Clinicopathological features and prognostic factors analysis of multiple synchronous distinct subtypes of primary thyroid carcinoma

To investigate the clinicopathological features, diagnosis, treatment, and prognosis of multiple synchronous distinct subtypes of primary thyroid carcinomas. The clinical data of 68 cases of synchronous carcinomas of the thyroid (STC) admitted to the Department of Thyroid Surgery in our hospital from January 2013 to December 2021 were reviewed. According to the pathological type, they were divided into the Synchronous differentiated and differentiated thyroid Carcinoma (SDDTC) group (42 cases), the Synchronous medullary and differentiated thyroid Carcinoma (SMDTC) group (18 cases), and the Synchronous Anaplastic and differentiated thyroid Carcinoma (SADTC)group (8 cases). The diagnosis, treatment, and survival of patients in each group were analyzed. Women with coexisting thyroid cancer were predominant (59 cases). Most of the symptoms were found on physical examination (47.1%) and neck mass (45.6%). The median age of patients in the SDDTC group, SADTC group, and SMDTC group was 47.5 (28–74) years old, 68.5 (26–75) years old, and 56.5 (39–74) years old. The age of the SADTC group and SMDTC group was older than that of the SDDTC group (P = 0.04, P = 0.03), and the rate of lymph node metastasis in groups SADTC (62.5%) and SMDTC (55.6%) was higher than in group SDDTC (21.4%). The disease course time, tumor location, clinical stage, and mortality of the SADTC group were significantly different from those of the SDDTC group and SMDTC group (P < 0.05). The overall survival of patients with synchronous carcinomas of the thyroid was 6–105 months, and the median overall survival was 38.5 months. The tumor-free survival was 0–90 months, 19.1% of patients developed distant metastasis, 11.8% of patients had postoperative recurrence, as well as the survival rate was estimated 91.18%. Cox model multivariate analysis showed that cervical lateral lymph node metastasis and tumor stage III/IV were independent risk factors for progression-free survival(PFS). The comparison results of the survival curves showed that the overall survival (OS)of the patients in the SADTC group was significantly worse (P < 0.01), while there was no significant difference in the PFS of different pathological types (χ2 = 5.024, P = 0.081).The OS of different treatment methods was significantly different (P = 0.002), but there was no significant difference in OS between local recurrence and distant metastases with or without surgery (χ2 = 0.954, P = 0.329). The STC has relatively unique clinical characteristics, and most patients can get a better prognosis after radical surgery. Pathological type, lateral cervical lymph node metastasis, tumor stage, and treatment are important factors which affect the prognosis of the disease. Since there are two distinct tumors with different aggressiveness, treatment options, and prognosis, individualized management is required.


Introduction
Thyroid cancer is the most common endocrine malignancy, and its incidence is increasing year by year, with an annual growth rate of about 6.2% [1,2]. There are four common pathological types of thyroid cancer: papillary carcinoma (70 to 80%), follicular carcinoma (10.0 to 27.8%), medullary carcinoma (5 to 10%), and anaplastic carcinoma (3 to 8%) [3], but its prognosis is generally good, accounting for less than 0.5% of all fatal cancers [4]. Different types of thyroid cancer have different biological characteristics and behaviors, and although the incidence of thyroid cancer has increased, simultaneous development of two thyroid tumors in the same patient is still a rare event, although it is rarely reported. The synchronous carcinomas of the thyroid(STC) is divided into two forms, the first form consisting of two distinct tumors separated by non-neoplastic thyroid tissue, the other is a mixed tumor with dual differentiation [5]. Lamberg et al. first reported MTC with PTC in 1981 [6], Hales first reported a mixed medullary and follicular thyroid carcinoma (MMFTC)in 1982 [7]. Such reports have aroused extensive discussion, and since then, the coexistence of multiple different types of thyroid cancer has gradually been reported and recognized. To date, the cases of synchronous squamous cell carcinoma of thyroid (SCCT), papillary (PTC), follicular (FTC), medullary (MTC), poorly differentiated (PDTC), and anaplastic(ATC) thyroid carcinomas have been reported, and both coexisting forms of thyroid cancer have also been implicated [5,[8][9][10][11][12]. However, due to factors such as rare cases and limited conditions, there are few clinical studies on the clinicopathological characteristics and prognosis of multiple synchronous, distinct subtypes of primary thyroid carcinoma. This paper retrospectively analyzed the clinical data of 68 patients with STC admitted to the First Affiliated Hospital of Zhengzhou University, in order to explore the clinicopathological characteristics, diagnosis, treatment, and prognostic factors of the multiple synchronous distinct subtypes of primary thyroid carcinoma and to raise the level of awareness of various combinations of thyroid cancer, thereby providing a new perspective for the clinical diagnosis and treatment of this rare disease.

