 The next type of invasive breast cancer I will discuss is infiltrating lobular carcinoma or ILC. This is the second most common type of invasive breast cancer. However, it only accounts for five to 10 percent of invasive breast cancers. The incidence is increasing faster than the rates of ductal carcinoma, and we think it is because lobular carcinoma is more strongly associated with hormone replacement therapy. As the incidence of use of hormone replacement therapy increased, we saw an increase in this type of cancer. Macroscopic appearance is identical to IDC. In many cases, no mass lesion is grossly evident. Microscopic extent of invasive lobular carcinoma may be greater than the gross extent. Invasive lobular carcinoma is defined by pathologists as having the lack of e-cadherin staining, and this is specifically what distinguishes it from IDC. E-cadherin helps tumor cells form a mass. Because infiltrating lobular carcinoma lacks e-cadherin, it tends to form sheet-like single file cell pattern. It does not form a necessarily mass with convex margins. This is one of the reasons for its imaging appearance and its difficulty to be detected on imaging, because it's a sheet, not a mass. So sometimes it will only be seen on one projection. So present as an asymmetry, which can lead people to think it's not as significant and maybe just overlapping tissue versus a solid mass. Again, it's microscopically characterized by small cells that infiltrate stroma and adipose tissue individually in a single file pattern. It induces minimal fibrous reaction, so you will not see a speculated mass the way you will with infiltrating ductal carcinoma. Here is an imaging example of infiltrating lobular carcinoma that is patient presented with multiple palpable areas. We do see a slightly asymmetric area in the upper outer breast, but it's very, very subtle. When we look on ultrasound, we see this actual slightly hypercoic lesion and on biopsy, this was invasive lobular carcinoma. This is another example of invasive lobular carcinoma. This is a slowly developing asymmetry. The mammogram up here was two years prior to this current mammogram. As we can see, there is some tissue in the retroarial region that is roughly symmetric at that time, a little more on the left than the right. But over time, within two years, we see a significant change in the amount of asymmetry in the retroarial region on the left side. Ultrasound showed in a regular hypoechoic mass, this was biopsy proven infiltrating lobular carcinoma. Let's compare infiltrating ductal carcinoma with infiltrating lobular carcinoma. ILC has a higher frequency of bilateral and multi-centric disease. For this reason, they usually recommend patients with ILC have an MRI prior to surgery to rule out bilateral and multi-centric disease. ILC is more likely to be mammographically occult than IDC because of the way the cells form. As I said, it's more likely to be an asymmetry, it's less likely to be an obvious mass forming lesion. ILC is seen in older women and are larger and better differentiated than IDC. ILC is usually ER positive as a rule. Outcomes are more favorable with ILC. ILC tends to metastasize later than IDC. Originally, it was thought that ILC was more aggressive than IDC actually, because patients were presenting at later stages. But the reason patients were being presented at later stages is we were not detecting it as readily on mammography because it wasn't the classic appearance that we thought of with IDC. But then, if you look at it, it actually metastasizes less if detected early. ILC usually also spreads to the peritoneum meninges and GI tract as opposed to IDC, which spreads to bone, liver, lungs, or brain. There are distinct differences between these two pathologies. That is it for invasive breast cancer.