Radiation therapy uses high-energy beams or radioactive materials to destroy cancer cells, and the process varies depending on whether the radiation comes from a machine outside your body, a source placed inside your body, or a radioactive drug that travels through your bloodstream. The most common form, external beam radiation, involves lying on a table while a machine directs precise beams at the tumor. The actual radiation lasts only one to five minutes per session, though each visit takes 30 minutes to an hour because most of that time is spent getting you into the exact right position.
The Three Main Types
External beam radiation is by far the most widely used. A machine called a linear accelerator rotates around you, delivering beams from multiple angles. Internal radiation, sometimes called brachytherapy, places a small radioactive source directly inside or next to the tumor. Systemic radiation uses a radioactive drug given as a pill or through an IV that travels through your blood to reach cancer cells wherever they are. Each type follows a different process, but all three begin with careful planning to make sure radiation hits the tumor and spares as much healthy tissue as possible.
Planning and Simulation
Before your first external beam treatment, you’ll go through a planning session called a simulation. This is where the radiation team maps out exactly where your treatment will be aimed. You lie very still on a table while imaging scans are taken of the area being treated. The team uses these scans to identify the precise shape and location of the tumor.
To make sure you’re in the identical position for every future session, the radiation therapist will place tiny tattoo dots or ink marks on your skin. These serve as alignment guides throughout your entire treatment course. Depending on where the cancer is, you may also be fitted with a device that keeps you from moving. For head and neck cancers, this is typically a mesh mask with air holes that clips to the treatment table. For other body areas, a custom mold is made: you lie on a bag filled with foam or small beads, and it hardens around the contours of your body so you fit into the same position every time. This planning session can take an hour or more, but it only happens once.
What a Treatment Session Looks Like
On treatment days, you change into a hospital gown and go to a room that’s noticeably cool. You lie on the treatment table (or sit in a special chair), and the therapist uses your skin marks plus any mold or mask to position you precisely. You may notice colored lights projected onto your skin. These are harmless alignment tools.
Once you’re positioned, the therapist leaves the room and goes to a control area nearby. They watch you on a monitor or through a window the entire time and can talk to you through a speaker. The linear accelerator’s gantry rotates around you, and a component called a multileaf collimator shapes the beam to match the contour of your tumor. You won’t feel the radiation itself. Some patients report hearing a buzzing or clicking sound from the machine, and a small number notice an unusual smell during the session, though the reason for this isn’t fully understood.
The radiation beam is on for only one to five minutes. You breathe normally the whole time. Most of the visit is setup, not treatment. The typical schedule is five days a week for several weeks, though this varies by cancer type and treatment plan.
How the Machine Stays Accurate
Radiation doses are tightly controlled. Every morning before patients arrive, staff run safety checks on the machine’s dose calibration and positioning features. The treatment settings, including the dose and beam shape designed during your planning session, transfer automatically from the planning computer to the linear accelerator. This eliminates the risk of manual entry errors. A record-and-verify system then cross-checks the machine’s programming against your approved treatment plan. If anything doesn’t match, the machine will not deliver the treatment. Each session’s delivery is also logged so the care team can track your cumulative dose.
How Brachytherapy Works
Brachytherapy places radioactive material directly inside or beside the tumor, which lets it deliver a concentrated dose to a small area while limiting exposure to surrounding tissue. There are two main approaches.
High-dose-rate brachytherapy is temporary. The radiation team inserts a delivery device into the treatment area. For vaginal cancers, this might be a tube or cylinder shaped to fit the body opening. For breast or prostate cancers, it could be thin needles or applicators placed directly into tissue. The device connects to a machine that feeds in a small radioactive source. It stays in place for just a few minutes, delivering a strong dose, and then it’s removed. You may need several sessions spread over days or weeks.
Low-dose-rate brachytherapy is permanent. Under anesthesia, the radiation oncologist uses thin, hollow needles to implant tiny radioactive seeds, each about the size of a grain of rice, directly into or near the cancer. Imaging like ultrasound or CT helps confirm the seeds are positioned correctly. The seeds emit a low level of radiation over several months until the radioactive material is spent. They stay in your body permanently but become inactive and harmless.
Systemic Radiation
Some cancers are treated with radioactive drugs that circulate through your entire body. Radioactive iodine for thyroid cancer, for example, comes as a liquid or capsule you swallow. Other systemic treatments are given through an IV. You might feel a pinch from the needle, but you typically won’t feel the drug entering your body.
Because you temporarily carry radioactive material inside you, your care team will give you isolation instructions for the first few days. These usually include sleeping alone, avoiding close contact with children and anyone who is pregnant, using a separate toilet when possible and flushing twice, and avoiding sexual contact. Your team will tell you how long these precautions need to last and when it’s safe to resume normal routines, including when birth control is no longer needed.
What Side Effects Feel Like
External beam radiation causes side effects gradually. Skin changes at the treatment site are among the most common. Many people don’t notice anything until a few weeks into treatment, roughly a quarter of the way through their course. The skin may become red, dry, or tender, similar to a sunburn. In some cases, skin reactions don’t appear until after the final session.
Fatigue is also common and tends to build over the course of treatment. Other side effects depend on where the radiation is aimed. Head and neck radiation can alter your sense of taste or smell, sometimes causing a persistent metallic taste. Radiation to the abdomen may cause nausea or digestive changes. These effects are generally limited to the area being treated, which is one advantage radiation has over treatments that affect the whole body.
Brachytherapy and systemic radiation carry their own side effect profiles, but the general pattern is similar: effects are most noticeable during and shortly after treatment, and most improve in the weeks that follow once the radiation exposure ends.