Abstract: Background and purpose: It is still controversial whether papillary thyroid cancer (PTC) patients with pulmonary metastasis but negative  ¹³¹ I whole body scan (WBS) can benefit from empiric radioactive iodine (RAI) therapy. This study aimed to explore the necessity of empiric  ¹³¹ I therapy for PTC patients with non-RAI-avid pulmonary metastasis. Methods: Forty-five PTC patients with only pulmonary metastasis treated in Department of Nuclear Medicine, Peking Union Medical College (PUMC) Hospital from 2008 to 2019 were included. The changes of serum thyroglobulin (Tg) levels before and after empiric  ¹³¹ I treatment in which the post-treatment whole body scan (RxWBS) showed that the metastatic lesions were non-RAI-avid, as well as Tg change rate and imaging changes of the same patient under the two schemes of empiric  ¹³¹ I treatment and thyroid- stimulating hormone (TSH) suppressive therapy alone were compared. The progression-free survival (PFS) according to the imaging change was observed during TSH suppressive therapy alone. Results: Serum Tg levels of 45 PTC patients were increased after  ¹³¹ I treatment (P=0.001). There was no significant difference in Tg change rate (P=0.123), as well as imaging change (P=1.000) between two schemes of empiric  ¹³¹ I treatment and TSH suppressive therapy alone. The median PFS was 54.4 (46.5, 66.2) months during TSH suppressive therapy alone. Conclusion: Empiric  ¹³¹ I therapy has little benefit to PTC patients with non-RAI-avid pulmonary metastasis, and it may contribute to disease progression. Terminating empiric  ¹³¹ I therapy in time should be recommended in such patients.

Key words: Papillary thyroid cancer, Pulmonary metastasis, Radioactive iodine therapy, Radioactive iodine avidity