What is Radiation Therapy?
Starting radiation therapy can feel intimidating, but it is important to remember why your care team is recommending this treatment. Radiation therapy (also called radiotherapy or RT) is one of the most effective tools available for reducing the risk of your breast cancer coming back after surgery. It uses high-energy beams to destroy microscopic cancer cells that may occasionally remain in your breast, chest wall, or nearby lymph nodes following surgery — even when your surgery has been successful. This is known as adjuvant therapy, and it helps reduce the chance of the cancer returning. Advances in radiation oncology have dramatically improved both the precision and safety of treatment. Modern techniques allow your radiation oncologist to target the treatment area more accurately than ever before while minimizing exposure to your surrounding healthy tissues. As a result, you will likely be able to continue your normal daily activities throughout treatment with relatively mild and manageable side effects. However, every person responds differently, and you may experience more noticeable effects than others. While radiation therapy can cause temporary changes such as skin irritation, fatigue, or breast discomfort, your oncology care team will work closely with you to monitor your symptoms and provide strategies to help you stay as comfortable as possible. Most side effects improve after treatment is completed, and many long-term complications have become less common with modern treatment approaches. Understanding what to expect can help reduce your anxiety and allow you to feel more prepared and in control. This guide reviews common side effects, practical self-care strategies, important symptoms to report, and key medical terms you may hear throughout your treatment and recovery.Side Effects During and Shortly After Treatment (Acute Side Effects)
These typically occur during treatment and resolve within a few weeks to months afterward.Radiation Dermatitis (Skin Reaction)
This is the most common side effect. Your care team may describe it by grade:- Grade 1: Mild redness (erythema) or dry, flaky skin (dry desquamation)
- Grade 2: Moderate redness, swelling, or moist, peeling skin (moist desquamation)
- Grade 3: Severe skin breakdown (ulceration) — this is uncommon with modern techniques
Breast Pain (Mastalgia) and Swelling (Edema)
You may experience mild to moderate soreness, tenderness, or swelling in your treated breast. This is common and usually temporary. Tips: Over-the-counter pain relievers (analgesics) like acetaminophen or ibuprofen can help. Let your care team know if your pain is hard to manage.Fatigue (Cancer-Related Fatigue)
You may feel more tired than usual during treatment. Your care team may call this cancer-related fatigue (CRF) — it is different from normal tiredness and may not improve fully with sleep alone. Tips: Light physical activity (like walking) can actually help with your fatigue. Rest when you need to, and don't hesitate to ask for help with daily tasks.Side Effects That May Develop Later (Late Side Effects or Late Toxicity)
These may appear months to years after your treatment ends.Fibrosis and Breast Changes
Your treated breast may feel firmer over time due to scar tissue formation (fibrosis). It may also look slightly smaller (atrophy) or develop visible small blood vessels on the skin (telangiectasia). Your doctor may describe the overall change as radiation-induced fibrosis. These changes are usually mild.Lymphedema (Swelling from Fluid Buildup)
Swelling can develop in your arm, hand, breast, or chest wall on the treated side. It is caused by a buildup of lymphatic fluid when lymph drainage is disrupted by surgery (especially axillary lymph node dissection, or ALND) or radiation to your lymph nodes (regional nodal irradiation, or RNI). Most cases appear within the first 3 years, but lymphedema can develop at any time.Your care team may measure your arms before treatment (baseline limb measurements) to detect early changes. Early-stage lymphedema (Stage 0–1) is often reversible with prompt treatment.
