Photon therapy is a type of radiation treatment that uses high-energy beams of light (photons) generated by machines called linear accelerators to kill cancer cells or shrink tumors. It is the most common form of external beam radiation therapy used in oncology today. The term can also refer to low-level light therapy used in dermatology and pain management, though the cancer treatment meaning is far more common in medical settings.
How Photon Therapy Works
A linear accelerator produces beams of high-energy photons, essentially concentrated X-rays, and aims them at a tumor from outside the body. When these photons pass through tissue, they damage the DNA inside cells. Cancer cells are less able to repair this damage than healthy cells, so they die or stop dividing. The beam enters the body, deposits energy along its entire path, and exits on the other side. This means some radiation inevitably hits healthy tissue both in front of and behind the tumor.
That characteristic distinguishes photon therapy from particle-based treatments like proton therapy. Protons stop at a precise depth and release most of their energy right at the tumor (a phenomenon called the Bragg peak), while photons distribute their dose more broadly. This difference matters most for tumors near sensitive structures, like in children’s brains or near the spinal cord, where minimizing radiation to surrounding tissue is critical.
What Cancers It Treats
Photon radiation is used across nearly every cancer type. It works especially well on tumors considered “radiosensitive,” meaning they respond strongly to radiation. Lung, breast, and germ cell tumors fall into this category and are commonly treated with standard fractionated photon therapy, where the total radiation dose is split across many sessions.
Tumors that resist radiation more stubbornly, such as melanoma, kidney cancer, colon cancer, and sarcomas, are often treated with a different approach: fewer sessions delivering much larger individual doses, a technique called radiosurgery. Advances in photon delivery have made it possible to treat even these resistant cancers effectively by concentrating higher doses more precisely on the tumor.
Modern Delivery Techniques
Photon therapy has evolved well beyond a simple beam aimed at a tumor. Two major techniques now dominate treatment planning. Intensity-modulated radiation therapy (IMRT) shapes the beam into complex patterns, adjusting the radiation intensity across the treatment field to match the tumor’s shape while reducing exposure to nearby organs. Volumetric modulated arc therapy (VMAT) takes this further by rotating the beam around the patient in an arc, dynamically adjusting its shape and intensity throughout the rotation.
Both VMAT and newer hybrid approaches (combining VMAT and IMRT) outperform standard IMRT in conforming radiation tightly to the tumor and sparing critical organs. In studies comparing all three techniques for treating tumors near the abdomen, VMAT and hybrid plans delivered significantly lower doses to the kidneys, stomach, and nearby blood vessels. A newer hardware innovation, flattening filter-free beam technology, further sharpens the dose by reducing stray radiation scattered inside the machine itself. This also shortens treatment times, which improves patient comfort.
What a Treatment Course Looks Like
A typical course of photon radiation therapy involves daily sessions, usually five days a week, over several weeks. Depending on the cancer type and treatment plan, a full course can run as long as 44 to 50 sessions spread over roughly nine weeks. Each individual session is relatively short. Most of the time in the treatment room is spent positioning you precisely so the beams hit the right spot; the actual radiation delivery often takes only a few minutes.
For certain situations, doctors use hypofractionation, delivering larger doses per session over fewer total visits. Stereotactic body radiation therapy (SBRT), for example, can deliver a full treatment in as few as three to five sessions. The tradeoff involves careful planning to ensure healthy tissue can tolerate the larger per-session dose.
Side Effects
Because photon beams pass through healthy tissue on their way to the tumor and continue beyond it, side effects depend heavily on which part of the body is being treated. Fatigue is the most universal side effect, affecting the majority of patients regardless of treatment location. Skin changes in the treatment area, ranging from redness to peeling similar to a sunburn, are also common.
Beyond those, side effects are regional. Radiation to the head or brain can cause hair loss, nausea, memory difficulties, and headaches. Chest radiation may lead to coughing, shortness of breath, or trouble swallowing. Pelvic radiation often causes diarrhea, bladder irritation, and sexual or fertility changes. Abdominal treatment can bring nausea and urinary problems.
Most side effects from damaged healthy cells resolve within a few months after treatment ends, as those cells repair themselves. Some effects, however, can appear months or even years later. These late effects vary by individual and treatment area but are an important consideration when weighing treatment options.
Photon Therapy vs. Proton Therapy
The core physical difference is dose distribution. Photon beams deposit radiation along their entire path through the body. Proton beams stop at a set depth and release the bulk of their energy at the tumor site, delivering less radiation to tissues in front of the tumor and essentially none behind it. This gives proton therapy a theoretical advantage in reducing collateral damage.
Clinical data reflects this in certain scenarios. For central nervous system tumors, one large analysis found five-year survival rates of 59% for adults treated with proton therapy compared to 37% for those treated with photon radiation. In pediatric brain tumor patients, the gap was 76% versus 45%. These differences likely reflect both the precision advantage of protons and the fact that proton therapy tends to be selected for patients and tumor types where that precision matters most, so the comparison isn’t perfectly apples-to-apples.
Proton therapy is also more sensitive to changes in body shape and tissue density along the beam path, which can shift where the dose lands. Photon therapy is more forgiving of these small day-to-day variations. Proton facilities are far less common and significantly more expensive, so photon therapy remains the standard for the vast majority of radiation treatments worldwide.
Low-Level Photon Therapy for Skin and Pain
Outside of cancer treatment, the term “photon therapy” sometimes refers to photobiomodulation, previously called low-level light therapy or cold laser therapy. This uses non-ionizing light at specific wavelengths, meaning it does not damage DNA or carry the risks of radiation therapy. Instead, it stimulates cellular activity to promote healing and reduce inflammation.
Three wavelengths have the strongest evidence behind them: blue light at 415 nanometers, red light at 633 nanometers, and near-infrared light at 830 nanometers. Blue light is primarily used for mild-to-moderate acne, where clinical studies have shown improvement after eight sessions over four weeks. Red and near-infrared light penetrate deeper and are used for wound healing, psoriasis, and pain relief. One pilot study combining 830nm and 633nm LED treatments showed benefit for psoriasis that had not responded to other therapies, using two 20-minute sessions per week for four to five weeks.
Photobiomodulation is now recommended as standard care for oral mucositis, a painful inflammation of the mouth lining that commonly develops during cancer radiation or chemotherapy. Clinical guidelines are being developed for broader applications in wound healing, dentistry, and neurological rehabilitation. Red light at 640 nanometers has also been shown to trigger pain-relieving immune responses by acting on specific receptors in immune cells, offering a potential mechanism for its analgesic effects.