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, because the equipment needs it. 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 machine instantly if you say you feel sick or need to move. You can breathe normally the whole time.

Does any part of it hurt?

Not the beam. Radiation has no sensation, the way an x-ray of a broken bone has none.

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 field can turn red, dry, itchy, or sore, much like a sunburn. Radiation to the chest or neck can inflame the swallowing tube, a problem called esophagitis, which makes swallowing painful. Radiation to the pelvis can cause diarrhea or bladder irritation. Fatigue builds over the weeks.

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.

Tell your team the same day 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.

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 at the planning session so you can be positioned identically every day. If they are tattooed, they are permanent. If they are drawn in ink, they fade, and you should tell the therapist rather than redraw them yourself.

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. It is snug, and people who dislike enclosed spaces should say so before the fitting, because there are ways to help.

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.cdc.gov/cancer-preventing-infections/patients/fever.html