The short-term side effects of external beam radiation usually start a few weeks into treatment.

• Fatigue is common and is usually mild or moderate, and resolves a few weeks after completion of radiation. It is almost never severe. Almost all patients can continue to go about their normal job or daily tasks, including driving.
• A radiation skin reaction that occurs towards the end of treatment is very common. It is probably best described as a sunburn, although it is a little different. The skin of the breast and lower armpit become red, itchy, and may hurt like a sunburn. It may also become swollen, and the swelling is often the most obvious around the nipple. During treatment, patients are given special ointments to keep the skin healthy and to prevent drying and cracking. Some patients' skin will peel, but the skin reaction heals completely in all cases within a few weeks of completing radiation.
• Mild pain is uncommon. Severe pain is rare. Most pain is from the skin reaction discussed above, and is usually mild. Over-the-counter pain medicines usually control this pain, but doctors will provide stronger medicines if needed. In addition to skin pain, some patients will report very brief shooting-type pains in the breast during the course of treatment. These may occur a few hours after each treatment, or may come at any time. Sometimes these shooting pains are felt into the arm or hand. These shooting pains are almost always very brief and go away after radiation is completed.

Long term side effects after external beam radiation for breast cancer are almost always mild. Serious long-term side effects are quite rare. One of the biggest concerns women have is about damage to their heart and lungs, organs which lie under the breast. Older radiation methods treated large parts of the heart and lung, and some heart and lung damage was seen as a consequence. Modern radiation methods treat very little of these organs, if at all. Today, heart or lung problems from external beam radiation to the breast are incredibly rare, and only the older radiation oncologists will remember such cases from the past.