Inside a LINAC
How a medical linear accelerator treats cancer. Step by step, I take you into the treatment room: the machine and its parts, the journey from an electron to a shaped beam, why the beam comes from many sides, a patient’s way through treatment, the safety built into every part, and the people who make it work.
About this page: General principles only, for curious visitors, patients’ families and students: not operating instructions, and not medical advice. The drawings are my own and show no particular maker’s machine. The links go to the publishers and official sources, and I earn nothing from them.
Five little cells, one of every color, signed up to be electrons for the day. They lined up at the start of the arm and ran, faster and faster, until they were nearly as fast as light. At the bend they turned down into the head and became a beam, shaped to the tumor so that it spared as much as it could around it. The patient lay still and felt nothing at all, while the team watched over every moment from the room next door. The five ran the same race every day for weeks, and every day the tumor gave up a little more ground. On the last day, the patient rang the bell. The five were already back at the start line, cheering.
Before you start
- This is an educational overview of general principles only.
- It gives no operating, calibration, quality-check or servicing instructions, and it is not medical advice.
- Real procedures depend on the machine’s model and on the site, and are set by the makers, the hospitals and each country’s rules.
- Only qualified, trained and authorized people operate these machines.
What this tutorial does not teach
- the controls, or how to switch the beam on
- treatment settings and doses
- how to calibrate the machine or carry out its quality checks
- how the safety systems are tested
- how to service or repair it
And whatever you learn here, never enter a treatment room without the staff’s permission.
Two small labels guide you: General principle, with its sources, for what holds for medical linear accelerators in general, and Varies by model or site, for what each machine and each center does in its own way, which this tutorial leaves out on purpose.
1 Radiotherapy in one minute
Radiotherapy treats cancer with radiation: rays with enough energy to damage the cells they pass through, so that cancer cells can no longer grow and divide. Healthy cells are hurt too, so every treatment is planned to give the tumor the dose it needs and the healthy tissue around it as little as possible.
In external beam radiotherapy, the radiation comes from a machine outside the body. The most common of these machines is the medical linear accelerator, or LINAC. It makes high-energy X-rays, or for some treatments a beam of electrons, shapes the beam to the outline of the tumor and aims it from several directions.
The treatment is usually given in small daily sessions, five days a week, for two to nine weeks; some treatments need only one to five sessions. A visit takes an hour or less, most of it spent placing the patient precisely: the beam itself is on for only a few minutes. The patient cannot feel, hear, see or smell the radiation, and this kind of treatment does not make anyone radioactive.
General principle Sources: External Beam Therapy (EBT); External Beam Radiation Therapy for Cancer; Radiotherapy: Cancer treatment with radiation; Linear Accelerator; Technical specifications of radiotherapy equipment for cancer treatment
2 Meet the machine
A treatment room, drawn simply: no real room or machine looks exactly like this. Choose a part of the drawing to read about it, or show them all as a list.
Gantry
The large arm that carries the treatment head. It turns around the patient, so the beam can come from any direction.
Treatment head
Where the beam comes out. Inside it, the X-rays are made and shaped: part 3 follows them. A light in it shows on the skin exactly the area the beam will cover, to help place the patient.
Multileaf collimator (MLC)
In the treatment head: many narrow metal leaves that slide in and out to give the beam the outline of the tumor, as seen from where the beam comes. This way the beam reaches the tumor, and the tissue beside it gets less. The tumor looks different from each direction, so the leaves are rearranged for each one.
Treatment couch
The table the patient lies on. It moves up and down, left and right, in and out, so that the tumor sits exactly where the beams meet.
Positioning lasers
Thin lines of light from the walls and the ceiling. They cross at the point the gantry turns around, the isocenter, and help the team place the patient in the same position every day.
Imaging arms
Arms that fold out from the gantry to take X-ray images, or a CT scan as the gantry turns, just before the treatment. The team compares them with the images made for the plan, to check that the tumor is where it should be. A detector opposite the head can also make images with the treatment beam itself.
Camera and intercom
While the beam is on, the patient is alone in the room, but never out of sight: the team watches on screens and can talk with the patient at any moment.
Shielded walls
The walls and the ceiling are very thick, usually of concrete, sometimes with steel or lead, so that the radiation stays inside. The entrance often turns a corner, called a maze, for the same reason.
