SUN-305 A case of Metastatic Mammary Carcinoma initially mistaken for papillary thyroid cancer

Abstract Disclosure: S.A. Marowa: None. S. Hossain: None. J. Delshad: None. T. Saikia: None. H. Liao: None. D. Rosenthal: None. S.C. Kumar: None. Background: The most common form of thyroid cancer is primary thyroid cancer, which includes: papillary, follicular, oncocytic (hurthle cell), anaplastic and medullary thyroid cancer. Among these, papillary thyroid cancer is the most common and metastatic disease to the thyroid is rare. Breast cancer, although one of the most commonly diagnosed malignancies, rarely metastasizes to the thyroid. The incidence of thyroid metastases identified through fine needle aspiration (FNA) is less than 0.2% (1). In this report, we present a case of metastatic mammary carcinoma to the thyroid that was initially misdiagnosed as papillary thyroid cancer in the FNA. Case Presentation: A 68 years old female with history of metastatic invasive ductal carcinoma of breast and lung cancer presented to the endocrinology clinic for evaluation of an incidental thyroid nodule identified on a chest CT scan. An ultrasound of the thyroid identified two nodules. Both categorized as TI-RADS category 2 (TR2). Typically, TR2 nodules are not considered suspicious and do not necessitate a biopsy; however, due to the size of the dominant nodule measuring 3.5 x 2.4 x 2.9 cm and the presence of underlying metastatic breast and lung cancer, further evaluation was warranted. A FNA was performed on the dominant nodule and the pathology report resulted as Bethesda category 5, indicating it was suspicious for papillary thyroid carcinoma. Following this, she was referred for total thyroidectomy. However, surgical pathology revealed metastatic mammary carcinoma. Immunohistochemical staining showed neoplastic cells that were positive for Gata-3, CK7, estrogen receptor, AE1/3, and CEA, while negative for calcitonin, TTF-1, thyroglobulin, P63, synaptophysin, and NSE. After surgery, patient was resumed on hormonal therapy and continued to be monitored by her oncology team. Conclusion: This case illustrates a significant discrepancy in outcomes between FNA and surgical pathology report where the initial FNA suggested primary thyroid cancer but surgical pathology confirmed metastatic mammary carcinoma. This indicates that a more comprehensive approach is needed regarding interpretation of FNA results in patients with complex oncological histories. Further genetic and molecular studies may provide additional insights into the behavior of such tumors and guide future management strategies. Additionally, TI-RADS may misclassify nodules that are metastatic to the thyroid rather than originating from thyroid cells, highlighting the need for clinicians to exercise caution when relying on TI-RADS in patients with known or suspected malignancies. Reference: 1.Wang Y,Zhou S,Y B,et al.Case Report and Review of Literature:Thyroid Metastases From Breast Carcinoma.Frontiers in Endocrinology.2021;12:631894.DOI: 10.3389/fendo.2021.631894.PMID: 33776925; PMCID: PMC7994513 Presentation: Sunday, July 13, 2025