Radiation therapy kills cancer cells by damaging their DNA so severely that they can no longer divide and grow. About half of all people diagnosed with cancer receive radiation at some point during treatment, either as the primary approach or alongside surgery and chemotherapy. The process is more precise than most people expect, and understanding the basic mechanics can make the experience far less intimidating.
How Radiation Damages Cancer Cells
Radiation works through ionization, a process where high-energy beams knock electrons out of atoms inside cells. This disrupts molecules the cell needs to survive, and the most critical target is DNA. When radiation hits DNA directly, it can break one or both strands of the double helix. A single-strand break is something a cell can usually patch up. Double-strand breaks are far harder to repair and often prove fatal to the cell.
There’s also an indirect route. Since your body is mostly water, radiation frequently hits water molecules first and splits them into highly reactive fragments called free radicals. These unstable molecules travel short distances and slam into nearby DNA, causing the same kinds of damage. In practice, both direct and indirect damage happen simultaneously during every treatment session.
Once a cancer cell accumulates enough DNA damage, it either self-destructs through the body’s built-in cell death programs or attempts to divide and fails. A cell that can’t copy its DNA correctly will die during division, sometimes not until it tries to split one or two more times. This is why tumors don’t vanish the day after treatment. They shrink gradually over weeks as damaged cells attempt division and fail.
Why Treatment Is Spread Over Multiple Sessions
Most radiation therapy is delivered in small daily doses over several weeks rather than one large blast. This approach, called fractionation, exploits four biological differences between cancer cells and healthy tissue.
Repair: Normal cells are generally better at fixing DNA damage between sessions than cancer cells are. Giving small doses with overnight gaps lets healthy tissue recover while tumor cells accumulate lethal damage they can’t keep up with. Slowly responding tissues like nerve and muscle have an especially strong repair capacity compared to most tumors.
Reoxygenation: Tumors often grow faster than their blood supply, leaving pockets of oxygen-starved cells deep inside. This matters because cells without adequate oxygen are two to three times more resistant to radiation. As treatment kills the outer, well-oxygenated layer of tumor cells, the inner oxygen-deprived cells move closer to blood vessels, become better oxygenated, and grow more vulnerable to the next dose. Even a small population of oxygen-starved cells can prevent a cure if they’re never exposed to adequate oxygen during treatment.
Redistribution: Cells are more sensitive to radiation at certain points in their growth cycle and more resistant at others. After a dose wipes out the sensitive cells, the survivors tend to be clustered in resistant phases. Given time, those remaining cells move through their cycle at different rates and re-enter sensitive phases before the next dose arrives.
Repopulation: Both normal and cancer cells recruit resting cells to replace those that were killed. For fast-turnover tissues like the lining of your mouth and gut, this repopulation is what allows those tissues to tolerate weeks of treatment without permanent damage. The tradeoff is that tumor cells repopulate too, which is one reason treatments are scheduled without unnecessary delays.
External Beam Radiation
The most common form of radiation therapy uses a machine called a linear accelerator that aims high-energy X-ray beams at the tumor from outside your body. You lie on a table, the machine rotates around you, and treatment typically takes 10 to 30 minutes per session, though the radiation itself is on for only a few minutes. You don’t feel anything during the beam. It’s similar to getting an X-ray, just longer.
Modern external beam treatments use sophisticated shaping techniques to concentrate the dose on the tumor and spare surrounding organs. Intensity-modulated radiation therapy (IMRT) adjusts the strength of the beam across its cross-section, creating a dose that conforms tightly to irregular tumor shapes. Volumetric modulated arc therapy (VMAT) takes this further by continuously adjusting the beam’s intensity, rotation speed, and shape as the machine sweeps in an arc around you. Both approaches allow oncologists to deliver a high dose to the target while keeping nearby healthy tissue below harmful thresholds.
Stereotactic body radiation therapy (SBRT) delivers very large, precise doses in just one to five sessions instead of the typical 25 to 30. It uses extremely tight targeting, sometimes accurate to within one to two millimeters, and is most often used for small, well-defined tumors in the lung, spine, liver, or brain. Because each dose is so large, the margin for error is smaller, and imaging verification happens before and sometimes during every session.
Internal Radiation (Brachytherapy)
Instead of aiming beams from outside, brachytherapy places a radioactive source directly inside or next to the tumor. This lets doctors deliver a very high dose to a small area while the radiation drops off sharply just a few centimeters away, protecting surrounding tissue.
The radioactive material comes in small seeds, pellets, wires, or capsules. For cervical cancer, a device is temporarily placed inside the uterus and delivers the dose over minutes to hours before being removed. For prostate cancer, tiny radioactive seeds about the size of a grain of rice are permanently implanted directly into the gland, where they emit radiation over weeks to months before becoming inert. Head and neck cancers are also commonly treated with brachytherapy. High-dose-rate treatments use powerful sources like iridium-192 or cobalt-60 that are inserted briefly through thin tubes and then withdrawn, often requiring just a few sessions.
How Proton Therapy Differs
Standard radiation uses X-ray beams (photons) that deposit energy along their entire path through the body. They deliver dose as they enter, pass through the tumor, and continue out the other side, exposing healthy tissue both in front of and behind the target.
Proton therapy uses charged particles instead of photons, and their physics are fundamentally different. As a proton slows down inside tissue, it deposits more and more energy. Right before it stops, energy release spikes dramatically in what’s known as the Bragg peak. Physicists at Brookhaven National Laboratory describe this peak as the feature that makes ion therapy advantageous over X-ray treatment: by tuning the proton’s initial speed, doctors can place that energy spike directly inside the tumor. Beyond that point, there is essentially no exit dose.
This makes proton therapy especially valuable for tumors near critical structures like the brain, spinal cord, or eyes, and for treating children, whose developing tissues are more vulnerable to stray radiation. The tradeoff is cost and availability. Proton facilities require massive particle accelerators and are far less common than standard linear accelerators.
What Side Effects Feel Like
Radiation side effects are mostly local, meaning they show up in the area being treated. If you’re receiving radiation to the chest, you might develop irritation in the esophagus that makes swallowing uncomfortable. Pelvic radiation can cause bowel changes. Head and neck treatment often leads to dry mouth or sore throat. Skin in the treated area frequently becomes red and tender, similar to a sunburn, usually peaking in the final weeks of treatment.
Fatigue is the one side effect that’s nearly universal regardless of where the beam is aimed. It tends to build gradually over the course of treatment and can linger for weeks afterward. Most acute side effects improve within a month or two after the last session as the fast-turnover tissues in the treated area repopulate and heal.
Late side effects, those appearing months or years later, are less common but depend on the total dose and which organs were in the treatment field. This is precisely why so much engineering goes into beam shaping and fractionation: every improvement in precision translates directly into fewer long-term complications for the tissues that didn’t need to be treated.