Control area
Outside the room: the desk from which the radiation therapists switch the beam on and watch the patient throughout the treatment.
General principle Sources: Linear Accelerator; Radiotherapy Facilities: Master Planning and Concept Design Considerations, Human Health Reports No. 10; Radiation Protection in the Design of Radiotherapy Facilities, Safety Reports Series No. 47; Introduction of Image Guided Radiotherapy into Clinical Practice, Human Health Reports No. 16; Radiation Oncology Physics: A Handbook for Teachers and Students; Technical specifications of radiotherapy equipment for cancer treatment
3 From an electron to a beam
Follow the beam through a simplified machine, from the electron gun to the patient, one step at a time.
Step 1 of 7
Electrons leave the gun
It all starts with electrons, tiny particles with a negative charge. In the electron gun, a hot metal gives them off, and they are sent into a long tube.
Microwaves speed them up
The tube is the accelerating waveguide, emptied of air. Powerful microwaves, like those of radar, push the electrons along it, faster and faster, until they move at nearly the speed of light. Speeding particles up along a straight line gives the machine its name: a linear accelerator.
A magnet turns them
In most machines the waveguide is long, so it lies along the gantry and the electrons fly sideways. A bending magnet turns them towards the patient. Some smaller, lower-energy machines have a short waveguide that points straight at the patient, with no magnet.
They strike a target and make X-rays
The fast electrons hit a small metal target, usually of a heavy metal such as tungsten. They stop suddenly, and part of their energy comes out as high-energy X-rays: the treatment beam. For an electron treatment, the target moves aside and the electrons themselves become the beam.
The beam is evened out and measured
The X-rays leave the target strongest in the middle. In most beams, a metal cone, the flattening filter, absorbs more of them in the middle, where it is thickest, and evens the beam out. Then two separate detectors, the monitor chambers, measure the beam: when the planned amount has been given, the beam stops.
Jaws and leaves shape it
In most machines, heavy metal blocks, the jaws, set the size of the beam as a rectangle, and the leaves of the multileaf collimator give it the outline of the tumor.
The beam reaches the tumor
The shaped beam passes through the body to the tumor. On the far side, a detector can make an image with what comes through: the team uses it to check the patient’s position.
Go deeper
For readers who know a little about electrons and energy.
How much energy?
A machine may offer X-rays of one energy or of several, and electron beams of several energies. Higher energies reach deeper tumors; for low-energy machines, 6 MV (megavolts, the usual measure of an X-ray beam’s energy) is the most common choice. Higher energies also need thicker walls, and from about 10 MV up the treatment head also gives off a few neutrons while the beam is on: the concrete walls absorb them, but the entrance, the maze or the door, may need extra shielding.
Where the microwaves come from
The microwaves come from a magnetron, which makes them, or from a klystron, which amplifies weak microwaves from a small source into powerful ones. As a rule of thumb, magnetrons power the lower-energy machines and klystrons the higher-energy ones, though some machines use a magnetron at high energy too.
How far the magnet turns the beam
In some machines the bending magnet turns the beam through about 90 degrees; in others through 270 degrees, in a loop that brings it out pointing at the patient.
Below the target
Just under the target, a thick block with a cone-shaped opening, the primary collimator, limits the beam to its largest circle. Some machines can also give their X-rays without the flattening filter: the beam is then strongest in the middle, but it gives its radiation much faster, which shortens many treatments.
Treating with electrons
Tumors on or not far below the skin are sometimes treated with the electrons themselves instead of X-rays. Electrons give their energy in the first few centimeters of tissue and then stop, so deeper tissue is spared more. For this, the target and the flattening filter slide out of the way, so the electrons reach the patient without being turned into X-rays, and a very thin metal sheet, the scattering foil, takes the filter’s place: it spreads the narrow electron beam so it covers a wider area evenly. Electrons also scatter in the air, so the beam gets its final shape from an applicator, which attaches to the treatment head and reaches close to the patient’s skin.
