Classic Papillary Thyroid Carcinoma with Tall Cell Features and Tall Cell Variant Have Similar Clinicopathologic Features

Abstract

Background: The tall cell variant of papillary thyroid carcinoma (TCVPTC) is more aggressive than classic papillary thyroid carcinoma (PTC), but the percentage of tall cells needed to diagnose TCVPTC remains controversial. In addition, little is known about the clinicopathologic features of classic PTC with tall cell features (TCF). Methods: We retrospectively selected and reviewed the clinicopathologic features and presence of the BRAF mutation in 203 cases of classic PTC, 149 cases of classic PTC with TCF, and 95 cases of TCVPTCs, which were defined as PTCs having <10%, 10-50%, and ≥50% tall cells, respectively. Results: TCVPTCs and classic PTCs with TCF did not vary significantly in clinicopathologic characteristics such as pathologic (p) T stage, extrathyroidal extension, pN stage, lateral lymph node metastasis, or BRAF mutations; however, these features differed significantly in TCVPTCs and classic PTCs with TCF in comparison to classic PTCs. Similar results were obtained in a subanalysis of patients with microcarcinomas (≤1.0 cm in size). Conclusions: Classic PTCs with TCF showed a similar BRAF mutation rate and clinicopathologic features to TCVPTCs, but more aggressive characteristics than classic PTCs.

Headnote

Background: The tall cell variant of papillary thyroid carcinoma (TCVPTC) is more aggressive than classic papillary thyroid carcinoma (PTC), but the percentage of tall cells needed to diagnose TCVPTC remains controversial. In addition, little is known about the clinicopathologic features of classic PTC with tall cell features (TCF). Methods: We retrospectively selected and reviewed the clinicopathologic features and presence of the BRAF mutation in 203 cases of classic PTC, 149 cases of classic PTC with TCF, and 95 cases of TCVPTCs, which were defined as PTCs having <10%, 10-50%, and ≥50% tall cells, respectively. Results: TCVPTCs and classic PTCs with TCF did not vary significantly in clinicopathologic characteristics such as pathologic (p) T stage, extrathyroidal extension, pN stage, lateral lymph node metastasis, or BRAF mutations; however, these features differed significantly in TCVPTCs and classic PTCs with TCF in comparison to classic PTCs. Similar results were obtained in a subanalysis of patients with microcarcinomas (≤1.0 cm in size). Conclusions: Classic PTCs with TCF showed a similar BRAF mutation rate and clinicopathologic features to TCVPTCs, but more aggressive characteristics than classic PTCs.

Key Words: Thyroid neoplasms; Histologic types; Classification; Tall cell features

Papillary thyroid carcinoma (PTC) is a disease with an indo- lent course, excellent overall prognosis, and a long-term surviv- al rate close to that of the general population;1 however, some variants of PTC have been associated with an increased risk of recurrent disease and aggressive behavior.2 The tall cell variant of PTC (TCVPTC) is the most common aggressive variant of PTC.2-4 The incidence of TCVPTC ranges from 4% to 17% of all PTCs, and its disease-free 10-year survival rate is believed to be 10% to 15% lower than that of classic PTC.5,6 The incidence of this disease in Korea has been reported to be up to 5%, al- though there are limited existing data.7,8 Some studies suggest that TCV PTC is still under-diagnosed worldwide.3-5,9,10 In fact, 1% to 13% of tumors originally diagnosed as classic PTC can be reclassified as TCV by thyroid expertise.11,12

TCVPTC was originally described by Hawk and Hazard9 in 1976 as a distinctive subtype of PTC. Tall tumor cells are at least three times as tall as they are wide and have moderate to abundant eosinophilic cytoplasm, basally oriented nuclei and nuclear features typical of classic PTC.2,10 TCVPTCs present later in life and have more aggressive pathologic features such as a larger size and higher frequency of extrathyroidal extension, lymph node metastasis, vascular invasion, and distant metasta- sis, when compared to classic PTC.5,9,11,13,14 However, some studies have shown that the worse prognosis of TCVPTC is re- lated to prognostic factors such as old age, tumor size, and ex- trathyroidal extension rather than the histologic type itself.4

There is still controversy with regard to what percentage of tall cells defines a TCVPTC.2,9,15 Thresholds ranging from 10 to 75% have been suggested by various studies.2,5,15-18 PTCs har- boring fewer tall cells than the required cutoff percentage are diagnosed as classic PTC with tall cell features (TCF). There- fore, the incidence and clinical results of TCVPTC may vary ac- cording to different diagnostic criteria and the pathologist's lev- el of experience.15,19

The goal of this study was to investigate the prevalence of TCVPTC in Korea and possible differences in clinicopathologic features between TCVPTCs and classic PTCs with TCF accord- ing to the percentage of tall cells.

