As a part of liquid biopsy, CTC has been increasingly used in the preoperative diagnosis and postoperative follow-up of lung cancer, breast cancer, gastric cancer and other malignant tumors. FR + CTC technology can accurately and quantitatively detect CTCs in peripheral blood by identifying folate receptors in tumor cells and adopting targeted PCR technology, which provides a reliable reference for each diagnosis and treatment stage of tumor patients. To our knowledge, this is the largest sample size study to date to evaluate the diagnostic value of CTCs in thyroid cancer.

The results of this study showed that the incidence of both malignant and benign thyroid tumors was higher in females than in males, which was consistent with the epidemiological characteristics of thyroid cancer. The mechanism may be related to estrogen [10]. The age and maximum tumor diameter of the malignant group were smaller than those of the benign group before PSM. Because this was a retrospective study, all the cases included were preoperative patients. Benign tumors did not need surgery until they had symptoms of neck compression, so the maximum tumor diameter of the benign group was generally higher than that of the malignant group [11]. Moreover, benign tumors progressed slowly, and it often took many years for tumors to reach the size requiring surgery, so the age of the benign group was also higher than that of the malignant group. Despite this, the number of CTCs in the malignant group was still significantly higher than that in the benign group, which fully demonstrated that more tumor cells were released into the circulation even if the malignant tumor was small or existed in the body for a short time. Previous studies have confirmed that a certain amount of CTCs can be detected in the blood samples of patients with early- to mid-stage tumors [12], even earlier than imaging indications [13]. The results were consistent with previous studies. However, after balancing age and tumor size, the number of CTCs was comparable in the two groups. This result indirectly suggests that the patient's age and tumor size may affect the release of CTCs. A prospective study that included 72 patients with DTC and 30 healthy controls suggested that the mean number of CTCs in patients with DTC with distant metastases was significantly higher than that in the healthy controls [14]. Another study that enrolled 67 thyroid cancer patients showed that the number of CTCs in thyroid cancer patients was largely increased compared to that in normal controls [15]. There were also differences in age between the groups of the above studies. Different from the present study, the control groups of the above two studies were healthy controls rather than patients with benign thyroid tumors, and the CTCs detection methods were also different. It has been pointed out that compared with elderly patients, young patients with thyroid cancer were more likely to find tumor invasion and focal metastasis at the time of diagnosis [16]. This may explain why before PSM, the age and tumor diameter of the malignant group were smaller, but the CTCs value was higher. But this needs to be confirmed by further research.

At present, thyroid cancer is often diagnosed by improving thyroid function tests, related antibody detection, color doppler ultrasound examination, contrast-enhanced ultrasonography, and aspiration biopsy. As an emerging diagnostic method, most studies have reported the efficacy of CTCs in monitoring postoperative thyroid cancer recurrence and radioiodine therapy [5, 17, 18]. There are few reports on the preoperative diagnostic value of CTCs in thyroid cancer, and most published studies have focused on lung cancer. FR + CTC is one of the detection methods of CTC. According to Na’s study [19], the sensitivity and specificity of FR + CTC for cancer diagnosis in the elderly population were 85.7% and 65.0%, respectively, with 10.0 CTC FU/3 mL. In combination with serum tumor biomarkers, the diagnostic efficiency of FR + CTC was improved. Yu et al. reported that the sensitivity of FR + CTC was 73.2% for non-small cell lung cancer (NSCLC) diagnosis [20]. Nevertheless, a prospective, multicentre, cohort study including 614 participants in France demonstrated that the sensitivity of CTC detection for lung cancer detection was only 26.3%, and concluded that CTC was unable to predict lung cancer or extrapulmonary disease [21]. In our study, the results showed that with the cutoff value of 7.55 FU/3 mL, the sensitivity of FR + CTC for thyroid cancer diagnosis was 65.6% after PSM. Binary logistic analysis indicated that CTC was not an independent influencing factor in thyroid cancer. These results suggest that detecting CTCs alone has a certain diagnostic efficacy for thyroid cancer, but it is relatively weak. On the one hand, this may be related to the lower release of CTCs in the blood than other malignant tumors, owing to thyroid cancer being considered to be an inert tumor. On the other hand, it may also be relevant to the expression of FRs in thyroid cancer cells. Although several studies have confirmed the specific expression of FRs in malignant tumor cells [22,23,24], there is no report on the expression level of FRs in thyroid cancer cells compared with other tumors. Further analysis showed that after PSM, the sensitivities of ‘CTC + ultrasound’ and ‘CTC + TgAb + ultrasound’ were 79.3% and 78.0%, respectively, the AUCs of which were comparable with that of ultrasound alone. This means that the diagnostic efficiency of CTC combined with other routine examinations was not improved.

This study also analyzed the relationship between the number of CTCs and lymph node metastasis. There was no significant difference in the number of CTCs between the N0 and N1 subgroups. Additionally, the binary logistic analysis indicated that CTC was not an independent influencing factor in lymphatic metastasis. A recent study found that neither lymph node metastasis nor distant spread had impacts on the number of CTCs [15]. The reason for this result may be attributed to the fact that PTC mainly metastasizes through lymph nodes instead of blood. With the development of oncogene diagnosis, the BRAF V600E gene mutation is considered to be related to extracapsular invasion, lymph node metastasis, multifocality and therapeutic efficacy of thyroid cancer [25]. In the present study, we innovatively analyzed whether the BRAF V600E mutation affects the number of CTCs. The results suggested that the number of CTCs was not related to the BRAF V600E mutation.

The advantage of this study is that the sample size is relatively large, and PSM was use to balance the confounding factors. The disadvantage is that it is a retrospective, single-center study. Further prospective randomized controlled studies are needed to confirm this conclusion.