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Differentiation of atypical lobular hyperplasia from lobular carcinoma in situ is based on the extent of proliferation and the distension of the lobular unit. In this case of atypical lobular hyperplasia (upper panel), all acini are filled with neoplastic lobular type A cells (arrows), yet very few are distorted. In contrast, the lower panel demonstrates that more than 50% of acini are filled and distended, indicating a diagnosis of lobular carcinoma in situ. Haematoxylin/eosin stain.
