RT therapy, short for radiation therapy, is a cancer treatment that uses high-energy beams to damage the DNA inside cancer cells, stopping them from dividing and growing. About 50% of all cancer patients receive radiation therapy at some point during their illness, and it contributes to roughly 40% of curative cancer treatments. Whether used on its own or alongside surgery and chemotherapy, it remains one of the most common tools in cancer care.
How Radiation Therapy Works
Radiation kills cancer cells by breaking both strands of their DNA. When a cell with this kind of damage tries to divide, the process fails catastrophically. The cell essentially self-destructs during division, which is the primary way radiation destroys tumors. This also explains why radiation works best against fast-growing cancers: the more rapidly cells divide, the more opportunities radiation has to catch them in the act.
Healthy cells near the treatment area can also be affected, but they’re generally better at repairing DNA damage than cancer cells. Modern treatment planning is designed to concentrate the radiation dose on the tumor while limiting what reaches surrounding tissue.
Types of Radiation Therapy
External Beam Radiation
This is the most common form. A machine directs beams of radiation from outside the body toward the tumor. It treats many types of cancer and can target tumors almost anywhere. For pelvic cancers, for example, abdominal organs in the treatment path typically receive doses in the range of 40 to 50 Gy (the unit used to measure absorbed radiation). Organs farther from the target, like the stomach, receive much lower doses.
Brachytherapy (Internal Radiation)
In brachytherapy, a small radioactive source is placed directly inside or next to the tumor. Because radiation intensity drops sharply with distance from the implant, organs right next to the source receive high doses while those even a short distance away are largely spared. This makes it a good option for cancers of the cervix, prostate, breast, head and neck, and eye. Compared to external beam therapy for the same area, brachytherapy can deliver five to ten times less radiation to nearby organs that aren’t being targeted.
Systemic Radiation
Some radiation treatments travel through the bloodstream. Radioactive iodine (I-131) is a well-known example used for certain thyroid cancers. Other systemic approaches include targeted therapies for advanced prostate cancer and some neuroendocrine tumors, as well as treatments that combine radioactive particles with antibodies to treat lymphoma. A specialized form called radioembolization delivers radiation directly to liver tumors through the blood supply.
Curative vs. Palliative Goals
Not every course of radiation therapy aims to eliminate a cancer entirely. Curative-intent radiation uses higher total doses, often around 60 Gy delivered in about 30 sessions over several weeks. The goal is to destroy the tumor completely or prevent it from returning after surgery.
Palliative radiation uses lower doses and fewer sessions to relieve symptoms like pain, bleeding, or pressure from a tumor. It’s common in advanced cancers where the priority is comfort and quality of life rather than a cure. Research in advanced lung cancer has shown that even patients with metastatic disease can gain a survival benefit from higher-dose radiation to the primary tumor compared to palliative doses alone.
What the Treatment Process Looks Like
Radiation therapy doesn’t start the day you’re diagnosed. There’s a careful planning phase designed to make treatment as precise as possible. The typical sequence involves several steps: a CT simulation scan to map the tumor and surrounding anatomy, physician contouring (where doctors outline exactly what to target and what to avoid on those scans), dosimetric treatment planning by specialists who calculate how to shape and aim the beams, quality assurance checks, plan verification, and then treatment delivery.
The simulation appointment often involves creating a custom mold or mask to keep you in exactly the same position for every session. You may get small, permanent ink marks (tattoos the size of a freckle) on your skin to help align the machine each time. This planning process can take one to two weeks before your first actual treatment session.
Each daily treatment session is usually short. You lie on a table, the machine positions itself around you, and the radiation is delivered over a few minutes. You won’t feel anything during the beam itself. Most curative courses run five days a week for several weeks, though shorter schedules are increasingly common for certain cancers.
Common Side Effects by Treatment Area
Side effects depend almost entirely on which part of the body is being treated. They fall into two categories: early effects that appear during or shortly after treatment, and late effects that can develop months or years later.
Head and Neck
Early side effects include skin redness, mouth sores, pain, and changes in taste. Salivary glands are particularly sensitive. Swelling and tenderness can start after the first session, and dry mouth often develops as treatment continues. Long-term, dry mouth can become permanent, leading to severe tooth decay, difficulty eating and speaking, and trouble wearing dentures. Some patients develop stiffness in the jaw muscles that limits how far they can open their mouth.
Chest
Radiation to the chest can cause a condition called radiation pneumonitis in the lungs, with symptoms like cough, shortness of breath, fever, and chest pain. Over months to years, inflammation can progress to lung scarring that permanently reduces breathing capacity. The heart is also at risk: short-term effects include inflammation of the sac around the heart, while long-term exposure increases the risk of heart disease by damaging blood vessels in and around the heart.
Abdomen and Pelvis
Nausea, cramping, and diarrhea typically begin two to three weeks into treatment. Radiation to the large bowel can cause high-volume watery diarrhea. Rectal treatment often brings increased mucus and a persistent feeling of needing to use the bathroom. Chronic effects can include ongoing diarrhea, nutrient absorption problems, and recurring bowel blockages.
Brain and Spine
Fatigue, nausea, and headaches are common early effects of brain radiation. Existing neurological symptoms may temporarily worsen because radiation can cause swelling around the tumor. Long-term concerns include changes in thinking and memory, hormonal imbalances, and a small risk of secondary cancers. Spinal cord radiation carries a risk of nerve damage that can range from mild tingling to, in rare severe cases, permanent loss of function.
Proton Therapy: A More Targeted Option
Standard radiation therapy uses photon beams (high-energy X-rays), which deposit energy along their entire path through the body. Proton therapy uses a different particle that can be tuned to release most of its energy at a precise depth, then stop. This means less radiation hits healthy tissue beyond the tumor.
The clinical benefit is clearest in certain situations. For children with brain and spinal cancers, proton therapy is associated with less impact on cognitive development, lower rates of thyroid problems, and fewer blood-related side effects compared to standard radiation. For head and neck cancers, patients treated with protons experience less long-term dry mouth and are less likely to need a feeding tube. Tumors in anatomically complex areas, where critical structures sit close to the target, are often strong candidates for this approach.
Proton therapy isn’t available everywhere and costs more than conventional radiation. It doesn’t offer a clear advantage for every cancer type, so it tends to be recommended when the potential to spare healthy tissue is greatest.