Patients
The study has been reviewed by the ethics committee of our hospital (batch number: 2021-KY-0202-002). Retrospectively collected 68 cases of STC with clear postoperative pathological diagnosis in our hospital from January 2013 to December 2021, analyzing patient clinicopathological features, auxiliary examinations, treatment, recurrence, metastasis, and death information. According to postoperative pathology, patients were divided into the Synchronous differentiated and differentiated thyroid Carcinoma (SDDTC) group (42 cases), the Synchronous medullary and differentiated thyroid Carcinoma (SMDTC) group (18 cases), and the Synchronous Anaplastic and differentiated thyroid Carcinoma (SADTC) group (8 cases).
Inclusion criteria: postoperative pathologically confirmed STC; complete clinical and follow-up data. Exclusion criteria: unclear pathological diagnosis; incomplete clinicopathological data; patients lost to follow-up.

Research methods
Collect patient-related data, including age, gender, symptoms at the time of visit, time from symptom onset to visit, preoperative examination results, surgical resection range, tumor size, and number, whether there is distant metastasis, tumor stage, comprehensive treatment, etc. All surgically resected specimens were confirmed by 2 senior pathologists, and the subtypes of thyroid cancer were diagnosed according to the World Health Organization (WHO) thyroid cancer pathological classification criteria [13] and was staged using the American Joint Committee on Cancer-Tumor Node Metastasis criteria (8th edition) [14].

Follow up
The postoperative conditions of the enrolled patients were obtained through follow-up visits, telephone follow-up, etc. The date of diagnosis of STC in the histopathological report was the starting time, and the month was the unit of measurement, and follow-up was conducted every 3 months within the first year after surgery, the follow-up interval is 6 months in the second year after surgery and beyond, and the follow-up deadline is June 17, 2022. Overall survival (0S) was calculated from the date of pathological diagnosis to the date of death or final follow-up. Progression-free survival (PFS) was defined as the time from the date of pathological diagnosis to the date of distant metastasis, recurrence, death, or the end of follow-up. Local recurrence or distant metastases are mainly based on imaging and biopsy pathology.
Statistical analysis SPSS 21.0 was used for data processing. Enumeration data was expressed as the number of cases and percentage (%), χ² test and Fisher 's exact probability method were used for comparison between groups, Kolmogorow-Smironov test was used for normality test, and measurement data that conformed to normal distribution were expressed as mean ± standard deviation. The t-test was used for comparison, the median (range) was used for measurement data that did not conform to the normal distribution, and the Mann-Whitney rank sum test was used for comparison between groups. Univariate and multivariate Cox regression analysis was used to screen the influencing factors of PFS; Kaplan-Meier was used to estimate the overall survival rate and progression-free survival rate; and the Log-rank method was used to test the significance of the survival rate. P < 0.05 was considered statistically significant.

