The prevalence of intrathyroid metastases of nonthyroid origin ranges from 1.9% to 25% [15–24]. Mortensen et al. account 4% of patients with metastatic neoplasm with secondary tumors to the thyroid gland . Silverberg meticulously examined the gland and found an incidence of 25% in patients dying from disseminated malignant tumor of other primary .
Primary carcinomas of the lung, kidney, breast, stomach are the most common tumors metastasizing to the thyroid [15–24]. Carcinomas of the colon, gynecologic tumors, oral cavity, esophagus, neuroendocrine cancers, and sarcomas have been rarely published only in case reports or small series [15–24]. In the study of Shimaoka et al., metastases to the thyroid gland occurred in 39% of melanoma patients, 21% of breast cancer patients, 12% renal cancer, 10% of lung cancer and 10% of patients with primary head and neck tumors . Chen et al. reported ten patients with thyroid metastases and 50% of these patients had metastases from renal cell carcinoma . Thus, renal cell carcinoma is by far the most common source of clinically relevant metastases to the thyroid gland .
However, considering that the reported metastases of autopsy cases included nonclinically metastasis (i.e. occult cancer or widespread cancer at the time of death), a better estimate of the incidence of clinically apparent metastases to the thyroid has been shown in clinical studies. The incidence of clinically significant metastases appears to be lower than the incidence found in autopsy. According to Shimaoka, the thyroid metastases were rarely clinically apparent in only 5% to 10% of the patients . Wychulis et al. described that only 10 of 20262 patients, who had undergone thyroidectomy at the Mayo Clinic, had symptomatic metastatic involvement of the thyroid gland .
In clinical practice, in a patient with a diffuse and bilateral multinodular goiter, a correct diagnosis is difficult, since there are no specific findings of metastatic thyroid tumor on ultrasonography or computer tomography scan investigations. Elliott and Frantz found 44 reported cases of metastatic carcinoma that had been misidentified as primary thyroid cancer .
Therefore, a correct diagnosis of metastatic thyroid tumors requires a careful consideration of patients with a history of cancer. This information immediately stratifies a patient into a high risk category.
Moreover, the presentation of a thyroid nodule years after the treatment of a primary cancer often poses a diagnostic dilemma. Often there is a latency period lasting years between the diagnosis of the primary cancer and the appearance of the thyroid mass . Latent intervals of up to 20 years have also been reported . This finding is especially true for renal primary tumors . In these cases with a long interval between the detection of the primary tumor and the development of the thyroid metastasis, the difficulty in making a correct diagnosis could increase as well.
On the other hand, there have been several reports on metastasis to the thyroid, which appeared prior to the primary tumor being detected .
Fine needle aspiration cytology (FNA) can allow for the preoperative diagnosis of a secondary tumor, thus changing the preoperative work-up of such patient [32–34]. Once the diagnosis of metastatic disease has been confirmed on FNA, the patient should undergo a metastatic work-up to rule out other distant metastases [32–34].
Finally, if technically feasible, thyroidectomy can be effective for local control . Surgical resection is regarded as the best treatment for a metastatic thyroid tumor, especially if the primary carcinoma appears to be controlled and there is no evidence of metastasis elsewhere . Moreover, considering the size and rapid growth of the thyroid tumor, even if the patient had already had other metastatic lesions, thyroidectomy would still be required in order to relieve tracheal compression. This is especially true for tumors that present years after the treatment of the primary cancer.
Survival time after diagnosis of the thyroid metastases is determined by the biology of the primary disease [32–36]. Our patient has no evidence of recurrence 5 months after thyroid surgery. Authors have demonstrated that for isolated thyroid metastases, thyroidectomy has prolonged survival. Chen et al. reported that 60% of the patients with solitary thyroid metastasis were still alive after a thyroidectomy during a median follow-up period of 5.2 years .
After surgical management, the administration of systemic therapy is recommended .