Lung Metastasis of Thyroid Cancer Appearing Diffuse and Disseminated on Imaging.

To the Editor: Thyroid cancer is the most frequent endocrine carcinoma, and its incidence is rising.[1] Metastasis of thyroid cancer to the lungs is unusual and can be misdiagnosed as other lung diseases. Here, we reported an unusual case of lung metastasis from thyroid papillary carcinoma (TPC), which appeared diffuse and disseminated on imaging.

A 66-year-old male complained of shortness of breath and postactivity fatigue lasting one year, as well as hemoptysis lasting one month. Physical examination revealed a firm, swollen thyroid that did not cause pain, as well as slight wet rales in the left lower lung. Computed tomography (CT) of the chest detected bilateral diffuse miliary lung nodules and enlarged upper mediastinal lymph nodes [Figure 1a]. Ultrasonography showed uneven swelling in the thyroid and multiple calcifications, as well as abnormal enlargement of the lymph nodes around the thyroid region. Laboratory results indicated elevated thyroid-stimulating hormone, normal levels of free triiodothyronine and free thyroxine, and a carcinoembryonic antigen level of 3.07 ng/ml. Lung biopsy was performed under CT guidance, and immunohistochemistry indicated TPC metastasis to the lungs, with positive staining for thyroglobulin [Figure 1b]. The patient was discharged without further treatment.

Figure 1

A 66-year-old male with lung metastasis from thyroid papillary carcinoma. (a) Computed tomography of the chest showing bilateral diffuse miliary lung nodules. (b) Immunohistochemistry of the lung biopsy tissue showing the presence of thyroglobulin (original magnification, ×400).

TPC is, together with thyroid follicular carcinoma, one of the two types of differentiated thyroid cancer (DTC).[2] TPC tends to enter the lymph system via lymph vessels, whereas thyroid follicular carcinoma tends to travel to lungs and bones via blood circulation. DTC does not show remarkable clinical symptoms when the disease is in early stages, and as it progresses, it is associated with enlarged cervical masses, hoarseness, and Horner's syndrome. Since the rate of distant DTC metastasis is only 5.2%, it can easily be misdiagnosed, especially when patients present with symptoms in other organs. In the present case, CT revealed numerous, diffuse, “popcorn”-like nodules across the lobules. Such an imaging feature is also associated with the more common conditions of miliary tuberculosis or other type of infectious pneumonia, resulting in high risk of misdiagnosis. In addition, CT may not always be sufficient for diagnosis; nuclear imaging may be required to detect diffuse changes.[3] Patients with suspicious imaging features should be biopsied to ensure accurate diagnosis.

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