Radiation vs. Chemotherapy: How They Differ

Radiation and chemotherapy are both common cancer treatments, but they reach the cancer in different ways. Radiation therapy is usually a local treatment. It uses high-energy beams or sources aimed at a specific area. Chemotherapy is usually a systemic treatment. Its drugs travel through the bloodstream to reach cells anywhere in the body. The two are used for different reasons, and they are often part of the same overall plan.

Radiation therapy uses high-energy beams to damage the DNA of cancer cells in a targeted area, so the cells stop growing or die. In some cases radioactive sources are placed inside the body instead. Because it is focused on one region, radiation treats a specific tumor or area rather than the whole body.

Chemotherapy uses drugs that kill or slow fast-growing cells. These drugs usually travel through the whole body. That is why chemo is used when treatment needs to reach cells beyond a single spot.

One treatment is focused and the other is body-wide, so they tend to cause different problems.

• Radiation side effects usually affect the treated area. Skin changes over that region are common, as is fatigue. They build up over a course of treatment.
• Chemotherapy side effects are often body-wide, such as hair loss, nausea, and low blood counts. The drugs reach healthy fast-growing cells as well.

Your care team tells you which effects are likely for your specific treatment.

Radiation and chemotherapy do different jobs, so plans often use both. Sometimes one follows the other. Sometimes they are given at the same time so they work alongside each other. Surgery, immunotherapy, or targeted therapy may also be part of the plan.

Neither treatment is simply "stronger." The choice, and the order, depends on the cancer type, its stage, and the goals of treatment. Ask your care team what each treatment in your plan is meant to accomplish. Ask which side effects are most likely for you, and how progress will be checked.

The two treatments raise different alarms, so it helps to know both.

• Call 911 or go to an emergency department if you have chest pain, sudden trouble breathing, heavy bleeding, a seizure, or new confusion or weakness on one side.
• Call your care team immediately, day or night, if you have a temperature of 100.4°F (38°C) or higher, or shaking chills, during chemotherapy. Low white cells let an infection turn serious within hours, and the CDC calls this a medical emergency. If you cannot reach your team quickly, go to an emergency department and tell them straight away that you are having chemotherapy.
• Call your care team the same day if you have that temperature during radiation alone, or unusual bruising or bleeding, or treated skin that blisters or breaks open.
• Call your care team within a day or two if nausea, vomiting, or diarrhea keeps you from drinking, mouth sores make eating hard, or fatigue is clearly worsening.

Treat a fever during chemotherapy as an emergency rather than an appointment topic, and do not take new medicines, herbs, or supplements without asking your team first.

• National Cancer Institute — Radiation Therapy to Treat Cancer
• National Cancer Institute — Chemotherapy to Treat Cancer
• National Cancer Institute — Infection and Neutropenia During Cancer Treatment
• Centers for Disease Control and Prevention — Fever during cancer treatment