WHY EARLY DETECTION IS IMPORTANT
Even though I had seen the American Cancer Society's posters urging women to perform breast self-exams, I never understood why it was so important to detect breast cancer early. I always figured that it would be better not to discover it and, in doing so, to put off chemo for as long as possible.
The reason why you want to detect breast cancer early is that, if you discover a tumor when it is small, you will have a much better chance of being cured. This means that it will be easier for a surgeon to cut it out of your body and get clean margins. Despite what you hear in the mainstream media, surgery is still the only way that most cancers are cured. You do not have to do chemo or radiation (although the doctors will try to sell it to you) and your breast can look relatively normal afterwards.
Breast cancer tumors grow like spiders. There are "legs" coming off of the main body of the tumor. So it will be hard or impossible for a surgeon to get clean margins if the tumor is very big. And once a cancer is large and has spread to other organs, it is almost impossible to cure.
Another important thing to remember is that, even if you are told you have a fibroadenoma (a benign, harmless tumor) or fibrocystic breasts, you should always get a second opinion. On the website, Young Survival Coalition, the women tell story after story of how they were misdiagnosed because they were in their twenties or thirties when they got breast cancer. This happens all of the time.
My sister died because she was misdiagnosed. She went to the hospital and they performed an ultrasound, physical exam, mammogram, and fine needle aspiration (biopsy of the tumor). And it turns out that every one of these exams failed to detect her cancer, which was already a cherry sized tumor. Later we found out that the pathologist never even bothered to look at the slides (which clearly showed cancerous cells), probably because my sister was only 29 and considered too young to have breast cancer. She was reassured by the advice nurses multiple times that, although it was growing, it was a benign tumor. And by the time she insisted on getting a second opinion, ten months later, the tumor was too big to cut out.
In summary, you should check your breasts for lumps and have your gynecologist do the same (a physical exam). If you find a lump, have it tested and then tested again.