A linear accelerator is a type of machine that uses electric fields to accelerate charged particles along a straight path. Variations of linear accelerators are used in scientific research, materials testing, and particle physics. One of their most important applications, however, is in medicine, where specialized linear accelerators are used to generate high-energy radiation beams to treat cancer and certain non-malignant conditions. 

The medical linear accelerator, often referred to as a LINAC, accelerates electrons and uses them either directly or to generate high energy x-rays (photons) by striking a metal target, typically made of tungsten. These radiation beams are shaped and directed with precision, allowing physicians to deliver effective radiation treatment to tumors or other target volumes while minimizing dose to healthy tissue and reducing related side effects. These machines are the primary workhorses for delivering radiation therapy today.   

If you or someone you love is starting radiation treatment, there is a good chance a LINAC is the machine doing the work. Here is what it actually does, without the physics degree. 

What is a linear accelerator? 

A LINAC is a large machine that rotates around a treatment table and delivers radiation from carefully chosen angles. It does not hold any radioactive material inside it. It produces radiation on demand using electricity, so when the beam is switched off, it no longer produces radiation. The patient does not become radioactive, and the room is safe to enter after the treatment has been delivered.   

The linear accelerator is the foundation of modern external beam radiation therapy, and many advanced external-beam radiation treatments utilize this technology. A LINAC can also produce electron or photon beams of different energies, allowing physicians to treat targets near the surface or at greater depth. Hospitals have relied on this technology for decades, and it keeps getting more precise as imaging and computing improve. 

The machine sits in a specially built room with thick concrete walls and a heavy door, which keep the radiation contained to the space where treatment happens. The shielding keeps radiation confined so that staff and others outside the treatment vault are protected, and exposure levels remain strictly within established limits.  

How does a linear accelerator work? 

Inside the machine, electrons are accelerated to nearly the speed of light along a straight path a few feet long, which is where the name comes from. Those high-speed electrons either treat the target directly, which works well for cancers close to the surface, or strike a metal target to produce high-energy X-rays that can reach targets deeper in the body. The care team chooses which kind of beam and how strong it is based on where the cancer is located. 

The arm that holds all of this, called the gantry, rotates in a full circle around the patient. That movement allows the beam to enter from many different directions over the course of a session, intersecting to deliver treatment doses to the target volume. 

As the beam leaves the machine, it passes through a collimation system that shapes the radiation beam. While there is more than one method of collimation, many linear accelerators are outfitted with a device called a multileaf collimator, or MLC. This is a set of thin metal leaves that move independently to shape the radiation beam to the treatment region as defined by the radiation oncologist. For some treatment techniques, the leaves even move during treatment to help optimize the dose to the target while protecting nearby critical structures such as the spinal cord, bladder, or rectum.  

Accuracy depends on the treatment target being in the same position the plan assumes, so many LINACs are outfitted with an onboard imaging system to align the patient for treatment. These images allow for better set up by adjusting for daily changes, minimizing treatment margins when compared to radiation treatments decades ago that lacked such information and improving the overall treatment for the patient.  Don’t be alarmed when a therapist states that they are taking x-rays, pictures, or images prior to the treatment. This is all part of the regular process and not to diagnose something new! 

How does the radiation hit the target volume and not everything else? 

Radiation works by damaging the DNA inside cells. Cancer cells tend to be worse at repairing that damage than normal tissue cells, so they take the bigger hit and eventually degrade, while much of the surrounding healthy tissue recovers. This is part of why many treatments have historically been “fractionated”, or broken up over multiple treatments, because it gives healthy tissue time to repair between treatments.  

Additionally, the care team works carefully on each patient plan to maximize dose to the target volume and minimize dose to “critical structures,” or healthy tissue volumes with historically determined tolerance levels. While the medical dosimetrist and medical physicist are often involved in creating an optimal treatment plan that is deliverable on the LINAC, the radiation oncologist oversees the process, determining the prescription dose and identifying regions where dose should be limited.  

