NUT Midline Carcinoma Mimics Anaplastic Thyroid Carcinoma

Introduction/Objective NUT carcinoma (NC) is an extremely rare, aggressive subtype of squamous cell carcinoma defined by rearrangement of the NUTM1 (aka NUT) gene. NC most commonly arises within the thorax and head and neck and predominantly affects teens and young adults. For the first time, we described a rare case of NC mimics anaplastic thyroid carcinoma. Methods/Case Report A 27-year female with no significant past medical history presented with neck pain and swelling for a few months and was noted to have a “thyroid” mass. An ultrasound guided fine needle aspiration of the mass showed a hypercellular specimen composed of highly atypical epithelial cells with irregular nuclear membrane and prominent nucleolus, high nuclear to cytoplasmic ration, dense cytoplasm, lying singly or in flat sheet mixed with acute inflammatory exudate (Fig. A, B). No thyroid follicular cells or colloid are present. It was signed out as suspicious for squamous cell carcinoma since there was no cell block for further workup. A following excisional biopsy demonstrated similar atypical epithelium detached or embedded in skeletal muscle and soft tissue with acute, chronic inflammation and necrosis (Fig C). By immunohistochemical stains, the atypical cells are positive for p40 (Fig D) and p53. The Ki-67 proliferation index is ranging from 40 % to 70 %. Scattered tumor cells are weakly positive for PAX-8 and TTF-1. These findings raise the possibility that the squamous cell carcinoma might be a component of anaplastic thyroid carcinoma. However, molecular studies revealed a NSD3-NUTM1 fusion, which is characteristically identified in a subtype of squamous cell carcinoma know as NUT “midline” carcinoma. Results (if a Case Study enter NA) NA Conclusion Diagnosis of NC can be established by positive NUT nuclear immunohistochemical staining. Recognizing the typical morphology and keeping low threshold for the NUT immunohistochemical staining will increase the diagnostic efficacy.