RIHT is one of the most common complications observed in patients with HNC that underwent radiotherapy. The median follow-up time for this study was 10 months (rang, 3-30 months), and the incidence of RIHT was 36.9%. Similar phenomenon was found in Sommat et al.’s study [
10]. Interestingly, 6 patients occurred hyperthyroidism, in which 3 patients gradually convert to be euthyroid, 1 patient eventually reversed to be hypothyroid. We thought the change from hyperthyroidism to hypothyroidism may be attributed to transient release of thyroid hormone caused by serve injury of thyroid parenchymal cells. The other change from hyperthyroidism to euthyroid could be interpreted by increased permeability of cellular membrane that were not changed [
11]. These are good explanations for the transient changes of thyroid function observed in our series patients.
Clinical factors associated with hypothyroidism occurrence
The significantly related clinical factors for RIHT confirmed in our study were female gender, use of chemotherapy, frequency of chemotherapy, N stage, surgery involving thyroid and smaller thyroid volume. In our study, female patients were more prone to RIHT compared with male (61.9% vs. 31.2%;
P = 0.01). Although the smaller volume of the female thyroid glands may be able to explain the sex difference, gender has no significant impact on thyroid volume(14.7 ± 8.6 and 12.1 ± 7.0, respectively;
P = 0.059) in our research. Fan et al. [
12] and Jain et al. [
13] reported consistent conclusion. Therefore, female patients with HNC may benefit from extra measures to protect their thyroid gland from radiation exposure.
Age’s impact on RIHT is not clear. Lee et al. analyzed 149 patients with NPC and showed that age has no significant effect upon RIHT [
14]. Several researches also believed that age’s effect upon RIHT is not significant, which was consistent to our study [
15,
16]. However, in Diaz et al.’s research, the risk for increasing age were 0.93 [
17]. Also, other study indicated contrary results that increasing age was subject to the higher incidence of RIHT, particularly for patients who were over 60 years old [
18]. The different age distribution of the study population may cause the above discrepancy.
It’s of great concern whether chemotherapy has an effect on the incidence of RIHT. Luo et al. found that 24.5% of the 155 patients who received chemotherapy developed RIHT; only 1 of the 19 patients without chemotherapy developed RIHT (5.26%) [
19]. In the Multivariate analysis, chemotherapy was indeed an independent risk factor for RIHT in patients with NPC, which is consistent with the results of our study (
P = 0.007). And we found that frequency of chemotherapy raised the risk of developing RIHT (
P = 0.021; 48.3% vs. 22%).
Whether patients with HNC undergo surgery before radiotherapy has a certain influence on the occurrence of RIHT. Vogelius et al. indicated that the risk of RIHT increases regardless of whether the surgery involves the thyroid, yet the incidence of RIHT was higher after surgery involving the thyroid, which was inconsistent with our study’s results [
20]. Alba et al. also found that laryngeal surgery affects the occurrence of RIHT in a significant way [
21]. Most of researchers believe that surgery treatment may decrease the blood supply of the thyroid. In our study, the effect of surgery on RIHT is not significant, but surgery involving thyroid significantly affect the development of RIHT (
P = 0.027; 34.7% vs. 83.3%), which accurately show that the impact of surgery on RIHT is attributed to the direct destruction of thyroid by the surgery.
N stage is usually a vital risk factor for RIHT in most studies. Zhou et al. found that patients with advanced N-staging (N2-N3) nasopharyngeal carcinoma had a 0.91-fold increased risk of RIHT compared with patients with early N-staging (N0-N1) (37.38% vs. 13.11%) [
22]. Similar conclusion was found in our study: advanced N-staging (N2-N3) significantly increased incidence of RIHT (
P= 0.050; 29% vs. 48.6%). Because the distance between metastatic lymph nodes and the thyroid may be more closer for patients with advanced N-stage [
23]. We should think highly of the change of thyroid function for patients with advanced N stage after radiotherapy.
Many studies have found a clear association between thyroid volume and RIHT: the incidence of RIHT increases with the decrease in thyroid volume. Diaz et al. reported that the incidence of RIHT decreased by 0.93 times for every 1 cm
3increase in thyroid volume (95% CI, 0.88–0.98) [
17]. In a retrospective research of 206 patients with nasopharyngeal carcinoma undergoing radiotherapy, thyroid volume ≤ 12.82 cm
3 was an independent risk factor for RIHT: when the thyroid volume was less than or equal to 12.82 cm
3, the incidence of hypothyroidism was 75%, and when the volume was greater than 12.82 cm
3, the incidence of hypothyroidism was 37.31% [
22]. Similarly, we found that thyroid volume ≤ 10.6cm
3 was an independent risk factor of development of RIHT(
P = 0.000; 72.5% vs. 14.8%). However, this study also found no significant correlation between thyroid volume and RIHT when patients with thyroid volume ≤ 10.6 cm
3 were excluded (
P = 0.304) (Table
6). Thus thyroid volume may be a confounding factor in the risk factors for RIHT. Chyan et al. suggested that thyroid volume may have an impact on dose limitation [
24]. For patients with thyroid volume larger than 8cm
3, setting the thyroid dose to VS
45 ≧3cm
3 could decline the incidence of RIHT. If the volume of thyroid is less than 8cm
3, more rigorous restriction are required, including Dmean < 49 Gy, V
50 < 45%, VS
45 ≧3cm
3 and VS
50 ≧3cm
3.
