Prognostic Assessment of BRAF Mutation in Preoperative Thyroid Fine-Needle Aspiration Specimens

Rita Abi-Raad, MD; Manju L. Prasad, MD; Jingwei Zheng, MD; Pei Hui, MD, PhD; Berrin Ustun, MD; Kevin Schofield, CT(ASCP); Guoping Cai, MD; Adebowale J. Adeniran, MD

Disclosures

Am J Clin Pathol. 2021;156(1):100-108. 

In This Article

Discussion

Although there are continuing reports about the relationship between BRAF mutation and aggressive tumor characteristics such as ETE, LVI, multicentricity, higher tumor stage, LN metastasis, and resistance to radioactive iodine therapy (131I),[8,13,14,16–20,28] numerous studies have failed to confirm such an association.[21–26] In thyroid FNA specimens, the utility of BRAF testing for diagnostic purposes has been widely documented.[11,29] Furthermore, some authors have advocated for preoperative BRAF testing to determine the appropriate extent of surgery.[8,20]

The current study showed that mutant BRAF in FNA specimens correlated significantly with ETE (P = .006). More recently, the eighth edition of the AJCC staging system for differentiated thyroid carcinoma has restricted the criterion of T staging to gross ETE based on increasing evidence that microscopic ETE does not affect prognosis.[27,30–32] We found that mutant BRAF was significantly associated with gross ETE as well (P < .001). Since the status of gross ETE is considered in determining tumor stage, mutant BRAF tumors associated with gross ETE were also associated with higher tumor stage compared with wild-type BRAF (P = .04). At variance to the results reported by others,[8,19,28] there were no significant differences in tumor size, LVI, or LN metastasis between BRAF mutant and BRAF wild-type tumors.

The relationship between BRAF status and LN metastasis is of particular interest. A positive association between BRAF mutation and LN metastasis has been described by some authors,[8,20] an observation challenged by others.[24–26,33] At our institution, even though patients with tumors harboring BRAF mutation were more likely to undergo LN dissection compared with BRAF wild-type, the difference was not statistically significant. Furthermore, there was no difference in LN metastasis between BRAF mutant and BRAF wild-type tumors.

It is difficult to reconcile the inconsistency of results as reported in the literature. The heterogeneity of the PTC histomorphologic subtypes in different series, in particular the relative proportion of classic, tall-cell, and follicular variants of PTC, has certainly played a role. This holds particularly true for the recently reclassified entity NIFTP, which has not been excluded in many of those studies.[13,16,17,19] The correlation between BRAF mutational status and PTC histomorphologic subtype is un- disputable;[13,19,21,23,34–37] FVPTC is known to harbor BRAF mutation with low prevalence, while the more aggressive classic and tall-cell variants carry the mutation with a much higher prevalence. Our study is in line with the literature; BRAF was more frequently mutated in classic (64%) and tall-cell variants (89%) compared with FVPTC (19%). Furthermore, the frequency of ETE was higher in classic (14%) and tall-cell variants of PTC (67%) than it was in FVPTC (0%). Given the high specificity of this mutation for some aggressive tumor phenotypes, it remains unclear whether BRAF mutation is the initiating event for high-risk pathologic features, or rather the association of BRAF mutation with aggressive pathologic features stems from its mere association with aggressive PTC subtypes. Alternatively, high-risk pathologic characteristics are virtually never encountered in encapsulated FVPTCs that rarely harbor the BRAF mutation. We did not observe significant differences in the BRAF distribution in relation to ETE, gross ETE, or T stage when the analysis was performed within PTC subtypes. Our results fit with those of many studies, which included PTC subtype in the multivariate analysis or performed their analysis within each morphologic subtype,[8,28,36,38,39] suggesting that BRAF is more likely associated with high-risk histomorphologic subtypes but is not an independent factor in predicting aggressive tumor features. Unfortunately, many studies that have reported a positive association did not perform subgroup analysis.[13,14,19]

Another possible explanation for those discrepant results is the inclusion of multifocal and bilateral PTCs in many studies. Multifocality of PTC is common, not infrequently displaying different histopathologic subtypes, and BRAF mutation status may vary among the different foci within the thyroid.[40] In patients with PTCs characterized as wild-type BRAF, the presence of an unsampled mutant BRAF PTC may alter the reported relationship incurred by the sole sampling of the wild-type PTC focus. To circumvent this factor, multifocal and bilateral PTCs were excluded from our study. As a downside of this exclusion, however, the association of BRAF mutation with multicentricity could not be assessed. In addition, BRAF mutation, contrary to prior concepts, does not always occur as the sole mutation. The combination of BRAF and TERT mutations has been shown to confer a poor behavior.[41] In those studies supporting a positive relationship between BRAF mutation and aggressive pathologic features, the concurrent presence of a high-risk driver mutation that is responsible for the apparent association of mutant BRAF with worse pathologic features cannot be entirely excluded. Similarly, our cases were only tested for BRAF mutation. Whether the pathologic features of wild-type BRAF cases are the result of a different molecular alteration is also unknown.

Another drawback of our study, similar to other retrospective studies, is the presence of selection bias that may have occurred, since only patients referred to surgery are included in our study cohort. While surgery is likely performed for patients with a diagnosis of PTC or indeterminate cytology diagnosis with mutant BRAF, patients with indeterminate cytology and wild-type BRAF may proceed to surgery only when clinical or radiologic suspicions are present, which could preoperatively correlate with worrisome pathologic features. On the other hand, patients with a more benign clinical and radiologic presentation, reflecting more benign preoperative pathologic characteristics, will not undergo surgery. This referral bias to surgery has probably underestimated the relationship of BRAF with the abovementioned pathologic features by excluding those wild-type BRAF tumors associated with low-risk pathologic characteristics. In addition, the relatively small number of cases in subgroup analysis, performed to correlate BRAF status with pathologic variables within each histomorphologic subtype, constitutes a limiting factor. While we found no differences in aggressive histopathologic features between mutant and wild-type BRAF groups within each subtype, the numbers might be too small to reach statistical significance.

In conclusion, our study demonstrates that the presence of BRAF mutation in thyroid FNA is associated with aggressive PTC subtype, ETE, gross ETE, and T stage. However, we found no difference in LN metastasis between BRAF mutant and BRAF wild-type tumors, which argues against the use of BRAF mutation as the sole and independent determinant of risk stratification to guide the extent of LN dissection.

processing....