General principle Sources: Linear Accelerator; Equipment (radiotherapy), Human Health Campus; Radiation Generators, Therapy Physics Review Course (P.J. Biggs); Radiological Safety Aspects of the Operation of Electron Linear Accelerators, Technical Reports Series No. 188; Selecting Megavoltage Treatment Technologies in External Beam Radiotherapy, Human Health Reports No. 17; Electron therapy; Basic Applications of Multileaf Collimators, Report No. 72; Radiation Oncology Physics: A Handbook for Teachers and Students
4 Shape the beam
Here is a tumor as the treatment head sees it, with a sensitive organ beside it. Move the leaves so that the opening covers all of the tumor and stays clear of the organ, then turn on the beam to see how well you did. There are three tumors, one after another.
Choose a leaf, then move it with the arrow keys or the buttons.
Tumor 1 of 3: beside the organ
- Tumor
- Sensitive organ
Move the leaves, then turn on the beam to see how well they fit.
That was the last of the three tumors. In a real plan, a computer works out the leaves for every direction of the beam, and the team checks them.
A game of shapes only: it shows no doses. Real plans are made by a team, on a computer, and checked before any treatment.
General principle Sources: Linear Accelerator; Selecting Megavoltage Treatment Technologies in External Beam Radiotherapy, Human Health Reports No. 17; Intensity-Modulated Radiation Therapy (IMRT)
5 Many angles, one target
Here is a slice across the body, shown the way CT scans are: as if you were standing at the foot of the bed, looking toward the patient’s head. A tumor sits near the middle of the slice. Add beams from different angles of the gantry and see where they overlap.
No beams yet.
One beam: the tumor gets radiation, but so does all the healthy tissue along the beam’s path.
Two beams: they overlap only at the tumor, so it gets more than the healthy tissue on either path.
Three or more: the tumor gets the most, while each stretch of healthy tissue gets only one beam’s share.
- Tumor
- Sensitive organ
That is why the gantry turns. Beams from several angles cross at the tumor, so the radiation adds up there, while the healthy tissue along each path gets only part of it, but never none. Many treatments go further: the leaves change the beam’s shape while the gantry turns around the patient.
General principle Sources: Radiotherapy: Cancer treatment with radiation; Linear Accelerator; Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT); Intensity-Modulated Radiation Therapy (IMRT)
6 Your treatment journey
Six stages, from the first visit to the follow-up, each with the people who look after it.
Here they are mixed up: put them in order with the arrows, then check.
Consultation
Who: The radiation oncologist
The radiation oncologist studies the patient’s history, tests and images, decides whether radiotherapy suits the cancer, and explains what it involves, its benefits and its side effects.
Simulation
Who: Radiation therapists, with the radiation oncologist
The patient lies in the position for treatment, often with a mask, a mold or pads that help to keep still, and has a CT scan. Small marks on the skin, of ink or tiny tattoos, help to find the same position every day.
Planning
Who: The dosimetrist, the medical physicist and the radiation oncologist
On the images, the radiation oncologist outlines the tumor and the healthy organs nearby. With a planning program, the team designs the beams; the radiation oncologist makes the final decisions.
Checks before the first session
Who: The medical physicist and the dosimetrist
The plan’s calculations are checked, and checked again, before any treatment is given.
Daily treatment
Who: Radiation therapists
Each day the therapists place the patient with the help of the marks and the lasers, take images to check the position, then switch the beam on from outside the room. The first session takes the longest: the radiation oncologist checks and approves the set-up.
Reviews and follow-up
Who: The radiation oncologist and the nurse
During the treatment, they see the patient regularly, often every week, to check how it is going and to help with side effects. Afterwards, follow-up visits check the recovery and how the tumor has responded.
Varies by model or site In order: this is the usual journey. The details and the timing vary from center to center, and from patient to patient.
General principle Sources: External Beam Therapy (EBT); External Beam Radiation Therapy for Cancer; Professions in Radiation Therapy; The Radiation Therapy Treatment Team
7 Safety by design
The machine and its room are built so that the beam stops whenever something is not right. Here is why the beam stops, not how the safety systems are tested.
The door
Try it on this pretend door, which controls nothing.
Door closed: beam on.Door open: beam off.Door closed. The beam stays off until the team switches it on again from the control area.The door is open: the beam can’t be switched on.
The door is linked to the machine by an interlock. If it opens while the beam is on, the beam stops at once, and it does not start again by itself when the door is closed. The interlock is fail-safe: if it fails, the beam stays off.
