General considerations

A secondary thyroid tumor is not infrequently detected at autopsy. Up to 24% of tumors have been found to have spread to the thyroid in patients who died of cancer (Silverberg 1966, Berge 1977). In clinical practice, however, it is a rare finding. The cytological presentation of secondary tumors of the thyroid does not differ essentially from that of the lymph nodes or other organs. The only difference is the possibility that, by using thyroglobulin and calcitonin immunochemistry or wash-out technique, we are able to prove the secondary nature of a carcinoma within the thyroid gland.

Tumors metastatisizing to the thyroid

A variety of tumors have been described as metastasizing to the thyroid. The most common are clear cell renal cancer, carcinoma of the lung, gastro-intestinal tract and breast carcinoma, and malignant melanoma (Watts 1987, Schmid 1991, Michelow 1995, Lin 1998). Renal cell carcinoma has been reported as the most frequent metastatic tumor to the thyroid (Friberg 1969, Ericsson 1981, Czeh 1982, Nakhjavani 1997, Lam 1998). However, renal cell carcinoma in only one has not yet been diagnosed in only one patient in our practice. This observation is similar to those of McCabe et el. and Lin et al. McCabe 1985, Lin 1998). We mean that the diagnosis is greatly influenced by the access to the diagnosis. We worked for more than 10 years in close cooperation with a pulmonary unit which explains the relatively great proportion of pulmonary origin among our secondary carcinoma cases.

Clinical history

While the knowledge of a malignant tumor outside the thyroid might seem to be of great importance (Ivy 1984), in our practice it has not proved significant. In around half of our cases, the first sign of the disease was the thyroid metastasis. Moreover, in contrast with the report of Lin et al. who found a mean tumor size of  5.7 cm in 14 patients (Lin 1998), the mean tumor size in our first 18 patients was only 3.1 cm. One of the possible explanations for these differences is the routine use of thyroid US in all patients sent for investigation.

Diagnostics of secondary thyroid carcinomas

A brief summary:

• The possibility must be first suggested by routine stained smears, and
• immunostaining or wash-out for thyroglobulin must be performed, if the situation is not unequivocal.

We must be aware of the possibility of a secondary malignancy if unusual atypical cells are seen in the smear, and the cytological picture does not meet the criteria for any primary thyroid disease. One of the most important auxiliary signs is the presence of necrosis. Suggestion of the possibility of the secondary nature of a thyroid tumor is not very difficult on this basis in a majority of the cases. The most important exception is clear cell renal cancer. The thyroid manifestation can be the first clinical sign of renal cell carcinoma. Hedinger et al. state that a clear cell tumor of the thyroid is more likely to be metastatic renal cell carcinoma than a primary tumor (Hedinger 1967); it is important to keep this fact in mind, and not mistake it for a primary thyroid tumor with a clear cell pattern (Halbauer 1991, Rikabi 1991, Schmid 1991). Another relatively frequent secondary tumor is colonic adenocarcinoma. As described in the literature, this can be mistaken for anaplastic thyroid cancer or the columnar cell variant of papillary cancer (Orell 1997). This occurred in one of our cases, when a review of the smears revealed a clear interpretation error. The presence of necrosis and palisading columnar cells could have pointed to the correct diagnosis. Misinterpretation of a metastatic cancer as anaplastic thyroid cancer has been reported in the literature (Erdogan 1994, Haugen 1994, Lin 1998). The typical clinical picture of anaplastic cancer may be of great help.

Thyroglobulin determination of the wash-out of the remnant in the aspiration needle or immunostaining for thyroglobulin must be performed as a rule if the possibility of the secondary nature of a thyroid tumor cannot be ruled out.