Metastasis of pulmonary adenocarcinoma to the thyroid closely mimics papillary thyroid carcinoma.

Metastases to the thyroid gland are rare, with reported incidence as low as 0.1%. The kidney, breast, and lung are known to be the most common origins of metastatic events in the thyroid.1 It can be very challenging to distinguish metastatic tumor from primary thyroid cancer when metastatic thyroid nodules are the initial presentation of the disease. A 69-year-old woman noticed a persistent, painless mass over the right aspect of the neck. Thyroid sonography revealed a right-sided nodule measured 3.5 cm with suspicious features including hypoechogenicity, irregular borders, and microcalcifications (Figure 1A). Multiple neck lymphadenopathies measured up to 2.2 cm were also found over the right level III to V. Fine-needle aspiration biopsy was performed on the right thyroid nodule and one of the suspicious lymph nodes. Upon cytological examination, aspirates from both sites showed small clusters of follicular cells with irregular nuclei, nuclear grooving (Figure 1B), and inclusions. The preoperative diagnosis was papillary thyroid carcinoma (PTC). The patient subsequently received a total thyroidectomy and neck lymph node dissection. Histopathological examination of the right lobe tumor was characterized by papillary and micropapillary structures (Figure 1C). The neoplastic cells were oncocytic cells with prominent hobnail features. Nuclear irregularity, vesicular chromatin, and nucleoli were noted. Necrosis was also identified (Figure 1D). Lymphatic channels within the thyroid tissue were invaded by tumor cells; 33 lymph nodes were involved. The preliminary pathological diagnosis of the surgical specimens was papillary thyroid carcinoma, hobnail variant with extensive lymph node metastasis. Postoperative chest computed tomography was performed as an evaluation of distant metastasis prior to radioiodine therapy. A contrast-enhanced spiculated lesion over the right lower lobe of the lung and multiple lymphadenopathies in the supraclavicular region and mediastinum were identified (Figure 1E,F). Primary lung cancer was therefore suspected. These unusual findings prompted a reexamination of the collected materials to clarify the nature of the thyroid lesion. Some intracytoplasmic vacuoles containing mucin were further identified (Figure 1G). Subsequent immunohistochemical studies (IHC) were performed. Tumor cells were positive for Napsin-A and TTF-1 but negative for PAX-8 or thyroglobulin in both histological and pre-operative cytological specimens (Figure 1H). All of these findings were consistent with metastatic pulmonary adenocarcinoma with prominent micropapillary and papillary growth patterns, which closely mimicked PTC. The rarity of metastasis to thyroid gland might be partly explained by being an immune-privileged organ. Thyroid metastasis can mimic PTCs if the neck mass was the initial presentation of malignancy. Infrequently, metastatic renal cell carcinoma presented as asymptomatic thyroid nodules with slow progression.2 Ultrasonographic findings of the metastatic tumors share several features with primary thyroid malignancies and are less of help to differential diagnosis.3 It is challenging to histologically distinguish metastatic pulmonary adenocarcinoma and PTCs due to the similarity of their growth patterns. However, pathologists should be aware of pathological changes that indicate the possibility of metastasis, such as necrosis, cytoplasmic vacuoles, and lymphovascular invasion. It is prudent to perform IHC in cases showing these unusual features before making the definite diagnosis of PTC.4 Despite the rarity of metastasis to the thyroid gland, it should be considered when unusual cytological and histological findings were presented. Given the much poorer prognosis of metastatic tumors, prompt diagnosis is crucial for appropriate and timely treatment.5 Pi-Jung Hsiao, MD, MS and Jen-Yu Hung, MD, PhD for providing the case. The authors declare no conflict of interest.