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Expression of thyroid transcription factor-1 (TTF-1) by primary breast carcinoma and ductal carcinoma in situ (DCIS) is extremely rare and can potentially lead to diagnostic confusion. Here, we present an instructive case, including the first report of mutation analysis on such a tumour, and discuss the ways in which diagnostic pitfalls can be avoided in this situation.
A 69-year-old female non-smoker with a past medical history of hypertension presented to the emergency department with cough and a right breast lump. Examination of the right breast revealed a 60 mm mass at the 12 o'clock position, with no skin tethering or fixation to deep structures. Mammography and ultrasound showed a well-defined lobulated mass measuring 30.5 mm in the upper half of the right breast, consistent with malignancy. In addition, chest radiography showed a right paramediastinal swelling of uncertain significance. Core biopsy of the right breast mass showed a grade 3 invasive ductal carcinoma with high-grade DCIS. Immunohistochemistry showed the tumour cells to be negative for oestrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) (figure 1). In view of the unexplained lung lesion on radiology, TTF-1 …