Baseline data
The baseline characteristics of 68 patients with STC are shown in Table 1. A maximum of 13 cases of STC can be seen in a year, and at least 1 case (Fig. 1). PTC-FTC is the most common (62%) of STC, followed by PTC-MTC (25%), FTC-MTC is the least common, only 2% (Fig. 2). Females were predominant (59 cases), ranging in age from 26 to 75 years, with an average of 51.72 years. Health examination findings and neck mass were the common symptoms. The disease course ranged from 1 day to 20 years. 12 patients had Hashimoto's thyroiditis, and only 1 patient had a family history of thyroid cancer. The tumor stage was mostly I/II stage (66.2%), 13 cases (19.1%) had distant metastasis and the postoperative recurrence rate was 11.8%. 99.7% of the patients had less than 3 cancer foci and 66.2% of the patients had tumors located on both sides of the thyroid lobes. Five patients had died at the end of follow-up, and the median overall survival was 38.5 months. There were 5 mixed tumor with dual differentiation in STC, and classic ultrasound features are shown in Fig. 3.

Comparative analysis
As presented in Table 2, the median age of patients in SDDTC group, SADTC group, SMDTC group was 47.5 (28-74) years, 68.5 (26-75) years, and 56.5 (39-74) years old, they had a statistically significant difference in age (P = 0.04, P = 0.03); there was no significant gender difference among the three groups (P = 1.00). The disease duration of SADTC was significantly longer than that of  SDDTC and SMDTC (P = 0.03, P = 0.03), and the maximum tumor diameter was also significantly longer than that of SDDTC and SMDTC (P < 0.01, P < 0.01). Compared with the SADTC, SDDTC and SMDTC foci were mostly located on both sides of the thyroid gland (P < 0.01, P < 0.01). Up to 78.6% SDDTC had no lymph node metastasis, while 7(87.5%) patients in SADTC group developed central lymph nodes metastases and 1 (12.5%) patient developed lateral lymph nodes metastases, meanwhile, the central and lateral cervical lymph nodes metastases rates in SMDTC group were respectively 37.5 and 29.2% (P = 0.02, P < 0.01). Among the patients with lymph node metastasis, 60% of patients with SMDTC had more than 5 metastatic lymph nodes, while no patients with SDDTC had more than 5 metastatic lymph nodes (P = 0.02). There was no significant difference in the distant metastasis rate and recurrence rate among the three groups (P > 0.05). All 8 SADTC were in stage III/IV and the differences were statistically significant compared with those of SDDTC and SMDTC (P < 0.01, P < 0.01). 50% of the patients with SADTC died, while the mortality rate of SDDTC and SMDTC was only 4.8 and 0% and the difference was statistically significant (P < 0.01, P < 0.01).

Prognosis and survival analysis of patients
Overall survival: All 68 patients were followed up, the overall survival of patients with thyroid coexisting cancer was 6-105 months, and the median overall survival was 38.5 months. As of June 2022, the tumor-free survival time was 0-90 months, 6 patients died and the survival rate of STC was 91.18%. 21 patients had recurrence and metastasis after surgery, including 8 patients with local recurrence alone and 12 patients with simple distant metastasis. There were 1 patient with both distant metastasis and locoregional recurrence. Table 3 summarizes the results of univariate and multivariate analyses of patients' PFS. Univariate analysis showed that the pathological type (P = 0.040), the largest tumor diameter (P = 0.017), the area (P < 0.001) and the number of lymph node metastasis (P = 0.020), the clinical stage of the tumor (P = 0.005) were significantly related to the patients PFS related. Cox model multivariate analysis showed that cervical lateral lymph node metastasis and tumor stage III/IV were independent risk factors for the prognosis of PFS. We compared the OS and PFS survival curves of the 3 groups of patients. Compared with the other two groups, the patients in the SADTC group had significantly worse OS (P < 0.01) (Fig. 4a), while there was no significant difference in PFS between the three groups (χ2 = 5.024, P = 0.081) (Fig. 4b), using Kaplan-Meier analysis, the OS of different treatment methods was significantly different (P = 0.002) (Fig. 4c), but the OS of local recurrence and distant metastases with or without surgery The results of the survival curve showed that there was no significant difference between surgery (5 cases) and no surgery (16 cases) (χ2 = 0.954, P = 0.329) (Fig. 4d).

