Radiation therapy is delivered in three main ways: from a machine outside the body (external beam), from a radioactive source placed inside the body (internal), or through a drug that travels through the bloodstream to reach cancer cells (systemic). The method your treatment team selects depends on the type of cancer, its location, its size, and how close it sits to sensitive organs. Here’s what each approach involves and what the experience looks like.
External Beam Radiation
External beam radiation is the most common form. A machine called a linear accelerator aims high-energy beams at the tumor from outside the body. The machine doesn’t touch you. You lie on a treatment table, the machine rotates around you, and each session typically lasts anywhere from 10 to 30 minutes. Most of that time is spent positioning you correctly. The actual radiation delivery often takes only a few minutes, and you won’t feel anything while it’s happening.
Before your first session, you go through a planning step called simulation. A CT scan maps the exact size and location of the tumor, and the treatment team uses that scan to calculate the precise angles and doses for your beams. Small marks or tiny tattoos may be placed on your skin so the team can line you up in the exact same position every time. Depending on where the tumor is, you might be fitted with an immobilization device like a thermoplastic mesh mask (for head and neck cancers) or a body mold that keeps you still during treatment.
Conventional external beam radiation is usually given five days a week over several weeks. Each daily dose is called a fraction, and spreading treatment across many small fractions gives healthy tissue time to recover between sessions.
Types of External Beam Techniques
Not all external beam radiation works the same way. Several techniques exist, each designed to shape the radiation more precisely to the tumor.
3-D conformal radiation uses imaging to map the tumor in three dimensions, then shapes the beams to match its contour. This makes it possible to deliver higher doses to the tumor while reducing exposure to surrounding tissue.
Intensity-modulated radiation therapy (IMRT) takes that a step further. It uses many smaller beams, and the strength of individual beams can be varied so that different parts of the tumor receive different doses. This is especially useful when tumors sit near delicate structures like the spinal cord or salivary glands.
Image-guided radiation therapy (IGRT) adds imaging scans not just during planning but during each treatment session. Because tumors can shift slightly due to breathing or normal organ movement, IGRT lets the team adjust your position or the radiation dose in real time. That extra layer of accuracy helps spare healthy tissue.
Stereotactic radiosurgery and stereotactic body radiation therapy (SBRT) deliver very high, precisely focused doses in far fewer sessions. Stereotactic radiosurgery, most often used for brain tumors, aims many small beams at the target from different angles. Each individual beam is weak enough to pass through healthy tissue without much effect, but where all the beams converge, the combined dose is powerful. SBRT applies the same concept to tumors elsewhere in the body. Treatment might be completed in as few as one to five sessions, compared to the 25 to 30 sessions typical of conventional radiation. For peripheral lung tumors, for instance, a common SBRT schedule is three sessions delivering 15 gray each, or four sessions at 12 gray each.
Proton Therapy
Standard external beam radiation uses photons (X-rays). Proton therapy uses protons instead, and the physics of how protons behave in tissue gives it a distinct advantage for certain cancers. Photon beams deposit energy along their entire path, entering the body, passing through the tumor, and continuing out the other side. Protons behave differently. They travel to a specific depth, release the bulk of their energy in a sharp burst called the Bragg peak, and then stop. Very little radiation exits beyond the tumor.
This means proton therapy can deliver a highly targeted dose while exposing less surrounding tissue to radiation. It’s particularly valuable for tumors near critical structures, such as cancers at the base of the skull, along the spine, or in children whose growing tissues are more sensitive to radiation damage. The treatment experience feels similar to standard external beam: you lie on a table, the machine positions itself, and the session is painless.
Internal Radiation (Brachytherapy)
Brachytherapy places a radioactive source directly inside or next to the tumor. Because the radiation travels only a short distance, it delivers a concentrated dose to the cancer while largely sparing tissue farther away. There are two main forms.
Permanent (low-dose-rate) implants involve tiny radioactive seeds, each about the size of a grain of rice, placed directly into the tumor. In prostate cancer, for example, 60 to 120 seeds might be implanted through thin needles during a single procedure. The seeds emit radiation slowly over weeks to months, then become inactive and stay in the body permanently. Urinary side effects like frequency and discomfort can last several months as the seeds do their work.
Temporary (high-dose-rate) implants use a single radioactive source that travels through thin tubes or applicators placed in or near the tumor. A computer-controlled machine pushes the source into position, holds it there for a precise amount of time, then retracts it. Treatment sessions last only minutes, and the radioactive material is removed from your body afterward, so there is practically no radiation exposure to people around you. Symptom duration from high-dose-rate treatment tends to be shorter, often weeks rather than months.
High-dose-rate brachytherapy offers more flexibility in shaping the dose after the applicators are in place, and it can treat larger tumors or tumors that have extended beyond the organ’s surface. Permanent seed implants require more extensive pre-planning and are generally limited to smaller, earlier-stage tumors.
Systemic Radiation
Systemic radiation uses radioactive drugs, called radiopharmaceuticals, that travel through the body to find and irradiate cancer cells wherever they are. Most are given through an intravenous infusion, though some are swallowed as a capsule or liquid.
Some radioactive substances are naturally drawn to specific organs. Radioactive iodine, for example, is absorbed by thyroid tissue, making it effective against thyroid cancer. It’s taken as a pill or liquid that you swallow. Radium targets bone, so radioactive radium is used to treat cancer that has spread to the skeleton. Other radiopharmaceuticals are attached to molecules designed to seek out specific markers on cancer cells, delivering radiation directly to tumors throughout the body.
Because the radioactive material circulates in your system, you may need to follow temporary precautions afterward, like limiting close contact with others for a period of time, depending on the specific drug and dose.
Radiation During Surgery
Intraoperative radiation therapy (IORT) delivers a single large dose of radiation directly to the tumor bed while the surgical site is still open. The surgeon removes the tumor, then the radiation team treats the exposed area before closing the incision. Because the surgeon can physically move healthy organs and tissue out of the beam’s path, IORT allows a high dose to reach the target with minimal collateral exposure.
Mobile linear accelerators small enough to fit in an operating room made this technique widely available starting in the 1990s. Some newer devices use low-energy X-rays with a steep dose dropoff, meaning the radiation intensity falls sharply just millimeters from the source. These machines don’t even require the heavy shielding that traditional radiation equipment needs. IORT is most commonly used in breast cancer, abdominal cancers, and certain brain tumors, and it can sometimes replace weeks of post-surgical external beam radiation with a single intraoperative dose.
What Determines Which Method Is Used
Your oncology team considers several factors when choosing a delivery method. Tumor location matters most. A cancer deep inside the body with no safe path for implants will likely be treated with external beam or systemic radiation. A cervical or prostate tumor that can be reached through natural body openings is a strong candidate for brachytherapy. Cancers that have spread to multiple sites may call for systemic treatment.
Tumor size and stage also play a role. SBRT works best on small, well-defined tumors. Brachytherapy seed implants are generally reserved for earlier-stage, smaller tumors, while high-dose-rate brachytherapy can handle larger ones. Prior surgeries in the area, overall health, and whether radiation is being combined with surgery or chemotherapy all factor into the decision.
Regardless of the method, the goal is the same: deliver enough radiation to destroy cancer cells while keeping exposure to healthy tissue as low as possible. Advances in imaging and computer planning have made every form of radiation therapy significantly more precise than it was even a decade ago, and most patients complete treatment as outpatients with manageable side effects.