Radiation therapy uses high-energy beams to destroy cancer cells by damaging their DNA beyond repair. Roughly half of all cancer patients receive radiation therapy at some point during treatment, making it one of the most common tools in oncology. It can be the primary treatment, a follow-up after surgery to kill remaining cells, or a way to shrink tumors and relieve symptoms when a cure isn’t the goal.
How Radiation Kills Cancer Cells
DNA is the principal target. When ionizing radiation hits a cell, it creates breaks in the DNA strands, particularly double-strand breaks, where both sides of the DNA ladder snap. These breaks are largely responsible for killing the cell. A single radiation session can cause simple breaks, complex breaks near other damage, and breaks triggered when the cell tries to copy its damaged DNA during division.
Healthy cells and cancer cells both sustain this damage, but healthy cells are generally better at repairing themselves. Cancer cells divide rapidly and often have faulty repair machinery, so they’re more vulnerable. After radiation damages their DNA, cancer cells hit internal checkpoints that stall their growth cycle. If the damage is too severe to fix, the cells either stop dividing permanently or die. This is why radiation is delivered in multiple sessions: each round catches more cancer cells in the act of dividing, while giving normal tissue time to recover between treatments.
External Beam Radiation
The most common form of radiation therapy directs beams from a machine outside your body toward the tumor. Several techniques exist, each designed to focus the dose more precisely on the cancer and away from surrounding tissue.
Intensity-modulated radiation therapy (IMRT) uses many small beams whose strength can be individually adjusted. This lets the treatment team deliver higher doses to certain parts of the tumor while lowering exposure to nearby organs.
Image-guided radiation therapy (IGRT) builds on IMRT by taking imaging scans not just during planning but during each treatment session. Repeated CT, MRI, or PET scans detect changes in the tumor’s size and position, so the beams can be adjusted in real time.
Stereotactic body radiation therapy (SBRT) targets small, isolated tumors outside the brain and spinal cord, often in the liver or lung. It delivers very precise, high-dose radiation in fewer sessions. Special equipment holds you completely still during treatment to ensure accuracy.
Internal Radiation (Brachytherapy)
Brachytherapy places a radiation source directly inside or next to the tumor. It’s often used for cancers of the head and neck, breast, cervix, prostate, and eye. Because the radiation travels only a short distance, it can deliver a strong dose to the tumor while sparing more of the surrounding tissue than external beams typically can.
There are three forms. Low-dose rate implants stay in place for one to seven days, and you’ll typically remain in the hospital during that time. High-dose rate implants are inserted for just 10 to 20 minutes per session, with treatments happening twice daily for two to five days or once weekly for two to five weeks. Permanent implants stay in your body for life, but the radiation weakens steadily each day until it’s essentially gone.
Proton Therapy
Standard radiation uses X-ray beams (photons) that pass through the body, depositing energy along their entire path, including behind the tumor. Proton therapy works differently. Protons release most of their energy at a specific depth, a phenomenon called the Bragg peak, then stop. There’s a sharp spike in the delivered dose followed by a rapid falloff, which means very little radiation exits out the other side. This allows treatment plans that deliver lower doses to normal tissue than photon-based methods can achieve.
The catch is that the Bragg peak must be placed precisely within the tumor to take advantage of this effect. Proton therapy is particularly useful for tumors near sensitive structures like the brain, spinal cord, or eyes, and for pediatric cancers where minimizing long-term radiation exposure matters most.
What the Treatment Process Looks Like
Before your first session, you’ll go through a planning step called simulation, which takes about an hour. The team measures your body, positions you exactly as you’ll be positioned for every future session, and takes a CT scan of the treatment area. In some cases, an MRI or PET scan is done the same day. These images become the map your treatment team uses to aim the beams.
You may need preparation beforehand: a specific diet, an enema, certain medications, or a full bladder depending on the area being treated. Immobilization devices like molds, casts, headrests, or knee supports keep you comfortable and still. The team marks your skin with a marker and often places small permanent tattoos so they can line you up in the exact same position every day.
Treatment sessions themselves are typically quick, often 15 to 30 minutes including setup, with the actual radiation lasting only a few minutes. You won’t feel the radiation while it’s being delivered. Traditional treatment schedules ran five to seven weeks of daily sessions, but a shift toward hypofractionation (fewer sessions with higher doses per session) has shortened many courses to three to four weeks. For breast cancer, some regimens now use as few as five sessions total.
Side Effects During and After Treatment
Side effects fall into two categories with very different timelines. Acute effects appear within a few weeks of starting treatment and primarily hit fast-dividing cells like skin and the lining of your mouth or throat. Depending on where you’re being treated, you might experience skin redness or irritation similar to a sunburn, fatigue, or soreness in the treated area. These generally fade after treatment ends.
Late effects are a different concern. They emerge months to years after treatment and tend to involve slower-turnover tissues like the heart, kidneys, liver, muscle, and bone. Late effects can include stiffness or scarring in the treated area, changes in organ function, or, rarely, a second cancer caused by the radiation itself. If acute side effects go untreated and persist, they can become what’s called consequential late effects, essentially early damage that never fully heals.
The location of the tumor largely determines which side effects you’ll experience. Radiation to the head and neck often causes dry mouth or difficulty swallowing. Radiation to the pelvis can affect bowel or bladder function. Your treatment team will outline the specific risks for your situation before you begin.
How Safety Is Built Into Treatment
Every treatment plan is designed around a principle called ALARA: keeping radiation exposure as low as reasonably achievable while still being effective. The CT-based planning maps described earlier let the team shape the beams to conform tightly to the tumor. The dose is verified before and during treatment, and imaging during sessions (as in IGRT) catches any shifts in tumor position.
For staff who work around radiation daily, protections include lead-lined shields that reduce doses by a factor of 10, leaded aprons that block 95% to 99.5% of scattered radiation, thyroid shields, and leaded eyeglasses that cut lens exposure by 90%. Staff also wear dosimeters that track cumulative radiation exposure, which the facility’s safety department reviews regularly. These precautions ensure that the people delivering your treatment aren’t accumulating harmful doses themselves.
Shorter Treatment Schedules
One of the biggest shifts in radiation therapy over the past two decades is the move toward fewer, more potent sessions. For breast cancer, the standard of care has shifted from 25 to 28 sessions over five to six weeks down to regimens lasting three to four weeks, with some patients now completing treatment in just five sessions. The total radiation dose is lower, but each individual dose is higher, and studies have shown equivalent cancer control with comparable side effects.
This matters practically. Fewer trips to the treatment center means less disruption to work, childcare, and daily life. It also frees up treatment machines for other patients, which can reduce wait times. Hypofractionation is now standard for several cancer types beyond breast, including prostate and lung cancers, and the trend toward shorter courses continues to expand.