Discussion
There are significant differences in clinical features and histological manifestations of different pathological types of thyroid cancer. Their coexistence in the same patient is a very rare event, and although related cases have been reported, to our knowledge this is the first large cohort study of such cases with long-term follow-up. In the author's unit, in the past 9 years, only 68 patients were diagnosed with STC, of which STC found in the past 5 years accounted for 73.53% of the total cases, and thyroid coexisting cancer combined with PTC accounted for 98.52%, advancements in ultrasound imaging have increased numbers of patients with PTC diagnosed with additional potential tumour foci coexisting with the primary tumour. Additionally, progress in pathologic diagnosis has resulted in the increased identification of small tumour foci previously undetectable before   surgery. Such patients are often treated more aggressively based on existing guidelines [15]. Regarding STC, two tumors with different pathological types have been reported in a single or two separate lesions. The former is called a mixed thyroid tumor, and the latter is called a thyroid collision tumor. The term "collision tumor" is described as the coexistence of two or more histologically distinct neoplastic morphologies separated by normal tissue in the same organ [16]. According to a review by Ryan et al. in 2015, only 444 thyroid collision tumors were reported in the literature [8]. In our study, 61 STC met the definition of thyroid collision tumor. Previous clinical reports on mixed thyroid carcinoma were mostly medullary-follicular carcinoma [11,17,18]. reports <60 cases [11]. In addition, mixed papillary and follicular carcinoma(MPFTC), mixed squamous cell and follicular carcinoma, mixed anaplastic carcinoma of the thyroid with papillary carcinoma (MAPTC)and Other types of mixed thyroid carcinoma are also occasionally reported [12,19,20], we found 7 mixed thyroid carcinomas, including 3 MMTC, 1MAPTC, and 1 MPFTC, 1 mixed papillary and poorly differentiated thyroid carcinoma, 1 mixed papillary thyroid carcinoma and squamous cell carcinoma.
Since the pathogenesis of the coexistence of two tumors of different origins in the same thyroid gland is unclear, various hypotheses have been proposed to explain this phenomenon, including the stem cell theory, the random collision effect theory, and the theory that one tumor induces another tumor [8,9,21,22]. The collision effect theory states that tumors are composed of two distinct, separate components. The authors supporting this theory have proposed the term "collision tumor" to refer to these tumors, and our patients mostly have this form-two tumors  [23]. Consistent with the Gurkan report [23] the presence of mixed thyroid carcinoma seems to support the stem cell theory. However, no single theory can fully explain the origin and pathogenesis of these rare tumors. Regardless of the pathogenesis, it is important for us clinicians and pathologists to be aware of its existence, because the histological manifestations of STC can be very variable and difficult to differentiate, all of which sometimes require uncertainty about the primary thyroid cancer in the diagnosis. Therefore, lymph node histological/ cytological analysis is sometimes required in patients with a history of indeterminate diagnosis or misdiagnosed thyroid malignancy. The primary tumor of thyroid cancer was missed in 3 patients, only one thyroid cancer was diagnosed, and the existence of another thyroid cancer was later found through the metastatic lymph nodes.
Most of the previous literature reported that Synchronous MTC and PTC [24,25], even once thought it was the most common thyroid collision tumor [26]. Collision tumors of PTC and FTC were considered to be rare, and the first case of PTC and FTC collision tumors was not reported until Plauche in 2013 [27], so there is little literature data on this combination, but most of our cases are Synchronous PTC and FTC, accounting for 62%, followed by Synchronous MTC and PTC accounting for 25%. Obviously, our large sample cohort study shows that Synchronous PTC and FTC is the most common, which is also consistent with the incidence of singleton thyroid cancer, PTC is the most common, followed by FTC, MTC, and ATC [28]. Obviously, STC usually has two cancer foci, occasionally multifocal cancer with more than 2 cancer foci, and a few mixed thyroid cancer with 1 cancer foci, and our study Fig. 4 a The overall survival time of patients in group SADTC was shorter than that in group SDDTC and group SMDTC, and the prognosis was worse (p < 0.01). b Patients in group SDDTC, SMDTC and SADTC may have disease progression and there is no significant difference in progression-free survival rate among three groups (p = 0.08). c The prognosis of patients treated with surgery combined with radiotherapy and chemotherapy was worse (p < 0.01). d There was no significant difference in overall survival between patients with or without reoperation for recurrence or metastasis (p = 0. 33) found that the patients with lesions located in the bilateral thyroid lobes were more than the lesions. Coexisting with only the number of patients within one lobe, we often find microcarcinomas in the contralateral lobe that were not previously identified in postoperative pathological specimens. It can be seen that for most thyroid-impinging tumors, thorough pathological examination of surgical specimens remains the only reliable diagnostic tool [21].
