Liver Metastasis Masking as Neuroendocrine Carcinoma (NEC) Predates Discovery of Intra/Peri-Thyroid Thymic Carcinoma (ITTC)

Abstract Introduction/Objective Intrathyroidal thymic carcinoma (ITTC) also known as Carcinoma showing Thymus-Like differentiation (CASTLE) is a rare malignancy comprising 0.1 to 0.15% of thyroid cancers. The WHO 5th Edition emphasizes that these tumors may also occur outside the thyroid typically at the lower pole Methods/Case Report Our aim is to show that NEC is a mimic of ITTC; that CD5, c-KIT (CD 117), p40, CK5/6 expression distinguishes ITTC from NEC, and that ITTC may at times express NE markers. Briefly, a 51-year-old man presented with hoarseness left neck mass and a PET avid liver lesion in 2016. Biopsy of the neck and liver masses revealed an infiltrating poorly differentiated “NEC” with similar morphology; positive for p53, chromogranin, and synaptophysin; negative for cam5.2, thyroglobulin, TTF-1, CD20, Pax5, CD3, CD5, S-100, calcitonin, CD56, p16, p63, c-KIT (CD117), AE1/3; and 100% staining for Ki-67. The patient underwent chemotherapy, responded well, remained under surveillance, and was symptom-free in the interim. A new 2.1 x 2.0 cm nodule appeared at the left thyroid lower pole after 6 years. The tumor was positive for CD5, CD117, p40, CK5/6, with > 95% Ki67 expression. Chromogranin, synaptophysin, CD56, thyroglobulin, calponin, TTF1, and Epstein Barr virus were negative and was overall diagnostic of ITTC Results (if a Case Study enter NA) NA Conclusion This shows the diagnostic dilemma of distinguishing ITTC from NEC which may share expression of neuroendocrine (NEC) markers and that positivity for CD5, c-KIT (CD117), p40, and CK5/6 are key diagnostic features of ITTC.