Therapeutic radiology is the medical specialty that uses radiation to treat cancer and certain other diseases. You may also see it called radiation oncology or radiation therapy. More than 50% of all cancer patients receive some form of radiation as part of their care, making this one of the most widely used treatment approaches alongside surgery and chemotherapy.
How Radiation Kills Cancer Cells
Radiation works by damaging the DNA inside cancer cells. When a high-energy beam passes through tissue, it can strike DNA directly or, more commonly, split nearby water molecules into highly reactive fragments that damage DNA within a tiny radius. This creates breaks in the DNA strands, including complex, clustered damage that healthy repair machinery cannot fix. When a cancer cell tries to divide with this level of damage, it fails and dies.
Healthy cells in the treatment area also sustain some DNA damage, but they generally repair themselves more effectively than cancer cells. Treatment plans exploit this difference by spreading radiation across multiple sessions, giving normal tissue time to recover between doses while accumulating lethal damage in the tumor.
Types of Radiation Delivery
There are three broad categories: external beam radiation, internal radiation (brachytherapy), and systemic radiation. Most patients receive external beam treatment, where a machine directs radiation into the body from outside.
External Beam Radiation
Several techniques fall under this umbrella, each designed to shape the radiation dose more precisely:
- Intensity-modulated radiation therapy (IMRT) uses many small beams aimed from different angles, with the strength of each beam adjusted so certain parts of the tumor receive higher doses while surrounding tissue is spared.
- Image-guided radiation therapy (IGRT) adds real-time imaging, such as CT or MRI scans taken during each session, so the treatment team can adjust for changes in the tumor’s size or position as treatment progresses.
- Stereotactic body radiation therapy (SBRT) delivers very precise, high-dose beams to small, isolated tumors, typically in the lung or liver. Because the body shifts with breathing and digestion, SBRT is usually given in up to five sessions rather than a single dose. It is often an option for patients who cannot have surgery.
Brachytherapy
In brachytherapy, a small radioactive source is placed directly inside or next to the tumor. This delivers a concentrated dose over a short distance, which limits exposure to surrounding organs. It is commonly used for cervical, prostate, and certain head and neck cancers.
Systemic Radiation
Systemic radiation uses radioactive drugs given by mouth or injected into a vein. These drugs travel through the bloodstream and target specific tissues. This approach treats thyroid cancer, certain bone cancers, prostate cancer, neuroendocrine tumors, and some types of non-Hodgkin lymphoma.
Proton Therapy
Traditional radiation uses photon beams (X-rays), which deposit energy along their entire path through the body. Proton beams behave differently: they release most of their energy at a specific depth and then stop, meaning less radiation reaches tissues behind the tumor. Modern proton systems “paint” the tumor layer by layer with precise spots of proton energy.
This physical advantage is most meaningful for tumors in sensitive locations. In head and neck cancers, proton therapy has been linked to less long-term dry mouth and reduced need for feeding tubes compared to photon-based IMRT. In brain tumor treatment, one comparison found that severe drops in immune cell counts occurred in 14% of proton patients versus 39% of those receiving photon therapy. Proton therapy is especially valued in pediatric cancers, where minimizing radiation to developing tissues matters most.
What the Treatment Process Looks Like
Before your first radiation session, you go through a planning process that typically takes a few days to a few weeks.
The first major step is a simulation appointment, which lasts about an hour. You lie on a treatment table while the team takes a CT scan to map your anatomy precisely. If you need to stay perfectly still during future sessions, they may create an immobilization device like a custom face mask or leg mold. Small permanent ink dots (tattoos, each about the size of a freckle) are placed on your skin so the beam targets the exact same area every time.
After simulation, your radiation oncologist designs a treatment plan tailored to your tumor’s shape, location, and size, choosing the type of machine, the total radiation dose, and how many sessions you will need. A dosimetrist uses specialized software to calculate how the dose will be distributed, ensuring the tumor receives enough radiation to be destroyed while nearby organs like the lungs, liver, or spinal cord receive as little as possible. A medical physicist reviews and verifies the plan before treatment begins.
Treatment sessions themselves are typically short, often 15 to 30 minutes including setup time. A radiation therapist positions you, confirms alignment, and operates the machine. You do not feel the radiation during delivery. Most external beam courses run daily, Monday through Friday, for several weeks, though schedules vary. SBRT courses may be as short as one to five sessions total.
The Treatment Team
Radiation treatment involves a coordinated group. The radiation oncologist is the physician who prescribes and oversees your care. The medical physicist ensures the equipment is calibrated correctly and that complex treatment plans are properly tailored. The dosimetrist creates the detailed dose maps, contouring both the tumor and nearby healthy structures on your scans. The radiation therapist is the person you see most often: they position you and deliver each daily treatment.
Side Effects: Short-Term and Long-Term
Side effects depend heavily on which part of the body is being treated, because radiation primarily affects the tissues in its path.
Short-term side effects appear within a few weeks and mostly involve fast-growing tissues like skin and the lining of the mouth or digestive tract. Skin redness similar to a sunburn is common. Patients receiving radiation to the head and neck often develop mucositis, a painful inflammation of the mouth or throat lining. Radiation to the abdomen or pelvis frequently causes nausea, diarrhea, or both. Brain radiation can trigger headaches and nausea from temporary swelling.
Late side effects emerge months to years after treatment and tend to involve slower-turnover tissues like muscle, bone, and organs. Lung radiation can cause a persistent cough and, over time, scarring (fibrosis) in the lung tissue. In the head and neck, long-term fibrosis of the jaw muscles may limit how far you can open your mouth. Pelvic radiation can lead to chronic bowel changes, including ongoing diarrhea or rectal bleeding. Brain radiation sometimes causes lasting fatigue or subtle changes in memory and thinking.
Most acute side effects resolve within weeks of finishing treatment. Late effects, when they occur, are typically managed but may be permanent. Your treatment plan is designed specifically to minimize these risks by limiting the dose to healthy tissues.
Beyond Cancer Treatment
Radiation is not exclusively a cancer treatment. Low-dose and moderate-dose radiation has been used for benign conditions for over a century, with the first documented cases involving juvenile arthritis in 1898. Today, radiation treats keloid scars (the most common non-cancer indication), Dupuytren’s contracture (a hand condition that curls the fingers inward), heterotopic ossification (abnormal bone growth in soft tissue), Graves’ disease, and trigeminal neuralgia. Certain benign tumors, such as paragangliomas and vestibular schwannomas, are also treated with focused radiation when surgery poses too much risk.