A study of multifocal papillary thyroid carcinoma found that bilateral multifocal papillary thyroid carcinoma occurred in 65% of cases, and that bilaterality was not associated with adverse outcomes in terms of local/regional recurrence or distant metastasis and death [29]. In 45 of our 68 cases, two thyroid cancer lesions were located on both sides of the thyroid lobe, accounting for 66.2%. Similar to his findings, no poor prognosis was found for bilateral STC. Consistent with published case reports and reviews [3,8,22,30], our study confirmed that STC often occurs in middle-aged and elderly women, mean age at diagnosis was 51.72  years, male to female ratio was 9:59.The most common manifestations of STC are neck masses and thyroid nodules found by color Doppler ultrasound in health examination. Most of the cases were in the early clinical stage, meaning 66.2% of the cases were in stage I/II, However, 19.1% of patients had distant metastasis, and 11.8% of patients had postoperative recurrence. It can be seen that STC is more aggressive, and the recurrence and metastasis rates are high, which is also consistent with Virginia's point of view [27].
The same case involves different pathology subtypes of thyroid cancer, so we can further study the clinicopathological characteristics of different STC by classifying these subtype combinations. The results showed that the age of SDDTC was younger than that of SMDTC and SADTC, and the average age at diagnosis of SADTC was the largest, followed by SMDTCr, which was similar to the age analysis results of singleton thyroid cancer [31][32][33]. There was no statistically significant gender difference among the three groups, SADTC not only has a longer maximum tumor diameter than SDDTC and SMDTC, but also has a significantly longer disease duration than SDDTC and SMDTC, previously reported that anaplastic carcinoma is highly aggressive, progresses rapidly, and the neck mass expands rapidly [34]. However, the SADTC in our case has a longer course of disease, up to 20 years. There are existing reports on SADTC and the course of the disease are also more than 20 years [9,12], may be due to undifferentiated progression from differentiated [35]. Epidemiological, clinical, and pathological evidence suggests that ATCs are in most cases transformed from preexisting or coexisting DTCs [36]. Considering that the transformation of thyroid cancer may take a long time, this is consistent with our findings of a longer course of SADTC. Interestingly, the foci of SDDTC and SMDTC are mostly bilateral, while the SADTC are mostly ipsilateral, and our findings are consistent with previously reported data [10,35,37,38].
Multiple studies have reported an increased risk of regional lymph node metastasis in patients with multifocal thyroid cancer [15,39]. In our study, lymph node metastasis of SMDTC and SADTC was more common than that of SDDTC. Both 66.7% SMDTC and SADTC had lymph node metastasis, while 78% SDDTC had no lymph node metastasis. Reviewed the literature and found that the lymph node metastasis of Synchronous FTC and PTC was lack of reports, which may be the first case report of lateral lymph node of metastasis of Synchronous FTC and PTC [40]. In our cases, the number of metastatic lymph nodes in the SDDTC was less than 5, while 60% SMDTC and 40% SADTC patients had more than 5 metastatic lymph nodes, which is in line with the relevant reports [5,41,42]. Although there was no significant difference in distant metastasis among the three groups of patients, SDDTC containing FTC was more common in terms of the number of cases with distant metastasis, accounting for 76.9% of STC cases with distant metastasis, It may also be related to the biological characteristics of FTC prone to distant metastasis [43]. But puzzlingly, the first report of distant metastases from synchronous Papillary and Follicular thyroid carcinomas will not appear until 2022 [44]. Therefore, the biological behavior changes of the Synchronous FTC and PTC need to be further studied. According to the clinical staging of the American Joint Committee on Cancer (AJCC)/International Union for Cancer Control (UICC), we found that SMDTC and SADTC are more common in stage III/IV than SDDTC, and more than 90% of the SDDTC are in stage I/II. Therefore, the clinical progression and prognosis of SDDTC are better than that of SMDTC and SADTC. During follow-up, the mortality rate of SADTC was 50% higher than that of SDDTC and SMDTC, this is also in line with the current reports on the mortality/prognosis of different subtypes of thyroid cancer and the understanding of tumor staging [45], The current analysis showed no significant difference in recurrence rates among the three groups of patients.
