Background: Although clinically evident metastases of nonthyroid malignancies (NTMs) to the thyroid gland are uncommon, it is important to suspect them in patients who present with a new thyroid mass and a history, however far back, of prior malignancy. In fact, metastases from NTMs to the thyroid gland have been reported in 1.4%-3% of all patients who have surgery for suspected cancer in the thyroid gland. Here we review the literature over the last decade regarding this topic.
Summary: Based on recent literature, the most common NTMs that metastasize to the thyroid gland are renal cell (48.1%), colorectal (10.4%), lung (8.3%), and breast carcinoma (7.8%), and sarcoma (4.0%). Metastases of NTMs to the thyroid are more common in women than men (female to male ratio=1.4 to 1) and in nodular thyroid glands (44.2%). The mean and median intervals between diagnosing NTMs and their metastases to thyroid gland are 69.9 and 53 months, respectively. In 20% of cases the diagnosis of the NTM and its metastases to the thyroid was synchronous. Recent reports indicate that there is a higher frequency of sarcoma metastasizing to the thyroid gland than reported in prior years. Fine-needle aspiration biopsy (FNAB) of thyroid masses is useful in diagnosis of thyroid metastases. However, this requires information about the NTM so that the proper antibodies can be used for immunohistochemical analysis; therefore it is of lesser utility if the NTM is occult. In patients with preexisting thyroid pathology the FNAB diagnosis can be more difficult due to more than one lesion being present.
Conclusions: It is important to keep in mind that the thyroid gland can be a site of metastases for a variety of tumors when evaluating a thyroid nodule, especially in a patient with a prior history of malignancy. In patients with thyroid lesions and a history of malignant disease, regardless of time elapsed since the initial diagnosis of the primary neoplasm, disease recurrence or progression of malignancy must be considered until proven otherwise.