Excellent RAI therapeutic response on a patient presenting skull metastasis of follicular thyroid cancer after 15 years.

Bone is the second most common site of metastasis for differentiated thyroid carcinoma (DTC). Bone metastasis (BMs) occur in about 10% of patients with DTC and is observed more often in follicular thyroid carcinoma (FTC) (7-28%) than papillary thyroid carcinoma (PTC) (1-7%). Bone metastasis is associated with unfavorable clinical outcomes mainly including skeletal-related events (SREs), such as pathologic fractures, bone pain, spinal cord compressions, and hypercalcemia, which negatively impact the quality of life of patients and reduce their life expectancy. Patients with BMs from DTC require comprehensive and multimodal treatment approaches, including radioiodine (RAI) therapy, palliative care, surgery, external beam radiotherapy, and targeted drug therapy. RAI therapy is the first-line treatment, despite being rather ineffective, especially in large BMs. The response to RAI therapy, either alone or in combination with BM focal treatment depends on iodine avidity. This study reports a rare case of metachronous skull bone metastasis from FTC in a 72-year-old female patient 15 years after initial treatment. The patient had an excellent response to RAI therapy, which resulted in the abnormal uptake disappearing. Following treatment, the patient has been disease-free for six years. This case confirms that a complete response to RAI treatment for BM depends on the degree of dedifferentiation of cancer cells, which highlights the need for long-term follow-up, especially for FTC patients.