A 58-year-old woman presents with an irregular nodularity that has developed in her right breast over the past 3 months. Mammography demonstrates irregular densities in both breasts. A needle biopsy of one breast lesion is shown. An excisional biopsy of the contralateral breast shows similar histology. Which of the following is the most likely pathologic diagnosis?

Lobular carcinoma in situ arises in the terminal duct lobular unit. Malignant cells appear as solid clusters that pack and distend the terminal ducts but not to the extent of ductal carcinoma in situ. The lesion does not usually incite the dense fibrosis and chronic inflammation so characteristic of intraductal carcinoma in situ and is, therefore, less likely to cause a detectable mass. It is not uncommon for lobular carcinoma in situ to be an "incidental" finding in a biopsy that was prompted by benign changes. As with intraductal carcinoma in situ, 20% to 30% of women with lobular carcinoma in situ receiving no further treatment after biopsy will develop invasive cancer within 20 years of diagnosis. However, about half of these invasive cancers will arise in the contralateral breast and may be either lobular or ductal cancers. Thus, lobular carcinoma in situ, more than ductal carcinoma in situ, serves as a marker for an enhanced risk of subsequent invasive cancer in both breasts. The histologic appearance is not consistent with any of the other choices.Diagnosis: Lobular carcinoma in situ