Lobular carcinoma in situ (LCIS) is the name for abnormal cells in the lobules (milk sacks) of the breast. The causes of LCIS are unknown. The abnormal cells stay inside the lobules of the breast and do not spread to other parts of the breast or body.  

LCIS is not life-threatening, and it does not require treatment if there are no other abnormal changes to the breast. But having LCIS does increase the risk of developing an invasive breast cancer, so close follow-up is important. LCIS cannot usually be felt but is usually only be found in a mammogram or when a biopsy is taken for another reason.^[5][6][4]