In recent years, gene mutation related to thyroid carcinoma has been intensively investigated, mostly in tumor tissue. In PTC, BRAF mutation was the one with the highest incidence. According to the studies in Chinese population, the incidence of BRAF mutation ranged from 59% to 72.4% in PTC patients [15-17]. Similar to those founding, the incidence of BRAF mutation in tumor tissue in our study was 75.8%. However, the incidence of BRAF mutation in ctDNA was only 15.2% in our study which was much lower than tumor tissues. We speculate that the difference between tumor tissue and ctDNA might be explained by the following reasons, first, tumor cells in certain patients might be confined to the local tumors sites without the release into the blood; second, although tumor cells might have been released into the blood at the time of surgery, the amount of the tumor cells might has significantly decreased along with the remove of the lesions, including primary tumor and local lymph nodes metastases in a portion of the patients; third, before 131I treatment, the post-operative patients usually need to wait for at least one month for the treatment preparation, in this period the amount of the tumor cell might further decreased. As the result of above occasions, gene mutation in ctDNA might not be detected at the time of blood sampling in certain patients. Similar finding also has been reported in previous studies. Pupill et al found that 71% of the patients of PTC originally had BRAF mutation in ctDNA became BRAF mutation free after surgery .
Post-operative 131I treatment is the most crucial adjuvant therapy that can significantly decrease the recurrence of PTC with medium or high risk after thyroidectomy, resulting in a better DFS . The ablation of residual thyroid tissue and the elimination of potential unresectable 131I-avid metastasis by 131I treatment could facilitate both follow-up using Tg and 131I uptake test and possible further therapy, i.e., next 131I treatment. A number of studies have found that BRAF mutation was significantly related to the downregulation of the expression of iodide-metabolism related gene and protein, e.g. sodium-iodide symporter (NIS) [20-22]. Yang et al found that in PTC patients with distant metastasis, the incidence of non-iodine avid foci was 84.2% in the patients with BRAF mutation, while this incidence was only 5.6% in the patients with wild-type BRAF . However, as for the relationship between BRAF mutation and the outcome of 131I treatment, no common consensus has been achieved so far. In 2015 ATA guidelines, for the first-time, molecular markers were introduced in the stratification of the recurrence risk of thyroid cancer. However, no recommendation was given concerning the relationship between BRAF mutations and postoperative recurrence in PTC due to the inconsistent findings in literatures. A study based on the data of 15 years follow-up demonstrated that BRAF mutation is an independent risk factor for the recurrence of PTC . On the contrary, other studies reported that BRAF mutation may have no influence on the outcome of 131I treatment in PTC patients [15, 24]. In our study, BRAF mutation was detected in both tumor tissue and ctDNA in 1 patient with lung and bone metastases and 3 patients without distant metastasis. However, compared with the patients without distant metastasis, the patient with distant metastases showed a higher titer of BRAF mutation in ctDNA. The results implied that ctDNA might be a useful indicator for the patients with distant metastases. However, since there was only one patient with distant metastases in our study, further investigation is needed for these occasions.
The relationship of BRAF mutation and malignant characteristics of PTC is still not fully understood [4, 16, 24, 25]. In accordance with the previous studies, in our study, none of the characteristics of age, gender, primary tumor size, multifocality of tumors, extrathyroidal extension, and lymph node metastasis showed significant relationship with BRAF mutation in both tumor tissue and ctDNA, implying that BRAF might be less relevant to the malignant biological characteristics of PTC. We detected multiple BRAF mutation sites in 1 patient in ctDNA. However, due to the small sample size, the clinical significance of multiple BRAF mutation sites needs further investigation.
Other than BRAF gene, a number of gene mutations such as TERT, PTEN, PIK3CA, TP53, RAS also have been investigated in the origination and malignant progression of thyroid cancer. It has been noticed that the co-existence of BRAF mutation and other mutations might be involved in the tumorigenesis and dedifferentiation of PTC and may be more predictive for the prognosis. Xing et al. reported that the PTC with the combination of BRAF mutation and TERT promoter mutation were the most aggressive and had the highest incidence of recurrence, compared with that with single BRAF mutation . In our study, only BRAF gene mutation was detected. To overcome this disadvantage, further investigations are demanded to elucidate the influence and the interaction of more gene mutations in PTC. Other limitations should also be noted in our preliminary study. First, the samples were not sufficient, especially for tumor tissue; Second, the sensitivity of our sequencing platform for ctDNA detection was relatively low (1637X), the mutation in the patients with trace amount of mutation might be failed to be detected. Our subsequent study will enroll more samples for both ctDNA and tumor tissue. In addition, more gene mutations will be investigated as the target, along with the expanding of the depth of sequencing and improving of sequencing sensitivity.
In conclusion, the value of BRAF mutation alone might be limited in predicting therapeutic outcome of the first 131I treatment in PTC. No certain relevance was found between BRAF mutation and malignant biological features in PTC.