Radiotherapy is a cancer treatment that uses high-energy radiation to damage the DNA inside cancer cells, making it impossible for them to grow and divide. More than 50% of all cancer patients receive radiotherapy at least once during their treatment, making it one of the most common tools in oncology. It can be used to cure cancer, shrink tumors before surgery, destroy remaining cells after surgery, or relieve symptoms when a cure isn’t possible.
How Radiation Kills Cancer Cells
Radiation works by breaking the DNA strands inside cells. A single treatment dose produces roughly 40 double-strand DNA breaks per cell, along with about 1,000 single-strand breaks and 10,000 instances of base damage. Double-strand breaks are the most lethal type. When both strands of the DNA helix snap at once, the cell often cannot repair itself and dies when it tries to divide.
Some of this damage happens directly, when radiation energy hits DNA. But a large share happens indirectly: radiation splits water molecules inside the cell into reactive fragments that then attack DNA. When the cell detects these breaks, it pauses its growth cycle and attempts repairs. If the damage is too severe, a self-destruct program kicks in. Cells that attempt to divide with unrepaired DNA undergo what’s called mitotic catastrophe, essentially falling apart mid-division.
Healthy cells near the treatment area also sustain damage, which is what causes side effects. But normal cells are generally better at repairing themselves than cancer cells, and most recover within a few months after treatment ends.
External Beam Radiation
The most common form of radiotherapy is external beam radiation, where a machine called a linear accelerator aims high-energy X-ray beams at the tumor from outside the body. Several techniques exist, each offering different levels of precision.
- 3D conformal radiation therapy uses imaging scans to build a three-dimensional model of the tumor’s exact shape and size, then aims multiple beams to match that shape while sparing nearby tissue.
- Intensity-modulated radiation therapy (IMRT) goes a step further by adjusting the strength of each beam across different parts of the treatment area. This lets doctors deliver higher doses to the tumor center and lower doses near sensitive organs.
- Image-guided radiation therapy (IGRT) takes images of the tumor immediately before and during each session. This is especially useful for cancers in areas that move with breathing, like the lungs, or tumors near the heart or kidneys.
- Volumetric modulated arc therapy (VMAT) swings the radiation beam in an arc around the patient, which significantly shortens each treatment session while maintaining precision.
All of these techniques share the same goal: concentrate radiation on the tumor and minimize what reaches everything else.
Proton Therapy
Standard external beam radiation uses X-rays (photons), which pass through the body and deposit energy along their entire path. Proton therapy uses charged particles that can be tuned to release most of their energy at a specific depth, then stop. This means less radiation hits the tissue behind the tumor.
That precision makes proton therapy particularly valuable for cancers near delicate structures: brain tumors, cancers at the base of the skull, spinal cord tumors, and eye cancers. It’s also favored for pediatric cancers because children’s developing tissues are more vulnerable to long-term radiation damage. For prostate cancer, a large study from the National Cancer Database found that proton-based treatment achieved higher 10-year overall survival than conventional photon-based treatment in certain patient groups. Proton therapy centers are less widely available and more expensive, so it’s typically reserved for situations where its precision offers a clear advantage.
Internal Radiation (Brachytherapy)
Brachytherapy places radioactive sources directly inside or next to the tumor. Because the radiation travels only a short distance, it delivers a high dose to the cancer while largely sparing surrounding tissue. It’s been used for over a century, with the first prostate brachytherapy described in 1911.
There are two main forms. Low-dose-rate brachytherapy involves permanently implanting tiny radioactive seeds into the tumor. These seeds slowly release radiation over several months as their radioactive material decays. High-dose-rate brachytherapy temporarily places a single powerful radioactive source inside applicators positioned in or near the tumor, delivering a concentrated dose in minutes before being removed. Brachytherapy is commonly used for cervical, prostate, and certain head and neck cancers.
Why Treatment Is Spread Over Weeks
Radiotherapy is almost always given in multiple small doses, called fractions, rather than one large blast. A typical course might involve daily treatments five days a week for several weeks. There are good biological reasons for this approach.
Between sessions, healthy cells have time to repair the damage they’ve sustained, while cancer cells, with their inferior repair machinery, accumulate unrecoverable damage over time. Spreading treatment out also helps with oxygen. Tumors often contain pockets of poorly oxygenated cells, which are more resistant to radiation. Between fractions, these oxygen-starved cells get a chance to reoxygenate, becoming sensitive to radiation again by the next session. This fractionation strategy improves the ratio between tumor killing and normal tissue damage.
Some newer approaches use fewer, larger doses. Stereotactic treatments deliver very high doses in one to five sessions, relying on extreme precision to protect surrounding tissue instead of relying on the repair advantage of fractionation.
What the Treatment Process Looks Like
Before your first radiation session, you’ll go through a planning appointment called a simulation. This visit typically takes one to three hours and involves no actual radiation. The care team positions you on the treatment table, sometimes creating a custom mold or mask to keep you in exactly the same position for every future session. They then take detailed imaging scans of the cancer site.
Those images go to a planning team that designs a radiation plan customized to your body, mapping out beam angles and doses to hit the tumor while avoiding nearby organs. Small marks or tiny tattoo dots may be placed on your skin to help align the beams precisely each time you come in. The planning process can take a few days to a couple of weeks before your first treatment session.
Each treatment session itself is usually quick, often 15 to 30 minutes in the room, with the actual radiation delivery lasting only a few minutes. You lie still on the table while the machine moves around you. It’s painless during delivery, similar to getting an X-ray.
Common Side Effects
Side effects depend heavily on which part of the body receives radiation. Fatigue is the most universal one, affecting many patients regardless of treatment location. It tends to build gradually over the course of treatment.
Skin in the treatment area often becomes red, dry, or irritated, similar to a sunburn. Radiation to the head and neck can cause mouth sores, dry mouth, and difficulty swallowing. Chest radiation may lead to a sore throat or cough. Pelvic radiation can cause bowel or bladder irritation. Most of these effects are temporary, with healthy cells recovering within a few months after treatment wraps up.
Late effects are less common but can appear months or years afterward. These happen when certain tissues sustain lasting damage. The specific risks depend on the treatment area and dose, and your radiation oncologist will outline the relevant ones before treatment begins. Modern techniques like IMRT and proton therapy have significantly reduced the likelihood of serious late effects by limiting radiation exposure to healthy organs.
How Radiotherapy Fits Into a Treatment Plan
Radiotherapy is rarely a standalone decision. It’s often combined with surgery, chemotherapy, or both. When given before surgery, it can shrink a tumor to make removal easier. After surgery, it can destroy microscopic cancer cells that might remain at the margins. When paired with chemotherapy, the two treatments can enhance each other’s effectiveness, though this combination also tends to intensify side effects.
For some cancers, radiotherapy alone is curative. Early-stage cervical cancer, certain head and neck cancers, and localized prostate cancer can all be treated with radiation as the primary approach. In advanced or metastatic cancer, palliative radiotherapy can relieve pain from bone metastases or reduce pressure from tumors pressing on nerves or airways, often in just a few sessions.