Tumor Biology is King: Secondary Tumors of the Thyroid From 2 Medical Centers in China

BACKGROUND: We studied the clinical characteristics, diagnosis, treatment, and prognosis of secondary tumors of the thyroid (STTs) and analyzed this rare phenomenon based on our clinical experience. METHODS: We reviewed 16 000 malignant thyroid tumors diagnosed at 2 medical centers in China from 1978 to 2018, including 55 patients with STTs. RESULTS: The most frequent primary tumor sites included lung (21.8%), gastrointestinal (18.2%), breast (14.5%), and kidney (12.7%). The median age at STT diagnosis was 56 years. The time from diagnosis of primary tumor to metastases to the thyroid ranged from 0 to 108 months, with the longest interval being for renal cell carcinoma (RCC; mean: 49 months). There were 22 cases of single metastatic foci and 33 cases of multiple metastatic foci. At the time of STT diagnosis, 42 patients had multiple organ metastases and 13 patients had only thyroid metastases. Thyroid function was examined in 50 patients, including 23 with Hashimoto’s thyroiditis. Metastases were diagnosed histologically and confirmed by negative immunohistochemistry for thyroid markers. Twenty-one patients were treated with resection, including total thyroidectomy in 14 and unilateral lobectomy in 7. Thirty-four patients were treated without resection, but 2 were treated with tracheotomy. The median survival time of all patients with metastasis was 10 months (range: 1-96 months). Patients with primary RCC had the best prognosis (median survival time: 52 months), followed by patients with breast cancer (33 months). Patients who underwent thyroid surgery had a better prognosis than patients without thyroid surgery. Patients with single metastatic foci or single organ metastases had a better prognosis than patients with multiple metastatic foci or multiple organ metastases. CONCLUSIONS: Metastasis to the thyroid is a rare clinical phenomenon, and sometimes a diagnosis of STT is difficult; so, we need to pay more attention to it. While prognosis appears to be related to surgery or some characteristics of metastatic spread, these data suggest it is more complex. Tumor biology is king; in fact, prognosis was mainly related to the biological behavior of the primary tumor. We cannot only opt for surgery; thus, case selection is important, and the treatment strategy for STT patients should be determined individually according to their specific biological behaviors.