During radiation therapy, a machine (or an implanted source) delivers focused energy that damages the DNA inside cancer cells, making it impossible for them to divide and grow. The actual beam is invisible and painless. Most of the time you spend in the treatment room, often around 20 minutes per session, is devoted to positioning your body precisely. The radiation itself is delivered in short bursts lasting only seconds.
How Radiation Kills Cancer Cells
Radiation works by breaking the DNA strands that cells need to copy themselves. This happens two ways. Sometimes a high-energy particle or photon hits a DNA strand directly, snapping the backbone of the molecule. More often, the radiation splits water molecules inside your cells, creating highly reactive fragments called free radicals. These free radicals then collide with nearby DNA, producing breaks and damage at multiple points along the strand.
When breaks occur on both strands of the DNA double helix close together, the cell faces what’s known as a double-strand break. This is the most lethal type of damage. The cell’s repair machinery may try to fix these breaks, but the process itself can create additional errors, especially when multiple damaged sites sit close together. Attempted repairs of clustered damage can accidentally convert single-strand breaks into double-strand breaks, compounding the destruction.
Normal, healthy cells have more effective repair systems than cancer cells. Research comparing tumor-derived cell lines to normal cells found that membrane repair either failed or was significantly less efficient in six out of seven cancer cell lines tested. Cancer cells also tend to have altered levels of the proteins responsible for sensing and repairing damage. This biological gap is the core principle behind radiation therapy: both healthy and cancerous cells sustain damage, but healthy tissue recovers more effectively between sessions while cancer cells accumulate fatal injuries.
The Planning Phase Before Treatment Starts
Before any radiation is delivered, you go through a planning session called a simulation. During this appointment, a CT scan maps the exact size, shape, and location of the tumor. The medical team uses these images to calculate beam angles and doses that concentrate energy on the tumor while minimizing exposure to surrounding organs.
Depending on the treatment area, you may be fitted with immobilization devices to keep your body in the same position for every session. For head and neck cancers, this typically means a custom thermoplastic mesh mask molded to your face, neck, and shoulders. For other body areas, you might use a form-fit body mold or specialized headrest. Small tattoo dots, each about the size of a freckle, are sometimes placed on your skin as alignment markers so the radiation therapists can reproduce your exact position each day.
What a Treatment Session Looks Like
A typical session begins with the radiation therapists positioning you on the treatment table using your immobilization devices and alignment marks. This setup phase takes up most of the visit. The therapists then leave the room and monitor you through cameras and an intercom from an adjacent control area. You can speak with them at any time.
The machine, usually a linear accelerator, rotates around you and delivers radiation from multiple angles. You won’t feel the beam. You might hear clicking or whirring sounds from the machine, notice flashes of light (particularly during brain treatments), or detect an unusual smell. These are all normal. The radiation comes in short bursts, not a continuous stream, and the entire delivery often takes just a few minutes within a session that lasts no more than 20 minutes total. Afterward, a technologist helps you off the table, and you may briefly meet with a nurse to discuss skin care or nutrition during your treatment course.
How the Machine Targets the Tumor
Modern linear accelerators use a component called a multileaf collimator to shape the radiation beam. This device consists of dozens of thin tungsten leaves, each only a few millimeters wide, that slide in and out independently to match the outline of the tumor from each angle. As the machine rotates, the leaf pattern changes continuously, conforming to the tumor’s shape as seen from every direction.
Newer systems with narrower leaves (2.5 mm instead of 5 mm) can reduce the volume of normal tissue exposed to high doses by roughly 13% to 30%, depending on the tumor’s size and location. The dose drops off steeply at the tumor’s edge, sparing healthy structures just millimeters away. For tumors near critical organs like the optic nerve or brainstem, the treatment team adjusts arc paths and beam angles to avoid those structures entirely.
Types of Radiation Therapy
External Beam Radiation
This is the most common form. A machine outside your body directs beams at the tumor. Within external beam therapy, there are several approaches. Intensity-modulated radiation therapy (IMRT) uses many small, shaped beamlets at varying intensities to sculpt the dose around complex tumor shapes. A typical IMRT course involves around 33 sessions delivered once daily over six to eight weeks, with each fraction delivering roughly 1.2 to 2 Gray of radiation.
Stereotactic body radiation therapy (SBRT) takes the opposite approach: fewer sessions with much higher doses per fraction, typically 6 to 10 Gray each. A full SBRT course usually involves about 5 sessions spread over one to two weeks, with a median treatment time of just 10 days. SBRT is used for smaller, well-defined tumors where extreme precision allows safe delivery of these concentrated doses.
Internal Radiation (Brachytherapy)
Instead of aiming beams from outside, brachytherapy places a radiation source directly inside or next to the tumor using a small catheter or a larger applicator. The source takes the form of tiny seeds, ribbons, or capsules. Low-dose-rate implants stay in place for one to seven days, delivering a continuous low level of radiation. High-dose-rate implants are inserted for just 10 to 20 minutes at a time, then removed. You may receive several high-dose-rate sessions over the course of treatment.
Doses and Fractionation
Radiation dose is measured in units called Gray (Gy). Total doses vary widely depending on the cancer type, location, and treatment goal. For prostate cancer treated with curative intent, for example, total doses of 80 Gy or higher have been associated with better outcomes for aggressive tumors. That dose is typically delivered in daily fractions of about 2 Gy each, meaning treatment stretches across many weeks.
Splitting the total dose into small daily fractions is deliberate. Each fraction damages both cancer cells and some healthy cells, but the gap between sessions (usually 24 hours) gives normal tissue time to repair. Cancer cells, with their inferior repair systems, accumulate damage faster than they can fix it. Over the full course of treatment, this difference compounds until the tumor can no longer sustain itself while surrounding tissue largely recovers.
Side Effects During Treatment
Because the beam passes through some healthy tissue to reach the tumor, side effects depend almost entirely on the body area being treated. Radiation to the head and neck often causes mouth sores, dry mouth, and difficulty swallowing. Chest radiation can irritate the esophagus. Pelvic radiation may cause bowel or bladder irritation. Skin in the treatment area frequently becomes red, dry, or sensitive, similar to a sunburn.
These effects tend to build gradually over the course of treatment rather than appearing after the first session. Most are temporary, peaking in the final weeks of therapy and resolving in the weeks to months afterward. Fatigue is the most universal side effect regardless of treatment site, and it typically worsens as treatment progresses.