Distant metastasis of well-differentiated thyroid cancer is not unusual during treatment, which may be diagnosed on the presentation of thyroid cancer or during follow-up 13–15. Unlike other malignancies, 131I is the first choice for papillary and follicular thyroid carcinomas with distant metastases, unless they lose the ability to trap iodine 16. Our study showed that 131I therapy was effective for controlling distant metastases of patients with well-differentiated thyroid cancer over a long-term follow-up period of 10 years. However, in our study, the remission rate was low. During treatment, the balance between the 131I effective dose and possible side effects from the accumulated 131I dose needs to be considered.
In our study group, follicular thyroid carcinoma had higher DSM rates than papillary thyroid carcinoma, although Kaplan–Meier survival curves showed no statistically significant difference between the two groups. Compared with papillary thyroid carcinoma, follicular thyroid carcinoma may be diagnosed and treated later, which is in accordance to the larger observed tumor sizes 26,27. More advanced distant follicular thyroid carcinoma metastases were found in our study. This is because, unlike papillary thyroid carcinoma, it is difficult to diagnose follicular thyroid carcinoma with preoperative fine-needle aspiration cytology 28.
During the long-term follow-up of patients with well-differentiated thyroid cancer, a secondary primary cancer diagnosis other than thyroid was not unusual 29,30. In clinical practice, the differential diagnosis between distant metastasis and secondary primary cancer is important for further treatment. In our study, 6.1% of patients with papillary and follicular thyroid cancer with distant metastases developed secondary primary cancer during the follow-up period. In our previous study, patients with well-differentiated thyroid carcinoma and metachronous secondary primary cancer had worse prognoses than patients without secondary primary cancer 31. In this study, the DSM of patients with secondary primary cancer was not affected by treatments with 131I doses over 600 mCi.
Patients with well-differentiated thyroid cancer with distant metastases have poor prognoses after long-term follow-up. 131I is an effective therapeutic modality for patients with well-differentiated thyroid cancer with distant metastases. After further follow-up, over a mean period of 10 years, more than 60% of patients experienced DSMs when high accumulated 131I doses were administered.
There are no conflicts of interest.
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