Radiation treats cancer by damaging the DNA inside tumor cells so severely that they can no longer divide and eventually die. About 80% of that damage happens indirectly: the radiation beam strikes water molecules in the cell, generating highly reactive fragments called free radicals that then attack DNA strands. The remaining 20% comes from direct hits, where the radiation energy breaks the DNA structure on contact. When both strands of the DNA double helix snap at the same spot, the cell usually cannot repair itself and is destroyed.
How Radiation Destroys Cancer Cells
Every cell in your body has built-in repair systems that fix minor DNA damage throughout the day. Radiation therapy overwhelms those systems by inflicting a concentrated burst of damage, particularly double-strand breaks, where both rails of the DNA ladder are severed. A single break is often repairable, but when multiple breaks cluster within a small stretch of DNA, the repair machinery fails. The cell either self-destructs through a programmed process or stalls permanently and stops growing.
Cancer cells are especially vulnerable for two reasons. First, most cancer cells divide faster than normal cells, and cells that are actively dividing (specifically in the phase right before and during division) are the most sensitive to radiation. Cells in their resting or DNA-copying phases are harder to kill. Second, many cancers have defective DNA repair pathways to begin with, so they’re less equipped to bounce back from radiation damage than healthy tissue is.
Why Treatment Is Spread Over Weeks
A typical course of radiation therapy runs five days a week for several weeks, with each session lasting roughly 10 to 30 minutes. Some shorter courses wrap up in one to two weeks. This scheduling isn’t arbitrary. It’s built on four biological principles that radiation oncologists have relied on for decades: repair, redistribution, repopulation, and reoxygenation.
Between sessions, healthy cells repair sublethal DNA damage more efficiently than cancer cells, which widens the gap between tumor destruction and collateral harm. Splitting the dose also catches cancer cells at different stages of their life cycle from session to session, since cells that were in a resistant phase on Monday may be in a vulnerable phase by Wednesday. Meanwhile, healthy tissue repopulates its lost cells between treatments. And tumors that have oxygen-starved cores (which resist radiation) gradually reoxygenate as outer layers of the tumor die off, making surviving cells more susceptible to the next dose.
The Therapeutic Ratio
The entire strategy of radiation therapy hinges on a concept called the therapeutic ratio: the gap between the dose that controls the tumor and the dose that causes unacceptable harm to surrounding tissue. Both tumor cells and normal cells respond to increasing radiation in an S-shaped curve, but ideally the tumor’s curve sits to the left, meaning it responds at a lower dose. When these curves overlap too much, treatment becomes risky and less effective. Every technique in modern radiation therapy, from beam shaping to dose scheduling, exists to push that ratio in the patient’s favor.
Total doses vary depending on the cancer. Head and neck cancers treated with curative intent typically receive 66 to 70 Gy (the unit used to measure absorbed radiation dose), delivered in daily fractions of about 1.8 to 2.0 Gy over six to seven weeks. Brain tumors may receive 50 to 60 Gy. Palliative courses aimed at relieving symptoms use substantially lower totals over fewer sessions.
External Beam Radiation
The most common form of radiation therapy fires high-energy beams from a machine outside the body. Several modern techniques make these beams increasingly precise.
Intensity-modulated radiation therapy (IMRT) aims many small beams at the tumor from multiple angles, with each beam’s strength adjusted so higher doses hit the densest parts of the tumor while lower doses reach its edges. This sculpts the radiation field to match the tumor’s shape. Image-guided radiation therapy (IGRT) builds on IMRT by taking imaging scans during the actual treatment session, not just during planning. If the tumor has shifted or changed size, the machine adjusts the beam in real time.
Stereotactic radiosurgery delivers a focused, high-energy dose to small, well-defined tumors in the brain or central nervous system. Despite the name, there’s no scalpel involved. Dozens of tiny beams converge from different directions so that each beam passes harmlessly through normal tissue, but the point where they all meet receives an intense dose. Treatment is often completed in a single session, or at most five.
Proton Therapy
Standard radiation uses photon beams (X-rays), which deposit energy along their entire path through the body, entering on one side and exiting the other. Proton beams behave differently. They release most of their energy at a specific depth, a phenomenon called the Bragg peak, and then stop. There is essentially no exit dose on the far side of the tumor.
This makes proton therapy particularly valuable for pediatric cancers and tumors in anatomically challenging locations, such as near the spinal cord, eyes, or brain stem, where even small amounts of stray radiation can cause serious long-term problems. Because fewer beam entry paths are needed and there’s no exit dose, proton therapy can often spare critical structures that photon beams would unavoidably hit.
Internal Radiation (Brachytherapy)
Instead of aiming beams from outside the body, brachytherapy places a radiation source directly inside or next to the tumor. Small seeds, ribbons, or capsules containing radioactive material are positioned precisely where they’re needed, delivering a high dose to the tumor while exposing very little surrounding tissue. It’s commonly used for cancers of the prostate, cervix, breast, head and neck, and eye.
Placement depends on the tumor’s location. For prostate cancer, radioactive seeds are implanted directly within the tumor tissue. For cervical or endometrial cancer, sources are placed inside the vaginal cavity. For eye melanoma, a small radioactive disc is attached to the surface of the eye.
Brachytherapy comes in three speeds. High-dose-rate implants stay in place for just 10 to 20 minutes per session, repeated over several days or weeks. Low-dose-rate implants remain for one to seven days, usually requiring a hospital stay. Permanent implants are left in the body indefinitely, but their radioactivity fades steadily over weeks until it’s essentially gone.
Systemic Radiation Therapy
Some cancers can’t be reached effectively with external beams or local implants, particularly when disease has spread to bone or multiple sites throughout the body. Systemic radiation therapy uses radioactive substances that travel through the bloodstream to find and irradiate cancer cells wherever they are.
One well-known example targets prostate cancer that has spread to bone. A radioactive form of radium, approved by the FDA in 2013, mimics calcium and is naturally taken up by bone tissue. Once there, it emits short-range particles that destroy nearby cancer cells while largely sparing surrounding soft tissue, since those particles travel less than a tenth of a millimeter.
A newer approach uses molecules engineered to lock onto a specific protein found on the surface of prostate cancer cells. When labeled with a radioactive element, these molecules circulate through the body, bind to cancer cells expressing that protein, and deliver radiation directly to the tumor. This targeted strategy, introduced in 2015, has become one of the most actively used systemic radiation treatments for advanced prostate cancer. Similar radiopharmaceutical approaches are being applied to other cancer types, using different molecular targets to guide the radiation to the right cells.