80.2/19.8)33/10 (76.7/23.3)
No evidence of disease, n (%)88 (96.7)36 (83.7)0.011b
Persistence, n (%)1 (1.1)4 (9.3)0.037
Recurrence, n (%)
 Overall2 (2.2)3 (7.0)0.195b
 5 years/10 years1/2 (1.1/2.2)1/2 (2.3/4.7)
Disease-specific mortality, n (%)
 Overall1 (1.1)1 (2.3)0.572b
 5 years/10 years1/1 (1.1/1.1)1/1 (2.3/2.3)

Values presented as median (interquartile range).

Log-rank P values.

TERT promoter mutations were significantly associated with distant metastasis (log-rank P = 0.001), in particular to the lung (log-rank P = 0.006), and advanced TNM stage (P = 0.045; Table 4). Furthermore, the patients with a TERT promoter mutation tended to have a poor prognosis and were less likely to remain disease-free (log-rank P = 0.001). The percentage of patients with persistent disease was higher in the TERT promoter mutation group, although this was not statistically significant due to its low incidence. Moreover, the overall recurrence and disease-specific mortality rates in TERT promoter wild-type vs mutant samples were significantly different (2.7% vs 28.6% and 0.9% vs 14.3%, respectively) (log-rank P, for recurrence = 0.002; for disease-specific mortality = 0.007).

Table 4

Clinicopathological characteristics and outcomes of follicular thyroid cancer according to the mutational status of TERT promoter.

TERT
Wild typeMutantP
Cases, n (%)113 (94.2)7 (5.8)
Follow-up duration, yearsa6.7 (4.9−13.5)6.4 (4.5−14.3)
TERT mutation type, n (%)
 C228T/C250T6/1 (5.0/0.8)0.002
Age at diagnosis, years44.6 ± 14.251.9 ± 15.70.191
Male sex, n (%)24 (21.2)2 (28.6)0.644
Body mass index, kg/m224.3 ± 3.822.6 ± 3.50.290
Tumor size (cm)3.5 ± 1.84.8 ± 2.60.071
Widely invasive FTC, n (%)22 (19.5)1 (14.3)1.000
Vascular invasion, n (%)22 (19.5)3 (42.9)0.157
Multiplicity, n (%)13 (11.5)1 (14.3)0.590
Extrathyroidal extension, n (%)21 (20.8)0 (0.0)0.341
 Microscopic/gross7/14 (6.9/13.9)0/0 (0.0/0.0)
Resection margin, n (%)4 (3.5)0 (0.0)1.000
Lymph node metastasis, n (%)1 (0.9)1 (14.3)0.114
Distant metastasis, n (%)6 (5.3)3 (42.9)0.001b
 At initial presentation2 (1.8)1 (14.3)0.166
 During follow-up4 (3.5)2 (28.6)
 5 years/10 years4/4 (3.5/3.5)1/2 (14.3/28.6)
Site of distant metastasis, n (%)
 Lung4 (3.5)2 (28.6)0.039
 Bone3 (2.7)1 (14.3)0.216
 Other sites1 (0.9)1 (14.3)
TNM stage, n (%)0.045
 I−II/ III−IV90/23 (79.6/20.4)3/4 (42.9/57.1)
No evidence of disease, n (%)106 (93.8)4 (57.1)0.001b
Persistence, n (%)4 (3.5)1 (14.3)0.263
Recurrence, n (%)
 Overall3 (2.7)2 (28.6)0.002b
 5 years/10 years2/2 (1.8/1.8)0/1 (0.0/14.3)
Disease-specific mortality, n (%)
 Overall1 (0.9)1 (14.3)0.007b
 5 years/10 years1/1 (0.9/0.9)1/1 (14.3/14.3)

Values presented as median (interquartile range).

Log-rank P values.

The only patient who showed the PAX8/PPARγ rearrangement was a 39.1-year-old man at diagnosis who had an 8.0-cm minimally invasive FTC without lymph node or distant metastasis. He underwent a lobectomy with completion thyroidectomy following RAI therapy (120 mCi) for remnant ablation and showed no tumor recurrence during 7.4 years of follow-up.

Association between the RAS and TERT promoter mutations and the effect of their coexistence on FTC recurrence

A significant association of RAS mutations with TERT promoter mutations was observed (P = 0.045; Supplementary Table 3). TERT promoter mutations were found in 2.5% of RAS wild-type cases vs 12.2% of RAS-mutated cases, and conversely, RAS mutations were found in 31.9% of TERT wild-type cases vs 71.4% of TERT-mutated cases.

The patients with a RAS mutation, especially with coexistent RAS and TERT promoter mutations, showed higher rates of recurrence or persistence than those without any mutations (Table 5). Two patients had a TERT promoter mutation alone, and FTC recurrence was not observed in them. The coexistence of two mutations had the highest recurrence rate and increased the recurrence risk 6.27-fold compared to the absence of mutations, although statistical significance was lost after adjustment for age at diagnosis and sex.

Table 5

Hazard ratios of RAS and TERT promoter mutations for recurrence of follicular thyroid cancer.

UnadjustedAdjusteda
Mutation statusCasesRecurrence/persistence, n (%)Recurrence, n (%)HR95% CIPHR95% CIP
RAS(−) TERT(−)773 (3.9)2 (2.6)11
RAS(+) TERT(−)364 (11.1)1 (2.8)1.130.10−12.460.9222.860.21−39.570.433
RAS(−) TERT(+)20 (0.0)0 (0.0)
RAS(+) TERT(+)53 (60.0)2 (40.0)13.591.88−98.550.0106.270.76−51.680.088

Adjustment for age at diagnosis and sex; HR, hazard ratio.

