US and CT findings of secondary neoplasms of the thyroid--a pictorial essay.

Although clinical series report a low (3%) incidence of metastases to the thyroid gland, autoptic studies have showed occurrences as high as 17%, probably because of the high vascularization of the thyroid. We selected 9 patients who had pathologically proven thyroid metastases at CT and US, during follow-up for a known primary neoplasm. The most common originating neoplastic primaries include mostly those that generally give rise to blood-borne metastases such as breast and lung cancers, mucoid adenocarcinoma of the stomach, colon cancer and renal cancer, as well as melanoma and leiomyosarcoma. Because of its diffusion, sensitivity, and noninvasiveness, ultrasonography can justifiably be introduced in the staging protocols of those neoplasm that more frequently give blood-borne metastases to the thyroid, but a US-guided biopsy is warranted for hypoechoic or otherwise suspicious nodules. This holds particularly true in disease-free patients or previously diagnosed with generally slow-growing malignancies (breast or kidney), since the secondary localization, often metachronous in our experience, may be effectively managed surgically. CT features, on the other hand, are extremely variable and are directly dependent on the histology of the primary lesion, as well as the size of the secondary lesions.