MATERIALS AND METHODS

Patients

We performed a retrospective review of a prospectively main- tained database of patients with PTC under approval by the In- stitutional Review Board of The Catholic University of Korea Seoul St. Mary's Hospital (KC13RISI0917). A total of 2,139 patients underwent surgery for PTC St. Mary's Hospital be- tween January 2012 and December 2013. We enrolled a total of 244 consecutive patients with TCVPTC (n=95) and classic PTC with TCF (n=149) and also selected 203 consecutive pa- tients with classic PTC as a control group. The age of the pa- tients at the time of diagnosis ranged from 22 to 79 years (mean 47.6 years for classic PTC, mean 51.0 years for classic PTC with TCF, and mean 47.1 years for TCVPTC).

Histopathologic review

All classic PTC, classic PTC with TCF and TCVPTC cases were reviewed by three board-certified surgical pathologists (C.K.J, G.S.P, and Y.S.L) with special interest in thyroid to iden- tify clinicopathologic characteristics and the percentage of tall cells. Tall cells were defined as cells with a height at least three times their width, abundant eosinophilic cytoplasm and nuclear features characteristic of classic PTCs such as enlarged, irregular, clear nuclei with grooves and pseudo-inclusions (Fig. 1).2,5,13,19 A tumor was classified as a classic PTC if it had any well-formed papillary structure and contained < 10% tall cells. A tumor was further defined as classic PTC with TCF if it harbored between 10% and 50% tall cells (Fig. 2) and as a TCVPTC if it con- tained 50% or more tall cells. The 50% criterion for TCVPTC was based on the World Health Organization classification and previous studies on TCVPTC.5,12,13,15,18-21 To determine whether samples reached the cutoff values of 10% and 50% of tall cells in the whole sections of tumor tissue, we used a digital training set which has the known percentage calculated from digitiza- tion of whole slide imaging. Discrepancies between the observ- ers were found in less than 10% of the reviewed slides. A con- sensus was reached when there was discrepancy between the ob- servers. We excluded any cases with tumor necrosis or marked mitotic activity (≥ 3 mitoses/10 high power field, 400× ) be- cause these pathologic features themselves could be related to tumor aggressiveness.5,6,19,20 In cases with multiple tumor foci, the largest tumors were selected as the primary lesions. TNM staging was performed according to the American Joint Com- mittee's Cancer staging manual 7th edition.

BRAF mutation analysis

Of a total of 447 PTC cases, 417 underwent BRAF mutation testing. Genomic DNA was extracted from two or three 10-μm thick paraffin tissue sections using the QIAamp DNA Mini kit (Qiagen, Hilden, Germany). The tissue sections were manually microdissected to enrich for tumor cells. We screened for muta- tions in exon 15 of the BRAF gene using polymerase chain re- action amplification and direct DNA sequencing, as described previously.8,22

Statistical analysis

We used Student's t-test to compare two different groups of continuous parametric data with a normal distribution. Pear- son's chi-square test was used to assess the relationship between categorical variables. For the multivariate analysis, we included all variables with a univariate probability (p) value of <.10 in a binary logistic regression test. Two-sided p-values < .05 were considered to be statistically significant. Statistical analysis was performed with SPSS ver. 16.0 (SPSS Inc., Chicago, IL, USA).

RESULTS

Out of a total of 2,139 patients with PTCs, 149 (7.0%) had classic PTC with TCF and 95 (4.4%) had TCVPTC.

