As abnormal cells continue to grow and divide, they begin to form a mass of tissue called a tumor. This tumor can take up space within the breast and begin to affect the surrounding healthy tissue.

As the tumor grows, it may press against nearby structures, which can lead to visible or physical changes. These may include swelling, thickening of the tissue, or changes in the shape of the breast. The skin over the area may also appear different, such as becoming dimpled or uneven.

Some tumors grow slowly and may not be noticeable for a long time, while others grow more quickly. Not all tumors cause pain, which is why they can sometimes go unnoticed.