Table 6
Analysis between thyroid volume and hypothyroidism
Volume > 10.6 cm3 | 52 | 9 | 0.304 |
Dosimertric factors associated with hypothyroidism occurrence
This study found that Dmean, VS
45, VS
50, VS
60 and V
30,60 were independent predictors. Many study stated a dose-dependent risk of RIHT. Many investigators believed that the mean dose of thyroid (Dmean) may be the most potential dosimetric factor. Zhai et al. showed that Dmean has an impact on the occurrence of RIHT for patients with nasopharyngeal carcinoma. Grouping according Dmean, they found that patients with Dmean > 45 Gy are almost five times more likely to experience RIHT than those with Dmean ≤ 45 Gy [
25]. Our results were similar to the above study. We showed that Dmean > 47.3 Gy was an independent predictor of RIHT ( 96.3% vs 23.1%). What’s more, the threshold of Dmean for the thyroid was aimed to be lower than 50 Gy in the latest international guideline [
26].
Some scholars insisted that the thyroid volume which were not disturbed by radiation is accountable for the generation of thyroid hormone and stands for thyroid hormone reserve[
14]. We believe that VS
x is better than Vx in predicting development of RIHT. Our study suggested at least 8.4cm
3 of thyroid should be spared form doses exceeding 60 Gy, at least 2.1cm
3 be spared form doses exceeding 45 Gy and at least 4.9cm
3 be spared from doses exceeding 50 Gy to decrease the occurrence of RIHT among HNC patients. After comparing AUC value, it was found that VS
60 was more important in predicting RIHT, which was coordinated with results of Lee et al.’study [
14]. They suggested patients with thyroid VS
60 > 10cm
3 had more latency and lower incidence of hypothyroidism. The other study also recommended that VS60 ≥ 10cm
3could become a viable dose limitation [
27]. Chow et al. identified 29 relevant studies involving 4,530 patients with HNC and also showed that VS60 > 10cm
3could be beneficial to decrease the incidence of RIHT [
28].
Although the parameter V
x can portray the dose distribution of thyroid well, repeated information exists among different V
x. Peng et al. believed that a new parameter V
a,b could be capable to decrease collinearity among dosimetry parameters in some degree. They analyzed 545 with NPC and suggested that V
30,60 ≦80% might be a practical dose constraint to conduct during IMRT planning for patients with thyroid volume ≦20cm
3 [
29]. Yet Our study also showed that V
30,60 was a reliable predictor of hypothyroidism after radiotherapy, we recommended that the threshold of V
30,60 of 57% could cut down the occurrence of RIHT. As is shown in Fig.
2, those with V
30,60 ≦57% had markedly lower incidence of RIHT than those with V
30,60 > 57%. All these researches offered us various dose limit targets. However, researches on the optimal dose threshold of the thyroid has not reached consensus.
We established the nomogram in order to predict the probability of RIHT individually. What’s more, the combined use of chemotherapy, the thyroid volume, Dmean, VS
40, VS
50, VS
60 and V
30,60in the nomogram demonstrated prominent predictive ability and calibration. Compared with other nomograms, the nomogram showed better predictive ability, and its AUC of 0.937(95%CI, 0.888–0.958) was apparently larger [
30,
31].
However, our research has some limitations. First, the follow-up time is short, only 10 months. Second, with a small amount of data about the antiTPO and antiTg, we found that patients with abnormal level of antiTPO and antiTg were more susceptible to RIHT. Thus we inferred that the level of antiTPO or antiTg may be significant to the development of RIHT and should be included in the nomogram. As is known to all, hashimoto disease is subclinical and underdiagnosed but highly prevalent. One of the reasons of RIHT is that radiation may induce the autoimmune reaction of the thyroid. However, the presence of inflammation of the thyroid may trigger more RIHT on the basis of the thyroid undergoing radaition. A research with a great quantities of the data of relevant thyroid antibodies should be conducted to probe into the correlation. In addition, internal and external data validation are needed to further identify the viability of the nomogram.