Prostate cancer is the second most frequent cancer diagnosis in men and the fifth leading cause of death worldwide.¹⁾ Most patients with prostate cancer are asymptomatic, and it may be accompanied by lower urinary tract symptoms such as hesitancy and nocturia. Distant metastasis is a poor prognostic factor for prostate cancer. Most frequently involved organs are bone, lungs, and liver.²⁾ In rare cases, metastasis to the cervical lymph nodes can also occur, accounting for approximately 10% of lymph node metastasis.²⁾

If cervical lymph node metastasis is present, ultrasonography (US) guided fine-needle aspiration biopsy (FNAB) can be preferentially performed to distinguish between head and neck malignancies. FNAB is a simple and cost-effective technique; however, its diagnostic accuracy varies depending on the clinician’s skill.³⁾ Therefore, core-needle biopsy (CNB) can be used to compensate for the limitations of FNAB. CNB is effective in reducing inconclusive results and improving diagnostic performance.

We present an unusual case of metastatic prostate cancer of the thyroid gland showing features of anaplastic thyroid cancer on US and computed tomography (CT). It was misdiagnosed as anaplastic thyroid cancer on US-guided FNAB; however, finally diagnosed using CNB. Here, we discuss the importance of CNB in the differential diagnosis of metastatic thyroid lesions from other primary carcinomas.

A 73-year-old man visited a local hospital presenting hoarseness and aspiration symptoms for 2 weeks. The patient underwent cryosurgery for prostate cancer 5 years ago and showed no signs of recurrence during regular follow-up. Laryngoscopy revealed left vocal cord palsy. Neck CT showed thyroid mass sized approximately 6.7×2.4 cm in the left lobe with nodule and focal calcifications of the right lobe which showed features of malignancy (Fig. 1A). Multiple heterogeneous enhancing or necrotic lymph nodes with focal conglomerations and extranodal extension, suggesting metastatic lymphadenopathy, were observed in the left level II-VI and superior mediastinum (Fig. 1B). US-guided FNAB was performed on the thyroid mass and the pathology report suggested anaplastic carcinoma.

The patient was referred to our hospital for further evaluation and treatment. In neck US, an approximately 6.6 cm sized heterogenous mass which occupied almost total left thyroid lobe that showed features of extrathyroidal extension to strap muscle and a 1.4 cm sized hypoechoic nodule with microcalcification in right lower pole showing K-TIRADS 5 were detected (Fig. 1C, D). Positron emission tomography (PET)-CT revealed hypermetabolic lesions in the left thyroid gland, left level II, III, IV, V, VI, supraclavicular region, and right level II. No suspicious lesions were identified in the other organs.

CNB of the left supraclavicular lymph node was performed based on the impression that it was likely a metastasis of anaplastic thyroid carcinoma. Micro-scopically, the biopsied tissue exhibited poorly-differentiated tumor cells diffusely infiltrating within the hyalinized stroma, with no evidence of lymph node structure (Fig. 2A). The presence of poorly-formed glandular structures led to the consideration of potential metastatic adenocarcinomas. Immunohistochemical stains were performed to determine the origin of metastasis. The tumor cells showed negative immunohistochemical stain results for TTF-1 (thyroid transcription factor-1) and thyroglobulin, excluding the possibility of thyroid or lung origin, positive staining for PSA (prostate-specific antigen) and AMACR (alpha-Methylacyl-CoA racemase) confirmed metastasis from the prostatic gland (Fig. 2B). A CNB of the thyroid gland was performed subsequently. Atypical cell clusters were observed between normal thyroid follicles, which also showed positive staining for PSA, confirming that it was a metastasis of prostatic adenocar-cinoma rather than a primary thyroid carcinoma (Fig. 2C, D).

The patient was referred to the Department of Oncology for hormonal therapy with a luteinizing hormone-releasing hormone agonist and a nonsteroidal anti-androgen. He had regular follow-ups in the outpatient clinic at intervals of 1 month up to 6 months after the hormonal therapy, but was lost to follow-up.

Prostate cancer frequently have metastasis to bone (90%), lungs (46%), liver (25%), pleura (21%), and adrenal glands (13%).²⁾ Thyroid gland is a rare organ for metastasis of prostate adenocarcinoma. To the best of our knowledge, only three cases of thyroid metastasis from adenocarcinoma of the prostate to the thyroid gland have been reported.^4-6) The difference from our case was that thyroid metastasis of prostate cancer was diagnosed using fine-needle aspiration in all three cases. Owing to its low incidence rate, clinicians may overlook thyroid metastasis in prostate cancer. Because thyroid cancer and metastatic lesions from prostate cancer require fundamentally different therapeutic strategies, it is important to accurately diagnose and differentiate them from one another.

On ultrasonography, anaplastic thyroid cancer usually appears as a poorly defined hypoechoic mass involving the thyroid gland with calcifications, tumor necrosis, or locally infiltrative borders.⁷⁾ Neck CT shows extrathyroidal tumor extension and regional lymph node involvement. Tumor is characterized as large neck mass with necrosis, calcification, and visceral space invasion, involving the esophagus, trachea, larynx, and lateral compartment lymphadenopathy being usually found.⁸⁾ In this case, both neck CT and US showed a 6 cm size huge thyroid mass in the left thyroid lobe with extrathyroidal extension and several enhancing or necrotic ipsilateral neck nodes. Due to the similarities between these imaging studies, this case could initially have been mistaken for anaplastic thyroid cancer.

In addition, similar to the FNAB results, which suggested anaplastic thyroid cancer, the patient’s chief complaints of hoarseness and signs of aspiration were similar to those of anaplastic thyroid cancer. Rapid enlargement of cervical masses is usually observed in patients with anaplastic thyroid cancer, and locally compressive symptoms, such as dysphagia, dyspnea, and neck pain are observed due to the mass effect.⁹⁾

However, considering the patient’s history of prostate cancer, thyroid and lateral neck metastasis from prostate cancer should be considered in the differential diagnosis. Although anaplastic thyroid cancer was suggested by US guided FNAB, CNB of the thyroid gland and lateral lymph nodes was necessary for a definite diagnosis. CNB results are typically superior to those of FNAB. Definitive diagnosis using FNAB often presents challenging problems. Diagnostic sensitivity as reported in the literature varies widely, ranging from 25 to 90%, and in a significant percentage of cases, FNAB results are suggestive but not diagnostic.^10,11) CNB was suggested to be the alternative to FNAB, since a sensitivity of up to 100% has been reported.¹²⁾ A recent report demonstrated that CNB was better than FNAB in providing a specific diagnosis (90% vs. 66%) and showed higher accuracy in identifying true neoplasms (100% vs. 93%) and detecting malignancy (99% vs. 90%).¹³⁾ CNB was able to reduce unnecessary diagnostic surgery in patients with anaplastic thyroid cancer by the virtue of its superior diagnostic sensitivity and positive predictive value compared to FNAB.¹⁰⁾ Likewise in our case, metastasis of prostate adenocarcinoma was confirmed after taking a CNB with immunohistochemical stain of thyroid and lymph node tissue.

In conclusion, misdiagnosis can be prevented if CNB is added to diagnose metastasis to head and neck lesions from other organs. In addition, clinicians must remember the history of other carcinomas before making a differential diagnosis of metastatic lesions.