Comparison of clinicopathologic characteristics among histopathologic types

When the clinicopathologic features of classic PTCs were compared with those of TCVPTCs and classic PTCs with TCF, patients with classic PTCs were younger at the time of surgery and showed lower pT and pN stages, a lower rate of extrathyroi- dal extension, a lower rate of lateral lymph node metastasis, and a lower frequency of BRAF mutations (Table 1). All BRAF mu- tations were V600E mutations. There were no statistically sig- nificant differences between TCVPTCs and classic PTCs with TCF with regard to mean age and clinicopathologic features such as pT, extrathyroidal extension, pN, lateral lymph node metastasis, or rate of BRAF mutation (Table 1). In multivariate analysis (Table 2), age group, extrathyroidal extension, lymph node metastasis, and BRAF mutations of classic PTC were sig- nificantly different from those of TCVPTCs and classic PTCs with TCF. The clinicopathologic features of TCVPTCs were not significantly different from those of classic PTCs with TCF.

Subanalysis of microcarcinomas (≤1.0 cm in size) according to histologic subtype

Patients with classic PTCs were younger at surgery and showed a higher frequency of single lesions, lower pT and pN stages, a lower rate of extrathyroidal extension, and a lower fre- quency of BRAF mutations, compared to patients with classic PTCs with TCF or TCVPTCs (Table 3). There were no statisti- cally significant differences in clinicopathologic characteristics between TCVPTCs and classic PTCs with TCF (Table 3).

Subanalysis of unifocal tumors according to histologic subtype

We only included unifocal cancers to eliminate any possible bias from tumor multifocality. We further analyzed the clinico- pathologic differences between classic PTCs, classic PTCs with TCF or TCVPTCs. The results for unifocal cancers were in the same range: clinicopathologic features of classic PTCs were sig- nificantly different from those of classic PTCs with TCF or TCVPTCs, but no significant differences were observed in clin- icopathologic characteristics between TCVPTCs and classic PTCs with TCF (Appendices 1, 2).

Subanalysis of histologic subtypes according to patient age groups (<45 years vs ≥45 years)

In a multivariate subanalysis by patient age group (Table 4), there were no significant differences in clinicopathologic vari- ables between patients under or over the age of 45 in the classic PTC with TCF and TCVPTC groups. The clinicopathologic features of classic PTCs were significantly different from those of classic PTCs with TCF or TCVPTCs across age groups.

DISCUSSION

We demonstrated that classic PTC with TCF (PTC with 10% to 50% tall cells) had the same demographic and clinico- pathologic characteristics as TCVPTC, and these tumors showed more aggressive features than classic PTC.

TCVPTC typically affects older patients and has a larger size, higher stage at presentation, higher rate of extrathyroidal exten- sion, greater risk of recurrence, and poorer disease-specific sur- vival compared to classic PTC.3,4,9,16,18,19,23 Bernstein et al.24 indi- cated that despite controlling for size, TCVs of papillary thy- roid microcarcinomas were still associated with higher pT and, pN stage and aggressive pathologic features such as extrathyroi- dal extension. In our study, we also demonstrated that micro- carcinomas with a tall cell component were more aggressive than those with classic PTC morphology.

There is controversy regarding the high incidence of lymph node metastasis in TCVPTC. Some authors have demonstrated that patients with TCVPTC have a higher rate of nodal metas- tases than patients with classic PTC.4,16 Bernstein et al.24 showed that microcarcinomas with TCVPTC have a higher rate of cen- tral lymph node metastasis than microcarcinomas with classic PTC (39% vs 13%). However, other authors did not find signif- icant differences in the lymph node positivity rate between clas- sic PTCs and TCVPTCs.3,17,19 For example, Ganly et al.19 dem- onstrated no differences in nodal metastases among TCVPTCs, classic PTCs with TCF and classic PTCs. Such discrepancies might be caused by different patient ages or different criteria re- garding the percentage of tall cells necessary for a diagnosis of TCVPTC. In our multivariate analysis according to the age groups, there were significant differences in the rates of lymph node metastasis between classic PTC and classic PTC with TCF in patients under the age of 45 years. However, the differences in lymph node metastasis rate disappeared in patients over the age of 45 years. Johnson et al.16 and Bernstein et al.24 used a 30% threshold of tall cells and showed a higher rate of nodal positivi- ty in TCVPTCs. Investigators who used a 50% to 75% thresh- old of tall cells for TCVPTC diagnosis did not find a significant differences in nodal positivity between TCVPTCs and classic PTCs.17,19

