External Radiation 101: A Guide to Your Treatment

External Radiation 101: A Guide to Your Treatment

Reviewed by Jasper Clinical Board

Last updated 5/24/21

What is external radiation therapy?

External radiation therapy is a medical treatment that kills cancer cells by shooting them with a high-energy beam of radiation. Doctors use many different types of radiation for this purpose (such as X-rays, gamma rays, and proton beams), but they all accomplish the same goal: zapping cancer cells so they die and your tumor shrinks.
The radiation you receive is shot through your body by a large, high-tech medical device (usually a linear accelerator) that’s positioned near the tumor. That’s why the treatment is called external radiation therapy. (With internal radiation therapy, on the other hand, a radioactive implant stays in your body to treat the tumor gradually.) Generally speaking, the procedure itself takes just 15 minutes or so.
It’s important to note that external radiation therapy does not make you radioactive. The radiation passes through your body (like a powerful light), and then it’s gone. You don’t have to worry about the radiation affecting anyone else’s health.

What is the procedure like?

When you check in for your radiation therapy appointment, you’ll be taken to a special room for treatment. Your healthcare team will have you either sit or lie down, then position the machine over the part of your body that’s being treated. If you’ve ever gotten an X-ray or CT scan, this part will feel familiar.
The technicians and nurses will then leave the room (so they’re not exposed to radiation) and start the machine. You won’t feel anything unusual, but you may hear clicking, buzzing, knocking, or whirring noises as the machine operates. You can typically breathe normally during treatment - though in some cases (such as for breast cancer treatment), your team may ask you to hold your breath (rest assured, they’ll walk you through exactly what to do).
It’s important that you remain very still while you’re being treated. Radiation can damage healthy cells as well as cancerous ones, so the entire procedure is designed to precisely target the problem areas (i.e. down to the millimeter). Your healthcare team may use an immobilization device to help you stay still, such as a mold, tape, face masks, or plaster cast. These devices aren’t always fun to wear, but research shows that they’re very important for your health outcomes.

What are the first steps?

Before your radiation therapy begins, you’ll meet with a radiation oncologist (here’s a list of questions you may want to ask). This is the doctor who will oversee your entire radiation treatment experience. They’ll assess your overall physical health, and review your scans (typically CT, MRI, or PET scan images that show the size and location of your cancer). Then they’ll come up with a treatment plan.
Let your oncologist know if you’ve been exposed to radiation in other parts of your life—such as military service, past treatments, or your occupation. The goal is to limit your lifetime exposure to radiation, so your doctor will take these factors into account while planning your treatment course. You should also let them know if you have metal implants (such as a pacemaker or cochlear implant), as these devices can be affected by the radiation.
Your next step is a special, one-time session called a simulation appointment (aka SIM). This is a bit like a dress rehearsal for the coming treatments. You’ll go to the hospital or treatment center, meet with the team, and get measured for an immobilization device if one is necessary. They may also take additional scans and mark your body with temporary ink (or even small, permanent tattoos) to help guide the radiation beam. You’ll practice lying under the machine, and your team will make sure it’s a comfortable position for you to stay in over the coming weeks. The actual radiation therapy will begin with your next appointment.
You can learn more here about what happens during a simulation session.

How to prepare for treatment

Radiation therapy can be challenging, both emotionally and physically, with fatigue being the most common side effect (more on that shortly). That’s why it’s helpful to begin gathering resources beforehand (in addition to the below, here’s more on what might be useful to wear and bring).
Are friends and family offering to help? Don’t be afraid to take them up on it! Ask loved ones for assistance traveling to and from appointments, cooking and cleaning, caring for pets, and other daily responsibilities. Your hospital or treatment center may also have social workers who can arrange this sort of assistance, so check with your healthcare team. Some people stop working entirely during treatment, while others reduce their working hours or make special arrangements (e.g. working from home; note that it’s 100% within your rights to ask for this). 
Finally, you may want to begin exploring meditation, breathing exercises, yoga, and other wellness practices. Many facilities will have an outlet nearby and allow you to listen to your own music, guided meditations, or audiobooks on your phone (bring headphones). In addition, these relaxation techniques may help you stay calm and patient during long treatment sessions (when some patients feel restless or claustrophobic)—and there’s research suggesting many of them may be helpful for your overall health outcomes, too.

What are the side effects?

Radiation treatment itself is painless, but almost everyone experiences side effects of one kind or another. The most common is fatigue, which gets more likely as your treatment goes on. The other very common side effect is skin damage around the treated area. Your skin may get dark or red, and be painful to the touch like a sunburn. Hair loss in the immediate area—both temporary and permanent—is also a possibility.
Other side effects depend largely on the part of your body being treated. For head and neck treatments, dry mouth, sore throat, nausea, tooth decay, and changes in taste are common. Chest treatments may cause difficulty swallowing, a persistent cough, and shortness of breath. Lower abdomen treatments are linked to nausea, vomiting, and diarrhea. And the side effects of pelvis-area treatments include diarrhea, frequent urination, and sexual difficulties.
Like we mentioned above, you may feel claustrophobic during treatment (and staying still in a strange position can be challenging as well). Bringing music or guided meditations to listen to on your phone can sometimes help, but also let your oncologist know upfront if you’re concerned about this. They may be able to prescribe an anti-anxiety medication that will help you manage the experience.

Staying healthy during treatment

During this challenging time, common-sense wellness practices are your best friend. Sleep as much as you need to, get moderate exercise when possible, drink lots of liquids, and eat as healthfully as you can. (If you’re having trouble chewing or swallowing, your care team may be able to connect you with a registered dietitian.) And as we mentioned above, wellness practices like yoga and meditation can also be very helpful.
But far and away, the most important thing you can do during external radiation therapy treatment is to work closely with your healthcare team. Listen to any instructions they give you, share your concerns, and never hesitate to ask questions!
During treatment, you’ll meet regularly with your radiation oncologist (usually once a week). They’ll assess your progress and make any necessary adjustments. This is a perfect time to report any unusual sensations or symptoms you’re experiencing. Even something that seems small (like dry mouth) can have major health implications. So please, don’t try to “tough out” a health issue or write it off as silly. It’s not.

The content on this website is intended to provide the best possible information for you, but should not be considered—or used as a substitute for—medical advice. If you have questions about your diagnosis or treatment, please contact your health care provider(s). For questions or comments about this content, please email us at