**1. Introduction**

Differentiated thyroid carcinoma (DTC) is the most common endocrine malignancy [1]. The prognosis of DTC is generally favorable, with a 10-year survival rate of over 95% [2,3]. However, in 5% to 25% of patients, distant metastases are detected at the time of diagnosis or during the disease's course. In patients with DTC, bone metastases (BMs) occur in 2% to 13% of all patients and in nearly half of the patients with distant metastases [2]. In patients with DTC, the bone is the second most common site for distant metastases after the lung [2,4]. The spine is the site where DTC-BMs are most likely to occur, and it is affected in almost half of patients with DTC-BM [5]. BM from DTC is associated with a worse overall prognosis than lung metastasis [6–9]. However, the prognosis of patients with BM from DTC is still favorable, with a 10-year overall survival (OS) rate of

**Citation:** Kato, S.; Demura, S.; Shinmura, K.; Yokogawa, N.; Shimizu, T.; Tsuchiya, H. Current Management of Bone Metastases from Differentiated Thyroid Cancer. *Cancers* **2021**, *13*, 4429. https:// doi.org/10.3390/cancers13174429

Academic Editors: Fabio Medas and Pier Francesco Alesina

Received: 6 August 2021 Accepted: 24 August 2021 Published: 2 September 2021

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35% to 47% [10,11], compared with that of patients with BM from other cancers. Despite this relatively favorable prognosis, patients with osteolytic BMs from DTC often have a significantly reduced performance status (PS) and quality of life (QOL), with intractable pain, neurological symptoms, and increased mortality [12–14]. Farooki et al. have reported a 78% occurrence of at least one clinical skeletal-related event (SRE) with a median of 5 months from the identification of BM to the first SRE in patients with DTC with BM. After a median of 10.7 months, 65% of patients sustained a second SRE [12]. Importantly, mortality is significantly higher in patients with BM who develop SREs [12]. The goals of treatment for BMs remain palliative, striving toward symptom palliation, and improved PS and QOL, besides the long-term local control of the tumor. Recent advancements in cancer therapy have dramatically improved OS across multiple cancer subtypes. Therefore, long-term local control of thyroid BMs is desired, especially in patients with a single metastasis or oligometastases, who are expected to live longer.

Here, we reviewed the current management options for DTC-BMs and especially focused on local treatment for long-term local tumor control, including surgical metastasectomy, from the orthopedic tumor surgeon's point of view.
