Breast cancer is the most common form of cancer among women. Most patients with this diagnosis have likely had the cancerous mass for 5 to 10 years before the diagnosis. Cancers can easily be felt in the breast when they reach the size of approximately 1 cm. A lump of this size contains one billion cells, which is the result of 30 doublings of a single cancer cell. Assuming that a breast lump grows at a doubling time of 100 days, it would take approximately 10 years to reach a point where it could be felt.