Correlations Between Percentage of BRAFV600E Alleles and Outcome in Thyroid Carcinoma

BACKGROUND Papillary thyroid carcinoma (PTC) generally shows an excellent prognosis with a 5-year survival of over 95%. Even when PTC metastasizes to the regional lymph nodes, a frequent occurrence, it generally has an excellent prognosis. However, there are some patients who present with an aggressive background or with specific clinicopathological features who exhibit aggressive developmental behavior. Several clinicopathological features, namely pathology subtype, age, distant and lymph node metastasis, extrathyroid extension, and completeness of resection can be used to predict prognosis and have been adopted in staging systems such as the AMES, MACIS, and AJCC TNM classifications.

The genetic background is a powerful prognostic determinant only when applied to some gene mutations such as TERT promoter and p53 mutations. The prognostic value of BRAFV600E, the most frequent oncogene in PTC, is weak. It correlates with greater extension, lymph node metastasis, and advanced stage (III/IV). Its correlation with outcome in PTC has been the object of many not always concordant studies. A large multicenter study was necessary to unequivocally demonstrate the association between BRAFV600E and cancer recurrence and mortality. The risk of PTC harboring BRAFV600E remains weak (recurrence hazard ratio 2.66) and the search of this mutation is not recommended in clinical management of low-risk patients with thyroid cancer.

Different research groups provided evidence on the heterogeneity of PTC that consist of a mixture of tumor cells with wild-type and BRAFV600E (5, 6). The importance of the percentage of BRAFV600E positive cells within the tumor was demonstrated in 168 PTC. The analysis of BRAFV600E/BRAFwild-type ratio was performed by pyrosequencing and higher ratio positively correlated with patient age, tumor volume, lymph node metastasis and with worst disease outcome. Patients with tumors with BRAFV600E/BRAFwild-type ratio ≥ 30% displayed an odds risk of tumor recurrence of 5.1, whereas it was only 1.75 in BRAFV600E positive tumors if the allele ratio was not considered. The qualitative analysis BRAFV600E positive versus negative PTC can explain the discordant correlations with clinicopathological features and outcome in different cohorts of patients with different ratio of BRAFV600E alleles, and ultimately the qualitative analysis BRAF mutational status can reduce the strength of its clinical significance and utility.

While the existence of intratumor heterogeneity in PTC is supported by many evidences, its extension, biological significance and clinical utility is questioned and must be further investigated.

Digital-droplet PCR (ddPCR) is an analytical method recently developed, suitable for absolute quantification of target DNA in a DNA mixture. It is the most accurate method to determine the BRAFV600E/BRAFwild-type allele ratio.