Various investigators have used different thresholds of 10%,15 30%,11,15,24 50%,5,12,13,15,18-21 70%,3 or 75%2,14,17 tall cells to de- fine TCVPTC. Ghossein and LiVolsi5 and Regalbuto et al.18 suggested a cut-off of at least 50% tall cells to classify TCVPTC. In our study, we used a 10% threshold for classic PTCs with TCF and a 50% threshold for TCVPTCs. Using this definition, lymph node metastases were significantly more frequent in clas- sic PTCs with TCF and TCVPTCs than in classic PTCs. We also demonstrated a statistically significant difference between classic PTCs and classic PTCs with TCF (10% to 50% tall cells) with regard to lymph node status such as pN and lateral lymph node metastasis (Table 1).

Beninato et al.15 reported that patients with classic PTC with TCF (tumors with ≥ 10% tall cells) have more aggressive tu- mor features such as older age at onset, higher stage, more ex- trathyroidal extension (3% to 14%), more lymph node metas- tases (40% to 68%), increased lymphovascular invasion (2% to 17%), and a higher frequency of positive surgical margins, compared with classic PTC. These aggressive features have been shown to be associated with a greater risk of recurrence and were maintained with increasing tall cell percentage (≥ 10%, ≥ 30%, ≥ 50%) within PTCs with tall cells.15 Other authors have suggested that classic PTCs with TCF may be significant- ly associated with older age at presentation, larger tumor size, higher frequency of extrathyroidal extension, and BRAF muta- tions regardless of the percentage of tall cells.13 We also demon- strated a statistically significant difference in clinicopathologic features and BRAF mutations between classic PTCs and classic PTCs with TCF (10% to 50% tall cells) (Table 1).

The more aggressive clinicopathologic features of classic PTCs with TCF and TCVPTCs at presentation translate into worse outcomes.19 Recent clinical guidelines also suggest that TCVPTCs require more aggressive surgical resection.25 The ag- gressive nature of TCVPTC may be related to a higher preva- lence of BRAF mutations.26 BRAF mutations in PTC have been correlated with aggressive tumor behavior, including ex- trathyroidal extension, tumor recurrence, advanced tumor stage at presentation and lymph node metastasis,27 even in microcar- cinomas.28 The BRAF oncogene is a strong activator of the mi- togen-activated protein kinase signaling pathway, which leads to uncontrolled cell proliferation and transformation into ma- lignancy.29 BRAF mutation also plays a role in extracellular matrix remodeling and is associated with an increase in matrix metalloproteinases, a desmoplastic stromal reaction, and inva- siveness.30 Finkelstein et al.21 demonstrated a significant associa- tion between the presence of the BRAF mutation and fibrosis, desmoplastic stromal reaction, and infiltrating tumor borders. The BRAF mutation occurs commonly in Korean patients with PTC, ranging in frequency from 52% to 87% of all cases.8 The prevalence of the BRAF mutation worldwide is reported to be as high as 80% to 100% in TCVPTCs.29 For example, Bern- stein et al.24 demonstrated that the BRAF mutation was detect- ed in 93% of all microcarcinomas with TCVPTC. In our study, the prevalence of the BRAF mutation was 96.9% and 96.4% in classic PTCs with TCF and TCVPTCs, respectively.

Age at presentation is the single most important prognostic factor in thyroid cancer.1 In our study, we found that classic PTCs with TCF and TCVPTCs were more frequent in patients over the age of 45. Interestingly, PTCs with ≥ 10% tall cells were pathologically more aggressive than classic PTCs regard- less of age groups (<45 years and ≥ 45 years).

Our study has some limitations, such as a retrospective de- sign, relatively small sample size, and the fact that the data were collected by one institution. Furthermore, we did not ana- lyze the outcomes and prognosis of patients because the follow- up period was too short.

In conclusion, PTCs with more than 10% tall cells show more aggressive clinicopathologic features than classic PTCs re- gardless of tumor size and age. Classic PTCs with TCF have a similar BRAF mutation rate and clinicopathologic characteris- tics to those of TCVPTCs. Therefore, we suggest that the pres- ence of any tall cells should be noted in pathologic reports be- cause of their clinical significance.

Conflicts of Interest

No potential conflict of interest relevant to this article was reported.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future plan- ning (2013R1A2A2A01068570).

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Appendix

(ProQuest: Appendix omitted.)