A linac, short for linear accelerator, is a machine that uses electromagnetic waves to accelerate charged particles in a straight line to extremely high speeds. In medicine, it’s the device most commonly used to deliver external beam radiation therapy for cancer. Outside of hospitals, linacs also play roles in scientific research, industrial manufacturing, and national security. But if you’ve encountered the term, there’s a good chance it came up in the context of cancer treatment.
How a Medical Linac Works
A medical linac accelerates electrons to kinetic energies between 4 and 25 million electron volts (MeV). These electrons travel through a series of microwave-powered cavities that push them faster and faster in a straight line. Once they reach the desired energy, the machine can use them in one of two ways: either directing the electron beam straight out for treating shallow tumors near the skin, or slamming the electrons into a tungsten target to produce high-energy X-rays (called photon beams) that can penetrate deep into the body to reach internal tumors.
The beam passes through a section of the machine called the treatment head, which contains the components responsible for shaping, steering, and monitoring the radiation. A bending magnet redirects the electrons toward the target. A flattening filter evens out the X-ray beam’s intensity. Dose monitoring chambers measure the output in real time to ensure the patient receives exactly the prescribed amount of radiation.
Shaping the Beam to Match the Tumor
One of the most important components inside the treatment head is the multileaf collimator (MLC). This device contains dozens of thin, individually movable tungsten leaves, each between 2 and 15 millimeters wide, that slide in and out of the beam’s path. By adjusting the position of each leaf independently, the machine shapes the radiation field to closely match the three-dimensional outline of the tumor. This replaced the older method of hand-placing or casting custom lead blocks for each patient.
The MLC can also change the beam’s shape continuously during treatment. This capability is what makes advanced techniques possible. In intensity-modulated radiation therapy (IMRT), the leaves shift between multiple beam angles to vary the radiation dose across the treatment area, delivering higher doses to the tumor while reducing exposure to surrounding healthy tissue. In volumetric modulated arc therapy (VMAT), the linac rotates around the patient in an arc while simultaneously adjusting the beam shape, dose rate, and rotation speed. VMAT generally delivers treatment faster than step-and-shoot IMRT, which reduces the effects of any small movements the patient might make during the session.
What a Treatment Session Looks Like
A typical radiation therapy session on a linac takes about 30 minutes, though the actual beam-on time is often just a few minutes. Most of the appointment is spent positioning you precisely on the treatment table and verifying alignment with imaging. Treatments are usually scheduled daily, Monday through Friday, and a full course can last anywhere from one week to several weeks depending on the type and stage of cancer being treated.
Before the beam turns on, the machine uses onboard imaging to verify your position. This process, called image-guided radiation therapy (IGRT), allows the clinical team to adjust your setup and correct for any shifts in the tumor’s position from day to day. Older systems relied on low-resolution X-ray images or cone-beam CT scans for this step.
MRI-Guided Linacs
A newer generation of machines combines a linac with an MRI scanner in one unit. These hybrid systems can image soft tissues in real time while delivering radiation, something conventional CT-based linacs cannot do. MRI provides higher-resolution images of soft tissue without using additional ionizing radiation, which is particularly valuable for tumors in the abdomen, pelvis, and brain where cone-beam CT images often suffer from artifacts caused by air pockets and scattered radiation.
The practical benefit is significant: the treatment team can see the tumor and nearby organs while the beam is on, monitor breathing and organ movement as it happens, and even adapt the treatment plan on the spot while the patient is still on the table. This real-time visibility allows for beam gating, where the machine only fires when the tumor is in the correct position during the breathing cycle. Conventional linacs with CT-based imaging can approximate this, but they lack the soft tissue contrast needed to track many tumors with the same precision.
The Room Around the Machine
Because a linac produces high-energy radiation, it operates inside a heavily shielded room called a vault. The walls and ceiling are made of dense concrete, typically around 120 centimeters (about 4 feet) thick for the primary barriers that face the direct beam, and somewhat thinner for secondary barriers that only need to block scattered radiation. The concrete used is specially formulated with a density of about 2.35 grams per cubic centimeter. Even the maze-like entrance corridor is designed to prevent radiation from reaching the hallway outside, with walls ranging from 30 to 70 centimeters thick depending on their line of sight to the beam.
The machine also undergoes daily quality assurance checks. Medical physicists verify that the X-ray output for every energy level stays within 3% of the expected value. These checks ensure the dose patients receive matches what their treatment plan prescribes.
Linacs Outside of Medicine
While cancer treatment is the most familiar application, linacs serve a wide range of purposes in other fields. In particle and nuclear physics, they accelerate particles for experiments that probe the fundamental structure of matter. They also power light sources that produce intense X-ray beams used across many scientific disciplines, from structural biology to materials science.
Industrial linacs use electron beams to modify material properties: cross-linking plastics for products like shrink wrap, hardening the surfaces of artificial joint components, sterilizing medical equipment, and irradiating food to destroy pathogens. In the semiconductor industry, ion-beam accelerators (a related technology) are essential for manufacturing computer chips. National security applications include cargo inspection at ports, where linacs generate the penetrating radiation needed to scan shipping containers, as well as characterizing nuclear materials for stockpile stewardship programs.
The Department of Energy estimates that hundreds of industrial processes worldwide rely on particle accelerators, making them one of the more quietly ubiquitous technologies in modern life.