Cancer is treated with several types of radiation, but the most common by far is external beam radiation using high-energy X-rays (photons). Other options include proton beams, internal radiation placed directly inside the body, and radioactive drugs that travel through the bloodstream. The type your oncologist recommends depends on the cancer’s location, size, and stage.
External Beam Radiation: The Most Common Approach
External beam radiation therapy accounts for the majority of radiation treatments. A machine called a linear accelerator aims beams of high-energy photons (essentially powerful X-rays) at the tumor from outside your body. These beams pass through the tumor and continue out the other side, which means some healthy tissue before and beyond the cancer also receives a dose. Treatment is painless and feels similar to getting an X-ray, though each session lasts longer.
Total doses vary by cancer type. Head and neck cancers typically receive 66 to 70 Gray (the standard unit of radiation dose) spread over several weeks of daily sessions. Brain tumors may receive around 50 to 60 Gray. Esophageal cancers generally fall in the 60 to 68 Gray range. Breaking the total dose into many smaller daily fractions gives healthy cells time to repair between sessions while still accumulating enough damage to destroy cancer cells.
Proton Therapy: Stopping at the Tumor
Proton therapy uses beams of proton particles instead of photons. The critical difference is physical: a proton beam deposits most of its energy at a precise depth and then stops, rather than passing through the body. This means tissue behind the tumor gets little to no radiation. That property makes proton therapy especially valuable for cancers near sensitive structures, such as tumors at the base of the skull, in the spine, or in children whose growing tissues are more vulnerable to radiation damage.
Proton therapy is not necessarily more effective at killing cancer cells than photon therapy. Its advantage is reducing collateral damage. The trade-off is availability and cost. Proton centers require massive, expensive equipment, so they exist at far fewer hospitals than standard radiation facilities.
Carbon Ion Therapy
Carbon ion beams share the same stopping-at-depth advantage as protons but pack a stronger biological punch. Carbon ions cause more severe DNA damage to cancer cells per unit of dose, making them particularly promising for tumors that resist conventional radiation, including certain oxygen-starved (hypoxic) tumors deep in the body. Only a handful of centers worldwide offer carbon ion therapy, mostly in Japan and Europe, and research is still clarifying which cancers benefit most.
Stereotactic Radiation: Precision in Fewer Sessions
Stereotactic radiation is not a different type of energy. It’s a way of delivering photon or proton beams with extreme precision. For tumors in the brain, it’s called stereotactic radiosurgery. For tumors elsewhere in the body, it’s called stereotactic body radiation therapy, or SBRT.
The key feature is tight margins. In lung SBRT, for example, the treatment area typically extends only about 5 millimeters beyond the visible tumor. Advanced imaging and motion-tracking technology keep the beam locked on target even as you breathe. Because the dose is so concentrated, SBRT is often completed in just three to five sessions rather than the five to seven weeks typical of conventional radiation. It works best for small, well-defined tumors, particularly early-stage lung cancers and certain liver or spine tumors.
Internal Radiation (Brachytherapy)
Brachytherapy places a radioactive source inside or right next to the tumor. This delivers a high dose to the cancer while sharply limiting exposure to surrounding tissue, since radiation intensity drops off rapidly with distance. The radioactive material used (iodine, palladium, cesium, or iridium) depends on the type and location of the cancer.
There are two broad approaches. Permanent brachytherapy involves implanting tiny radioactive seeds, about the size of a grain of rice, directly into the tumor. This is common for early-stage prostate cancer. The seeds emit radiation over weeks or months and then become inactive, staying harmlessly in place. Temporary brachytherapy uses a stronger source placed through a catheter or applicator for minutes to hours, then removed. It’s widely used for cervical cancer, where an applicator delivers radiation directly to the cervix during a series of treatment sessions.
Systemic Radiation: Radioactive Drugs
Systemic radiation therapy uses radioactive substances, called radiopharmaceuticals, that you swallow or receive through an IV. These drugs travel through the bloodstream and seek out cancer cells based on specific biological markers, delivering radiation from the inside.
Radioactive iodine is the oldest and best-known example. Thyroid cells naturally absorb iodine, so a radioactive form concentrates in thyroid tissue and destroys it. This has been a standard treatment for thyroid cancer for decades. More recently, newer radiopharmaceuticals have expanded this approach to other cancers. One targets a protein on prostate cancer cells to treat advanced prostate cancer that hasn’t responded to other treatments. Another locks onto receptors found on neuroendocrine tumors, including rare pancreatic and carcinoid tumors. A third uses a bone-seeking element, radium, to treat prostate cancer that has spread to the bones. Each drug is designed to concentrate where the cancer is while largely sparing the rest of the body.
Radiation During Surgery
Intraoperative radiation therapy, or IORT, delivers a single large dose of radiation directly to the tumor bed while you’re still in the operating room. Once the surgeon has removed as much cancer as possible, the radiation is aimed at the exposed area where microscopic cancer cells might remain. This is particularly useful for cancers in the pelvis or abdomen, where tumors sit close to sensitive organs, and for cancers that have recurred after prior treatment. Many patients who receive IORT also get conventional external beam radiation before or after surgery.
Side Effects Depend on Location, Not Just Type
Regardless of the radiation type, side effects are driven primarily by which part of the body is treated and how much radiation is used. Some people experience very few side effects, while others have more.
Certain effects are common across all treatment sites: skin irritation in the treated area (similar to a sunburn), fatigue, and hair loss at the treatment site, which is sometimes permanent. Beyond those, effects are location-specific. Radiation to the head and neck often causes dry mouth, difficulty swallowing, changes in taste, and mouth sores. Chest radiation can lead to cough and shortness of breath. Abdominal treatment commonly causes nausea, vomiting, and diarrhea. Pelvic radiation frequently triggers bladder irritation, frequent urination, diarrhea, and sexual dysfunction.
Most of these side effects develop during or shortly after treatment and gradually improve. Late side effects can appear months or years later. In rare cases, radiation treatment itself can contribute to a new, unrelated cancer developing years or even decades down the line, though this risk is small relative to the benefit of treating the original cancer.