3.2.2. Stereotactic Radiosurgery

The development of SRS, which can be used to deliver significantly high radiation doses with submillimeter accuracy, has changed the treatment paradigm, especially for patients with oligometastases, including BMs. It can deliver high-dose radiation (14–16 Gy in a single fraction) to the target volume, while sparing adjacent at-risk critical organs [58]. Owing to these characteristics, SRS can offer favorable outcomes and allow the re-irradiation of previously treated sites if necessary.

Recently, several studies have reported the efficacy of SRS for DTC-BM, although treatment protocols of SRS are different [59–63]. Table 2 presents studies of SRS for BM from thyroid carcinoma, mainly DTC, with detailed clinical results, including information about post-treatment survival rates and/or local tumor control in the treated lesions [59–61,63]. Bernstein et al. prospectively evaluated the efficacy of frame-based SRS in 23 patients with thyroid cancer, with 27 spinal lesions, as primary or adjuvant/salvage therapy. They reported that the local tumor control rates were 88% and 79% at 2 and 3 years, respectively. Pain flare was observed in 30% of patients in the median follow-up of 29 months [59]. Ishigaki et al. retrospectively evaluated the efficacy of SRS using the Cyberknife system and reported the local control rate of 97% at 1 year in 13 patients with DTC with 60 skeletal lesions, including only 7 symptomatic lesions [60]. Meanwhile, a recent retrospective study of 12 patients with 32 spinal lesions treated with Cyberknife reported a lower local tumor control rate of 67% at 1 year [61]. This difference between clinical outcomes could be due to the baseline characteristics of the BM lesions (a proportion of large and/or spinal lesions associated with significant symptoms and local tumor control). Another retrospective study reported that the use of Cyberknife SRS for DTC-BM was considered successful [62]. The largest series, including 67 patients and 133 skeletal lesions, reported excellent outcomes of 96% and 82% in 1- and 5-year local control rates, respectively [63].

**Table 2.** Four studies that included detailed outcomes of SRS for BM from thyroid carcinoma.


BM, bone metastasis; DTC, differentiated thyroid carcinoma; F, female; Fr, fraction; FTC, follicular thyroid carcinoma; FU, follow-up; Gy, gray; LC, local control; LR, local recurrence; M, male; N/A, not available; No., number; PTC, papillary thyroid carcinoma; pub., publication; SRS, stereotactic radiosurgery.

> In all the previously cited studies, SRS was effective and safe without the occurrence of spinal cord injury. However, a potential risk of vertebral compression fractures after treatment has been reported. Risk factors for fractures include older age, baseline fracture or pain, osteolytic lesion, higher tumor burden, higher radiation dose, and spinal deformity [64,65]. In patients with these risk factors and high SINS or Mirels scores, prophylactic stabilization should be considered before applying SRS to avoid the complication [64,65]. For patients with epidural disease, separation surgery focused on circumferential spinal cord decompression is performed to create an adequate distance (typically 1–2 mm) between the tumor and the spinal cord to safely provide optimal dosing in the following SRS [66,67].

> SRS treatment is reported as showing a trend toward a significant improvement in PFS and OS rates in patients with oligometastatic disease from other cancers [68]. However, the effect of this treatment on survival rates among patients with DTC-BM remains unclear, in contrast to the effect of metastasectomy. A recent nationwide multicenter study has reported no significant effect of EBRT in decreasing the overall mortality of patients with DTC-BM [17]. Future studies are required to identify patients amenable to SRS and its effect on survival.
