Ah-Young Kim, MD, Sung Bin Park, MD, Hee Seok Choi, MD and Jae Cheol Hwang, MD

Department of Radiology, Dongguk University International Hospital, Dongguk University, College of Medicine, Goyang-si, Korea. Drs Park and Hwang are currently with the Department of Radiology, Ulsan University Hospital, University of Ulsan, Ulsan, Korea.

Abbreviations: CT, computed tomography

Metastatic thyroid carcinoma is clinically silent and found only at autopsy in most instances. The overall incidence, not surprisingly, varies from 1.25% in unselected patient autopsy series to 24% in autopsies of patients with widespread malignant neoplasms.^1–3 Although the thyroid gland is a hypervascular organ, and most metastasis to thyroid gland occurs hematogenously, metastasis to the thyroid gland is rare. In both clinical and autopsy series, renal cell, breast, and lung carcinomas are the most frequent sources of metastasis to the thyroid gland.^3,4

Renal cell carcinoma is the most common, constituting 56% of all secondary malignancies.³ Furthermore, after nephrectomy, renal cell carcinoma tends to metastasize as a solitary lesion without local tumor recurrence or distant metastasis to other organs, unlike other cancers.^5–7

We report a case of isolated thyroid metastasis from renal cell carcinoma 3 years after nephrectomy without local tumor recurrence or metastasis to other organs.

A 64-year-old man had a palpable mass in the left neck area, which showed relatively rapid growth over a few months. The patient had slight dysphagia but no other symptoms such as dyspnea or hoarseness. His medical history included a left radical nephrectomy for renal cell carcinoma for 3 years previously. Physical examination revealed a 3 x 4-cm hard nontender fixed mass without associated palpable cervical lymph node enlargement. Results of initial investigations, including a complete blood cell count and electrolyte and thyroid function tests, were within normal ranges. Fine-needle aspiration biopsy by palpation was performed. The results indicated a few scattered atypical or malignant cells suspicious for carcinoma.

Sonography of the thyroid gland showed an irregular hypoechoic mass replacing the left thyroid gland with invasion of the surrounding structure (Figure 1A). The sonographic differential diagnosis was relative rapidly growing anaplastic primary thyroid carcinoma, lymphoma, or metastasis. Sonographically guided core needle biopsy via an 18-gauge semiautomated gun was performed (Figure 1B). Neck computed tomography (CT) scan showed an inhomogeneous enhancing left thyroid mass with invasion of the thyroid cartilage, cricoid cartilage, larynx, and upper trachea. Extension to the muscular and subcutaneous areas was also evident. Abdominal and pelvic computed tomography showed no local tumor recurrence or metastasis to other organs (Figure 1, C–F). Pathologic results indicated metastasis from renal cell carcinoma identified from the previous radical nephrectomy specimen of clear cell carcinoma (Figure 1G). Thus, we finally diagnosed the tumor as isolated metachronous distant metastasis to the thyroid gland from renal cell carcinoma 3 years after the left nephrectomy. The patient refused surgery and died 2 months later.

The exact incidence of metastasis to the thyroid gland that is discovered clinically is not well documented, but secondary malignancies of the thyroid gland are thought to constitute less than 1% of thyroid cancers.³ Metastasis to the thyroid gland has been considered to show hematogenous spread.¹ The thyroid gland receives approximately half the volume of arterial blood received by the entire liver.¹ However, metastasis in the thyroid gland has been reported to be rare in comparison with other organs. Reasons for the relative rarity of hematogenous metastasis to the thyroid gland have been postulated as follows: first, tumor cells in the bloodstream are screened by the lung, which prevents them from reaching the thyroid gland; second, even if tumor cells do reach the thyroid gland, they are unlikely to be fixed there because of the abundant high-velocity bloodstream; and third, thyroid gland tissues are rich in oxygen and iodine, which prevent the proliferation of fixed tumor cells.¹

The clinical findings of metastatic thyroid carcinoma are nonspecific, including a neck mass, weight loss caused by hypercalcemia, dysphagia, and hoarseness. Absence of symptoms is also common.⁸

Takashima et al⁸ described radiologic findings of metastasis to the thyroid gland in 11 cases. According to the above report, the metastases were solitary (n = 5) or multiple (n = 6), and about half of them measured less than 2 cm in diameter. These tumors typically had well-defined margins and no calcification and sometimes had cystic portions. On sonography, the metastases appeared as hypoechoic or markedly hypoechoic areas without haloes; on CT, they appeared as low-density areas; and on magnetic resonance imaging, they appeared as areas of varying signal intensities. None of them showed marked hyperintensity on T2-weighted images or marked enhancement on gadolinium-enhanced T1-weighted images, often seen in benign thyroid nodules. Half of the metastases showed hypointensity on T2-weighted or gadolinium-enhanced T1-weighted images.

Conversely, in our case, sonography and contrast-enhanced CT showed a poorly defined mass with strong enhancement and massive invasion into the adjacent structures. This difference might be due to the small masses in the 11 patients of Takashima et al⁸ and the fact that renal cell carcinoma was not included among the cases. A hypervascular appearance on color Doppler sonography and contrast-enhanced CT would be expected because of the hypervascularity of primary renal cell carcinoma and was present in our case.⁶

Although thyroid metastasis from renal cell carcinoma is considered rare, more than 40 cases have been reported in the literature.⁶ In fact, most clinically recognized thyroid metastases are due to renal cell carcinoma.^3,4,6 Furthermore, many of these lesions presented long after radical nephrectomy, even 20 years later.^5–7 This certainly can pose a diagnostic dilemma to physicians caring for these patients. As in our case, the prognosis associated with renal cell carcinoma metastasis to the thyroid gland is poor, but curative surgical treatment can prolong survival.⁵

In our case, a diagnosis of a malignant thyroid tumor was easily made on the basis of the destruction of the thyroid and cricoid cartilage with invasion to the trachea. However, there seemed to be no radiologic features to differentiate primary and secondary thyroid malignancies. Therefore, sonographically guided core needle biopsy could be used to diagnose thyroid metastasis from renal cell carcinoma. Because sonographically guided fine-needle aspiration or biopsy is useful for differentiating primary malignant tumors from benign tumors,^9,10 it could be useful for diagnosing metastasis to the thyroid gland.

In summary, we have described a case of isolated thyroid metastasis from renal cell carcinoma 3 years after nephrectomy without local tumor recurrence or metastasis to other organs. If a patient with a history of nephrectomy for renal cell carcinoma subsequently has a solitary thyroid mass, one should consider isolated thyroid metastasis as well as a primary thyroid tumor.

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