Early signs to watch for include a feeling of heaviness, tightness, fullness, or mild pitting edema (swelling that leaves an indent when you press on it). Tips: Report any new swelling or heaviness to your care team right away — early treatment works best. Stay active; exercise does not cause or worsen lymphedema. Maintain a healthy body mass index (BMI), as obesity increases your risk.Cardiovascular Disease (Heart-Related Effects)
Radiation near your heart — especially if you are being treated for left-sided breast cancer — can slightly increase your long-term risk of heart problems, including coronary artery disease (CAD) and heart failure (cardiomyopathy). Your care team may discuss your mean heart dose (the average radiation dose your heart received) as a measure of your risk. Modern techniques such as deep inspiration breath hold (DIBH) — where you take a deep breath during treatment to move your heart away from the radiation field — and intensity-modulated radiation therapy (IMRT) have greatly reduced cardiac exposure. Recent large studies have shown that with these contemporary techniques, the historical excess cardiovascular risk associated with left-sided breast radiation has been largely eliminated. Tips: Keep up with heart-healthy habits — eat a balanced diet, stay physically active, don't smoke, manage your blood pressure (hypertension) and cholesterol (hyperlipidemia), and get 7–9 hours of sleep per night. Tell your doctor about any new chest pain (angina), shortness of breath (dyspnea), or unusual fatigue.Radiation Pneumonitis (Lung Inflammation)
A small number of patients develop inflammation of the lung tissue (pneumonitis) a few weeks to months after treatment, causing a dry, nonproductive cough or shortness of breath (dyspnea). This is usually temporary and treatable with corticosteroids (anti-inflammatory medication). In rare cases, it can lead to permanent scarring (pulmonary fibrosis). Tips: Contact your care team if you develop a new dry cough or difficulty breathing.Secondary Malignancy (New Cancer from Radiation)
There is a very small long-term risk of developing a new cancer in or near the treated area, such as a soft-tissue sarcoma (angiosarcoma). This is rare and typically would not appear until 10 or more years after treatment. Your care team will continue to monitor you with regular follow-up visits and imaging.When to Call Your Care Team
Contact your doctor or nurse if you experience any of the following:- Skin that blisters, cracks, or becomes very painful (severe dermatitis)
- New or worsening swelling in your arm, hand, or breast (possible lymphedema)
- Signs of infection (cellulitis): redness, warmth, fever, or chills in the treated area
- New chest pain, shortness of breath, or persistent cough (possible cardiac or pulmonary toxicity)
- Fatigue that does not improve with rest
Quick Reference: Key Medical Terms
- Adjuvant therapy — Treatment given after surgery to reduce the risk of cancer returning
- Erythema — Redness of the skin
- Desquamation — Peeling or shedding of the skin (dry or moist)
- Fibrosis — Thickening or scarring of tissue
- Telangiectasia — Small, visible blood vessels on the skin surface
- Lymphedema — Swelling caused by a buildup of lymph fluid
- Pneumonitis — Inflammation of lung tissue
- Dyspnea — Shortness of breath
- Angina — Chest pain related to reduced blood flow to the heart
- DIBH (Deep Inspiration Breath Hold) — A technique to protect your heart during radiation
- IMRT (Intensity-Modulated Radiation Therapy) — A precise radiation technique that shapes the beam to reduce exposure to your healthy tissue
- Hypofractionation — Delivering radiation in fewer, larger doses per session (now the standard approach for most breast cancer patients)
Key Takeaways
- Most side effects are manageable and improve over time.
- Modern radiation techniques (DIBH, IMRT, hypofractionation) are designed to minimize your side effects.
- Staying active, maintaining a healthy weight, and following heart-healthy habits can reduce your long-term risks.
- Early reporting of symptoms — especially swelling or skin changes — leads to the best outcomes.
- Your care team is here to help you through every step of your treatment and recovery.
References
- Chua, B.H., Link, E.K., Kunkler, I.H. et al. (2022) ‘Radiation doses and fractionation schedules in non-low-risk ductal carcinoma in situ in the breast (BIG 3-07/TROG 07.01): A randomised, factorial, multicentre, open-label, phase 3 study’, The Lancet.
- Citrin, D.E. and Timmerman, R.D. (2026) ‘Effects of radiotherapy in normal tissue’, The New England Journal of Medicine.
- Darby, S.C., Ewertz, M., McGale, P. et al. (2013) ‘Risk of ischemic heart disease in women after radiotherapy for breast cancer’, The New England Journal of Medicine, 368(11), pp. 987–998.
- Jagsi, R., Griffith, K.A., Vicini, F. et al. (2020) ‘Toward improving patients’ experiences of acute toxicity from breast radiotherapy: Insights from the analysis of patient-reported outcomes in a large multicenter cohort’, Journal of Clinical Oncology, 38(31), pp. 3640–3648.
- National Comprehensive Cancer Network (2026) Survivorship. NCCN Clinical Practice Guidelines in Oncology. Updated 8 April 2026.
- Verginadis, I.I., Citrin, D.E., Ky, B. et al. (2025) ‘Radiotherapy toxicities: Mechanisms, management, and future directions’, The Lancet.
- Zhang, L., Iliescu, C., Ferencik, M. et al. (2026) ‘Coronary atherosclerosis in patients with cancer and survivors: A scientific statement from the American Heart Association’, Circulation.