Radiation therapy uses high-energy beams to damage the DNA inside cancer cells, stopping them from growing and dividing. It’s one of the most common cancer treatments, used in roughly half of all cancer cases, and the process involves careful planning, precise technology, and a team of specialists working behind the scenes before a single beam is ever delivered. Here’s what actually happens from start to finish.
How Radiation Kills Cancer Cells
Radiation works at the molecular level by breaking the DNA strands inside cells. When DNA is damaged badly enough, the cell can no longer copy itself and eventually dies. The damage happens in two ways. Sometimes the beam hits DNA directly. But more often, especially with standard X-ray-based radiation, the beam splits water molecules inside the cell into highly reactive particles called free radicals. These free radicals then attack nearby DNA, causing breaks in one or both strands of the double helix.
Double-strand breaks are the most lethal type of damage for a cell. Healthy cells have better repair machinery than cancer cells, so they can usually fix themselves between treatment sessions. Cancer cells are worse at this repair, which is why radiation is given in multiple smaller doses over days or weeks rather than all at once. This approach, called fractionation, gives normal tissue time to recover while steadily destroying the tumor.
The Simulation: Planning Your Treatment
Before any radiation is delivered, you go through a planning session called a simulation. This visit can take 30 minutes to over an hour and sets the blueprint for every treatment that follows.
You’ll lie on a treatment table, usually on your back with one or both arms raised above your head, though some treatments require lying face down. The team then creates a custom device to keep you perfectly still in the same position every single day. For head and neck cancers, this is a mesh mask molded to your face that you’ll wear for about 10 to 12 minutes per session. For other areas, it might be a foam cradle shaped to your body.
Once you’re positioned, the team takes a CT scan (and sometimes a PET or MRI scan) of the treatment area. Your doctor may order contrast dye through an IV to make certain structures show up more clearly on the images. If your tumor is near your lungs or throat, you may be given specific breathing or swallowing instructions to practice during the scan and repeat at every future session. Finally, the team makes small ink marks on your skin so they can line you up in exactly the same spot each time. In some cases, these are tiny permanent dots, similar to tattoos.
How the Treatment Plan Is Built
After simulation, a team of specialists uses your scans to design a custom radiation plan. The radiation oncologist, the doctor overseeing your care, identifies the tumor and outlines exactly how much radiation the target should receive while setting limits on how much nearby healthy tissue can tolerate. A medical physicist then generates possible treatment plans, calculating how to deliver the prescribed dose as precisely as possible. A dosimetrist handles much of the detailed math: assembling your scan data, computing dose distributions, and figuring out exact treatment times. The physicist reviews all of this work, and then the oncologist and physicist together select the final plan.
Radiation dose is measured in units called grays (Gy). A typical curative treatment course delivers the total prescribed dose broken into daily fractions, often given five days a week over several weeks. Palliative treatments, designed to relieve symptoms rather than cure, use fewer and sometimes larger fractions.
What Happens During External Beam Treatment
The most common form of radiation therapy is external beam radiation, delivered by a machine called a linear accelerator (LINAC). This machine uses microwave technology to accelerate tiny particles called electrons to near light speed, then slams them into a heavy metal target. That collision produces high-energy X-rays, which are shaped into a beam aimed at your tumor.
The beam exits from a rotating arm called a gantry, which can swing around you to hit the tumor from multiple angles. Inside the machine’s head, a device called a multileaf collimator uses dozens of small metal “leaves” that slide into position to sculpt the beam so it matches the shape of your tumor. The treatment table also moves to fine-tune your position. The result is a beam that conforms tightly to the cancer while sparing as much surrounding tissue as possible.
Each session is painless. You won’t feel heat, pressure, or anything at all from the beam, much like getting an X-ray. The time you spend in the treatment room varies depending on the technique, but the actual beam delivery often takes only a few minutes. Most of the session involves getting you positioned and verified.
Precision Techniques: IMRT, SBRT, and Proton Therapy
Not all external beam radiation is the same. Intensity-modulated radiation therapy (IMRT) carefully shapes the energy beams to match the three-dimensional contour of the tumor, adjusting the intensity across the beam. It’s typically given in daily treatments over several weeks.
Stereotactic body radiation therapy (SBRT) takes precision a step further. It uses many beams aimed from different angles, all converging on the tumor so the intersection point receives a very high dose. Because each fraction is much stronger, you need far fewer sessions. For prostate cancer confined to the prostate, for example, SBRT treatment typically takes about two weeks instead of the five to nine weeks common with IMRT. Studies show roughly 95% of men treated with SBRT remain cancer-free at least five years out, with success rates similar to IMRT. SBRT is slightly more likely to cause short-term urinary and bowel side effects compared to longer courses, but by two years after treatment, side effects even out.
Proton therapy uses protons instead of X-rays. Protons deposit most of their energy at a specific depth and then stop, which can reduce radiation exposure to tissue beyond the tumor. This makes it particularly useful for cancers near sensitive structures, like the brain or spinal cord, and for treating children.
Internal Radiation: Brachytherapy
Instead of beaming radiation from outside the body, brachytherapy places a radioactive source inside or right next to the tumor. There are two main approaches. In interstitial brachytherapy, small radioactive implants (often called “seeds”) are placed directly into the tumor tissue. This is common for prostate cancer and some soft tissue cancers. In intracavitary brachytherapy, the radioactive source is placed inside a body cavity near the tumor, which is frequently used for cervical, uterine, and vaginal cancers.
Implants can be temporary or permanent. Temporary implants deliver a high dose over minutes to hours and are then removed. Permanent implants, like the tiny seeds used in some prostate treatments, stay in place and gradually lose their radioactivity over weeks to months.
Are You Radioactive Afterward?
With external beam radiation, no. There is no radioactive material inside your body at any point, so you pose zero risk to the people around you during or after treatment.
With brachytherapy, it depends. If you have a temporary implant, you stop emitting any radiation the moment it’s removed. If you have a permanent implant, your body does give off a small amount of radiation for a period of time, but it usually doesn’t travel far beyond the treatment area. Your care team will let you know if any short-term precautions are needed, such as limiting close contact with pregnant women or small children for a set number of weeks.
Side Effects by Treatment Area
Fatigue is the most universal side effect. It can hit all at once or build gradually over the course of treatment, and it varies widely from person to person, even among people receiving the same treatment.
Beyond fatigue, side effects depend almost entirely on which part of the body is being treated:
- Head and neck: mouth sores, difficulty swallowing, taste changes, reduced thyroid function
- Brain: memory and concentration problems, nausea, headaches, blurry vision
- Chest: sore throat, cough, shortness of breath
- Breast: skin irritation, swelling, tenderness
- Stomach and abdomen: nausea, vomiting, diarrhea
- Pelvis and rectum: diarrhea, bladder irritation, sexual and fertility changes
Skin changes in the treatment area, ranging from mild redness to peeling similar to a sunburn, and hair loss in the treated zone are common across most treatment sites.
Most side effects from damaged healthy cells improve within a few months after treatment ends, as those cells repair themselves. Some effects, however, can appear months or years later. These late effects depend on the area treated, the total dose received, other treatments you’ve had, and individual factors like genetics and smoking history.