This is Fibroadenoma. Note the tumor is encapsulated and well-demarcated from adjacent breast tissue. As the name implies it is composed of both fibrous and glandular tissue. It is benign. A firm, rubbery, painless mass in the breast of a woman under 25 will almost always be a fibroadenoma. If left alone it can continue to grow and ulcerate through the skin. It is by far the most common breast tumor in women under 25. Some fibroadenomas are polyclonal hyperplasias. Polyclonal fibroadenomas occur in almost half the women receiving cyclosporin for renal transplantation.