Prognostic analysis showed that STC including anaplastic thyroid carcinoma had significantly poor overall survival, suggesting that pathological type and tumor composition influence prognosis. Thyroid collision tumors consisting of papillary thyroid cancer and anaplastic thyroid cancer imply a poor prognosis due to the presence of anaplastic tumor fractions and should be evaluated and treated by surgery as early as possible [46]. Treatment of STC is challenging, and it often contains components with different aggressiveness, treatment options, and prognosis. Currently, there is no specific guideline, and individualized treatment can only be carried out according to the current guidelines, usually including surgical resection and adjuvant therapy. In our study, according to whether the patients were combined with other adjuvant therapy after surgery, the treatment methods were divided into three types: surgery alone, surgery combined with iodine-131 therapy, and surgery combined with radiotherapy and chemotherapy. We found that the prognosis of patients treated with surgery combined with radiotherapy and chemotherapy was poor. This is not difficult to explain, generally, only patients with undifferentiated or advanced thyroid cancer are combined with radiotherapy and chemotherapy. Like other cancers, STC can also experience tumor progression such as local recurrence, distant metastasis, or death. Although the OS of the SADTC was worse, there was no significant difference in the progression-free prognosis between the three groups. Hao Li and Jie Zhou reported cases in which lymph node recurrence and distant metastasis of SMDTC were treated with surgery for recurrence and metastasis, respectively, and good curative effects were obtained [47,48], there are also case of radiotherapy for distant metastases of SDDTC without surgery to achieve good prognosis [16]. However, Marcos reported that a case of SADTC recurrence was still ineffective after surgery and progressed rapidly and died [9]. Of 21 patients with recurrence or metastasis, 5 underwent surgery for recurrence or metastasis and 16 did not undergo reoperation. There was no statistically significant difference in the prognosis of recurrence or metastases with or without reoperation, which may be related to the lack of data we included and the choice of the timing of reoperation, or even the combination of different pathological types.

Conclusion
To sum up, STC is relatively rare, and it is easy to miss diagnosis and treatment in clinical practice. It is often found by health examination. In our study, the tumors of 45 patients (66.2%) were located in different lobes of the thyroid gland. We should be vigilant in preoperative examination and diagnosis. Experienced sonographers can help them make a preoperative diagnosis. The final diagnosis requires histopathology after complete resection. Whether it is ultrasonography or pathological examination, all glandular lobes must be carefully examined to improve the ability to diagnose various types of thyroid cancer and prevent the omission of lesions. The treatment of thyroid coexisting cancer is mainly based on surgical resection. The follow-up treatment comprehensively considers the characteristics of the two cancers, and adopts a more active plan for treatment and follow-up according to their respective guidelines. Pathological type, tumor stage, and treatment are important factors affecting the prognosis of the disease. This study is helpful to improve the diagnosis and treatment of STC, but there are still some shortcomings: It is a retrospective study with limited clinical and prognostic data due to the low incidence and small number of cases. Moreover, for such patients, the genetic testing has not been perfected, and the pathogenesis needs to be further studied. How to establish effective preoperative screening methods, standardize the scope of surgery and explore other effective treatment methods, and individualized treatment for patients with multiple synchronous distinct subtypes of primary thyroid carcinoma is an urgent problem that needs to be solved.