Does the machine touch me during external beam radiation?

No. The machine never touches you during external beam radiation therapy, and the treatment itself does not hurt.

The machine is large and often noisy, and it rotates around the table so it can aim at the tumor from several directions. It stays a short distance away the whole time. The National Cancer Institute puts it plainly: you will hear the machine and see it moving, but you will not be able to feel, hear, see, or smell the radiation.

What is happening in those few minutes

The beam is on for only 1 to 5 minutes. Most of the visit is spent lining you up. NCI says a typical appointment runs 30 minutes to an hour, and most of that hour is positioning.

You lie on a treatment table or sit in a chair, depending on where the cancer is. Expect the room to be very cool. Colored lights may shine on your skin. Those are alignment guides and are harmless.

The therapist steps out just before the beam starts and runs the machine from the next room. You are not abandoned there. The therapist watches you on a screen or through a window and talks to you through a speaker, and can stop the treatment at any time. You can breathe normally the whole time.

Does any part of it hurt?

Not the beam. NCI says you cannot feel, hear, see, or smell the radiation itself.

What can hurt comes later and comes from the tissue, not the machine. Radiation damages the DNA of cells in the treated area, and healthy cells nearby take some of that damage. Skin in the treatment area can dry out, peel, itch, or turn red or darker, and it may look sunburned. In some people it gets painful sores that can become infected. Radiation to the chest, head or neck can cause throat problems such as trouble swallowing. Radiation to the pelvis can cause diarrhea or bladder problems. Fatigue is common, and NCI says it can come on all at once or slowly.

These effects show up gradually because the biology is gradual. NCI notes that radiation does not kill cancer cells right away; it takes days or weeks of treatment, and cells keep dying for weeks or months after the course ends.

Call your care team right away if skin in the treatment area blisters or breaks open, if you cannot swallow liquids, if you run a fever of 100.4 °F (38 °C) or higher, or if you have new numbness or weakness. Those are not things to save for your next weekly check. NCI's own fever figure for mouth and throat problems is 100.5 °F (38 °C); use the lower number. If you are also getting chemotherapy, the CDC says to call your doctor immediately at 100.4 °F.

The things that do touch you

Not the machine, but a few other items will.

Small ink dots, about the size of a freckle, are placed on your skin to mark the treatment area, so you can be positioned the same way each day. If they are tattooed, they are permanent. If they are drawn in ink, they can fade. NCI says to be careful not to remove them and to tell the therapist if they fade.

A body mold made of plastic or plaster may be made to hold the treated part still. For head and neck treatment, a mesh mask is fitted to your face and clipped to the table. It has many air holes. If enclosed spaces bother you, tell your team before the fitting.

One more thing worth knowing: external beam radiation does not make you radioactive. You can be around other people, including pregnant women, babies, and young children, on the same day you are treated.

Sources

https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy

https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy/external-beam

https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy/side-effects

https://www.cancer.gov/about-cancer/treatment/side-effects/skin-nail-changes

https://www.cancer.gov/about-cancer/treatment/side-effects/mouth-throat

https://www.cdc.gov/cancer-preventing-infections/patients/fever.html