👉 Mastocarcinoma is a type of cancer that occurs in the breast tissue. It's a common form of breast cancer and can grow very quickly, causing pain or swelling in the breast, especially at night. Mastocarcinomas are most likely to be found in women who have had a previous history of breast cancer. They can spread to other parts of the body if they don't receive treatment quickly enough. If you suspect that you may have a mastocarcinoma,