Radiation Therapy can treat breast cancer in two ways

Radiation therapy uses high-energy rays to kill cancer cells. It affects cells only in the part of the body that is treated.
Radiation therapy may be used after surgery to destroy breast cancer cells that remain in the chest area. Women usually have radiation therapy after breast-sparing surgery, but it's sometimes used after mastectomy too.
You can get radiation therapy to treat breast cancer in two ways:

• Machine outside the body (external radiation therapy): The radiation comes from a large machine outside the body. You'll go to a hospital or clinic for treatment. Usually, women get treatment once a day, 5 days a week for 3 to 6 weeks. Each treatment session lasts only a few minutes. External radiation is the most common type used for breast cancer.
• Material inside the body (brachytherapy): The doctor will place one or more thin tubes inside the breast through a tiny incision. A radioactive substance is loaded into the tube. The treatment session may last for a few minutes, and the substance is removed. When it's removed, no radioactivity remains in your body. This method of radiation therapy may be repeated every day for a week.

Side effects depend mainly on the type of radiation and how much is given. Ask your health care team to describe what you can expect.
It's common for the skin in the treated area to become red, dry, tender, and itchy. Check with your doctor before using lotion, cream, or deodorant on the treated area. After treatment is over, the skin will slowly heal. However, there may be a lasting change in the color of your skin.
With either type of radiation therapy, your breast may feel heavy and tight. Internal radiation therapy may make your breast look red or bruised. These problems usually go away over time.
Bras and tight clothes may rub your skin and cause soreness. You may want to wear loose-fitting cotton clothes during this time.
You're likely to become tired during radiation therapy, especially in the later weeks of treatment. Although getting enough rest is important, most people say they feel better when they exercise every day. Try to go for a short walk, do gentle stretches, or do yoga.
You may wish to discuss with your doctor the possible long- term effects of radiation therapy. For example, radiation therapy to the chest may harm the lung or heart. Also, it can change the size of your breast and the way it looks. If any of these problems occur, your health care team can tell you how to manage them.