Most of the so-called signet-ring cell carcinomas of the breast are variants of invasive lobular carcinoma. In classic invasive lobular carcinoma, a variable proportions of tumor cells have intracytoplasmic lumina. When these lumina are prominent, they compress the nucleus and create a signet-ring appearance. In this image, the signet-ring cells form a sheet surrounding the terminal-duct lobular unit.
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