Discussion

The age at the diagnosis of FTC increased in the late 1990s, and the extrathyroidal extension of tumors significantly decreased recently. Moreover, the prevalence of RAS mutations also decreased in recent years, while that of TERT promoter mutations did not change. RAS mutations were associated with distant metastasis, persistent disease and frequent TERT promoter mutations. The coexistence of RAS and TERT promoter mutations was associated with a higher rate of recurrence, suggesting that they had additive effects on the prognosis, similarly to BRAF and TERT promoter mutations (13, 14).

The increase in small and low-risk PTCs has been attributed to the recent increase in thyroid cancer screening with ultrasonography (3, 16). In our study, the older age at the diagnosis of FTC after the late 1990s might be attributed to the introduction of a cancer screening program in the late 1990s. However, most other clinicopathological characteristics, including tumor size, were unchanged over time. This is because FTC usually does not have specific suspicious features of malignancy on ultrasonography (17), and in such cases, fine-needle aspiration (FNA) is usually performed when the nodular size is larger than 1–2 cm (18, 19). Furthermore, because FNA cannot distinguish between follicular adenoma and carcinoma on the basis of cytology, surgery is not immediately performed in some wait-and-see cases. However, a possible improvement in the aggressiveness of FTC was found; the total frequency of extrathyroidal extension decreased despite recent advances in pathologic evaluation. Recent increases in microscopic extensions might be related to more cautious pathologic examinations; however, gross extensions became markedly less common in the most recent decade, which may suggest a prognostic improvement in recently diagnosed FTCs. We propose more long-term observations to clarify this.

Recently, next-generation sequencing (NGS) has improved our understanding of the genetics and biology of thyroid cancer, including FTC and PTC (20, 21, 22), and RAS point mutations are the most representative driver mutations in FTC. As the results of our study, RAS mutations have been reported to be associated with distant metastasis (11, 12) and poor prognoses in FTC (9, 10). This prognostic impact of RAS mutations on FTC seems to be in contrast to the favorable prognosis of PTC with RAS mutations, which is usually FVPTC without aggressive tumor behavior (23, 24). Moreover, a similar frequency of RAS mutations was also observed in follicular adenoma (20, 25). Therefore, we suggest that the same RAS mutations might play different prognostic roles depending on the type of cancer.

Interestingly, as was found for BRAF mutations in PTC, RAS mutations were also associated with TERT promoter mutations in FTC, and their coexistence was associated with worse prognoses. TERT promoter mutations have not been detected in benign thyroid nodules, and have only uncommonly been found in cancer with a benign nature (26). In this study, patients with TERT-mutated FTC showed more frequent instances of distant metastasis, and higher recurrence and mortality. Although we could not demonstrate the effects of TERT promoter mutations alone, when they coexisted with a RAS mutation, the hazard ratio of recurrence was increased by more than 6 times (Table 5). Thus, TERT could be a useful prognostic marker of malignancy, especially for RAS-mutated follicular neoplasm.

The prevalence of the PAX8/PPARγ rearrangement has been found to vary geographically, ranging from 0% to 57% of FTCs (27); notably, a Japanese study failed to identify a single PAX8/PPARγ rearrangement in FTC (28). Its prevalence in Asia has been reported to be lower than that in other regions, but this is not conclusive because of the small number of studies in Asia. In this study, we observed a similarly low frequency of the PAX8/PPARγ rearrangement in FTC (2.1%).

Regarding other oncogenes of FTC not tested in this study, Nikiforova et al. (29) tested 12 cancer genes using targeted sequencing panel in 36 FTCs and identified that the second most common mutations after RAS (n = 12) were TSHR (n = 4) and TP53 (n = 4) mutations in conventional and oncocytic FTCs, respectively. However, in the two recently published studies of FTC genomics in Korea using a NGS approach, including a study of our institute (20, 22), the prevalence of TSHR and TP53 mutations were relatively very low: there was no FTC patient harboring TSHR mutation in both studies, and only one of 30 patients (3.3%) with TP53 mutation who had a favorable prognosis in our study (20). Moreover, according to the recent NGS studies on anaplastic or poorly differentiated thyroid cancer (30, 31), TP53, unlike TERT, might be related to the aggressiveness of undifferentiated thyroid cancer rather than well-differentiated thyroid cancer.

Our study had some limitations. Genetic testing was not performed in consecutive patients because of the limited availability of FFPE samples (14). Nevertheless, the characteristics of FTC samples performed genetic testing were not significantly different from the overall samples of FTC in the corresponding periods of our study, which could be representative. Regarding the histologic report, some FVPTC cases could have been included in the past, because FVPTC was first described in 1953 (32) and broadly recognized in the mid-1970s (33, 34), and more detailed examinations have been performed in recent decades.

Notwithstanding these limitations, to the best of our knowledge, this study is the first comprehensive study of the long-term changes in the characteristics and prognosis of FTC. In addition, the clinicopathological variables were extensively examined in all subjects, which might be useful for evaluating the effects of RAS and TERT promoter mutations on the clinical outcomes of FTC.

In conclusion, the age at diagnosis of FTC and the frequency of extrathyroidal extension have changed over the last four decades. Moreover, the prevalence of RAS mutations decreased. RAS mutations may be associated with poor clinical outcomes in FTC, especially in coexistence with TERT promoter mutations.

Supplementary data

This is linked to the online version of the paper at http://dx.doi.org/10.1530/EJE-17-0456.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this study.

Funding

This research was supported by the Seoul National University Hospital and the Seoul National University College of Medicine Research Fund (grant number 800-20120032).

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