Carcinoma of the breast, infiltrating lobular: The second most common type of invasive breast cancer. Infiltrating lobular carcinoma starts in the glands that secrete milk (lobules). On a mammogram, a lobular carcinoma can look similar to a ductal carcinoma, appearing as a mass with fine spikes radiating from the edges (spiculation). Infiltrating lobular carcinoma can cause a thickening of the breast tissue. Lobular carcinoma can occur in more than one site in the breast or in both breasts at the same time. Treatment may include radiation, chemotherapy, and surgery.