Mechanically, the LINAC provides flexibility to the treatment planner to optimize the treatment. The gantry, collimator, and table can all be rotated to various angles, overlapping in the treatment region to deliver the prescribed dose while minimizing dose to surrounding tissues. Modern LINACs provide many ways to shape and direct radiation. The care team can adjust beam angles and use beam shaping devices to conform the dose to the target while protecting nearby organs. Advanced techniques such as IMRT and VMAT allow the beam shape and intensity to change during treatment, improving dose conformity while keeping treatment times efficient. Each plan is mapped to the patient anatomy from a CT scan, so the high-dose area matches the shape of the treatment region as specified by the radiation oncologist rather than a generic outline. 

Treatment is often divided into a number of smaller sessions spread over days or weeks instead of delivered all at once. This spacing, called fractionation, gives healthy tissue time to repair itself between visits while the cancer cells, which recover less well, fall further behind with each session. It is why a course of radiation often means coming in according to the schedule provided, and why keeping every appointment matters. Depending on the type of cancer and the treatment goals, radiation may be delivered over several weeks or in a shorter period using approaches such as hypofractionation or stereotactic radiation therapy. A radiation oncologist can help to determine the best approach for a given diagnosis.  

Who makes sure it all works? 

A LINAC is only as good as the team running it, and several specialists shape the patient care path before the first treatment is ever delivered. The radiation oncologist is the physician who determines the appropriate prescription and target volume(s), reviews and approves the treatment plan, and oversees treatment. A medical dosimetrist creates a detailed plan, determining the beam angles and shapes that deliver the prescribed dose to the treatment volume while sparing nearby organs. A radiation therapist positions the patient on the table and delivers the dose during treatment. 

Behind all of it is the medical physicist. The role of the medical physicist is to bridge the clinical and the technical, making sure that what the radiation oncologist prescribes actually happens on the treatment table. Physicists calibrate the machine so that what it delivers matches the treatment planning system, and they establish and oversee the quality assurance program. Radiation therapists perform quality control checks on the LINAC every day before treatment begins. If something is meaningfully outside the allowed range, patients are not treated until the physicist evaluates the issue and confirms the machine is safe to use clinically. The physicist also reviews these checks routinely and typically performs the more detailed monthly and annual LINAC testing. Every patient treatment plan undergoes an independent quality review and verification by a physicist before treatment begins. Most of this happens before any patient walks in for treatment, and none of it is visible from the waiting room. 

That quiet, continual oversight is what keeps radiation accurate and safe, and it is the part of cancer care most people never see. At CAMP, it is the work our medical physicists and dosimetrists do for hospitals across the Mountain West, often at smaller community and rural centers that rely on outside physics expertise so their clinical teams and patients can keep their focus where it belongs. 

Frequently asked questions 

• Is a linear accelerator the same as chemotherapy? No. Chemotherapy uses drugs that travel through the whole body. A LINAC delivers targeted radiation to a specific area, so its effect is concentrated where the radiation is aimed. 
• Does the machine touch the patient during treatment? No. The LINAC is designed to rotate around the patient without making contact. The patient simply needs to lie still while it moves around the table. 
• Does the patient leave the treatment radioactive? No. External beam radiation therapy delivered by a LINAC does not make patients radioactive. Once the beam turns off, the radiation stops, and it is safe to be around others.  
• Why do patients get breaks on weekends and holidays? Breaks on weekends and holidays give normal tissues time to recover between treatments. The radiation oncologist accounts for this schedule when designing a treatment course. If a holiday or unexpected closure changes the schedule, the care team will adjust as needed so the overall treatment remains appropriate. 
• What kinds of cancer are treated with a linear accelerator? LINACs treat many types of cancer, including breast, prostate, lung, brain, and head and neck cancers. They also form a foundation for more advanced techniques such as IMRT, VMAT, and stereotactic radiosurgery. 

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Last updated: June 2026  

Clinical Disclaimer  

This resource is provided for general educational and informational purposes only and is not intended to constitute medical advice, diagnosis, or treatment, nor to replace the independent clinical judgment of a licensed physician or other qualified healthcare professional. Individual treatment plans, safety precautions, and clinical recommendations vary based on patient-specific factors and clinical context. Readers should consult their own licensed healthcare provider regarding any medical condition or treatment decisions.