Other safety measures
- A warning light at the entrance shows when the beam is on.
- The staff switch the beam on from outside the room, and see and hear the patient throughout: the treatment can be stopped at any moment, for example if the patient feels unwell.
- Emergency switches, in the room and at the control desk, must work before any treatment.
- Thick walls keep the radiation inside the room.
- The two monitor chambers stop the beam once the planned amount has been given; if one fails, the other stops it with very little extra, and a timer backs up both.
General principle Sources: Radiation Protection in the Design of Radiotherapy Facilities, Safety Reports Series No. 47; Radiotherapy Facilities: Master Planning and Concept Design Considerations, Human Health Reports No. 10; Selecting Megavoltage Treatment Technologies in External Beam Radiotherapy, Human Health Reports No. 17; Linear Accelerator; Intensity-Modulated Radiation Therapy (IMRT); Radiation Oncology Physics: A Handbook for Teachers and Students; Technical specifications of radiotherapy equipment for cancer treatment
Who checks the machine, and when
Varies by model or site A machine that treats patients is checked often, by people trained for it. Before a new machine treats anyone, medical physicists test it with the supplier and measure its beams. Which checks, and how, each center decides within its country’s rules.
Every morning
Radiation therapists
Before the first patient of the day, the therapists check that the machine and its safety systems work as they should.
Every month
Medical physicists
More detailed checks of the machine.
Every year
Medical physicists
The most complete checks of all.
After a repair
Medical physicists
Before the machine treats patients again, a medical physicist checks it and authorizes its use.
Three principles of radiation protection
Recommended by the International Commission on Radiological Protection (ICRP), they guide every use of radiation, in medicine too.
Justification
Any exposure to radiation must do more good than harm. For a patient, the treatment’s benefit must outweigh its risks.
Optimization
Exposures are kept as low as reasonably achievable. For a patient, the tumor gets what the plan needs, and the healthy tissue as little as possible.
Dose limits
Limits on how much radiation a person may receive apply to the staff and to the public, but not to the medical exposure of a patient: for patients, the first two principles do that work.
General principle Source: Fundamental Principles of Radiological Protection, ICRPaedia
Who licenses it in Iran
Varies by model or site
In Iran, work with radiation is governed by the Law on Protection against Radiation, passed in 1989 (1368). The regulator is the National Radiation Protection Department (NRPD), one of the key departments of the Iran Nuclear Regulatory Authority (INRA), which is part of the Atomic Energy Organization of Iran (AEOI).
A medical center needs licenses before it treats anyone. A joint commission of the regulator and the Ministry of Health reviews the request; the regulator issues the permit to work with radiation, in the names of a responsible person and a radiation protection officer, and the Ministry of Health issues the final license to set up the unit. Radiation workers have medical checks before and during their work, and no one under 18 may do radiation work.
Sources: Law on Protection against Radiation (1989), as published in FAOLEX; Country Nuclear Power Profiles: Iran, Islamic Republic of
8 Who does what
A radiotherapy department is a team. These are the people a patient meets, or is looked after by without meeting them.
Radiation oncologist
A doctor specialized in treating cancer with radiation. They decide whether radiotherapy is right, prescribe it, mark out the area to treat and choose the technique, oversee the patient’s care through the treatment and see the patient in follow-up.
Medical physicist
A scientist who works with the doctor on planning and giving the treatment. Medical physicists are responsible for the machines’ quality checks and safety tests, and make sure that each plan will be given as prescribed.
Dosimetrist
Designs the treatment plan with a planning program and calculates it, with the physicist and the doctor. Mainly a North American profession: in many countries, including Iran, physicists or radiation therapists do this work.
Radiation therapist
Gives the daily treatments, as the doctor prescribes: places the patient, runs the machine and watches over the patient. Also called a radiation therapy technologist or a therapy radiographer. Seeing patients every day, therapists are often the first to notice a problem.
Radiation oncology nurse
Looks after patients during the treatment, explains it and its side effects and how to manage them, and supports their families.
Medical-equipment engineer
Keeps the machines and their systems working: planned inspections and preventive maintenance, finding and repairing faults, often together with service engineers from the maker or an independent service company for the deeper work, and a record of every intervention. After any maintenance or repair, the engineer hands the machine back to the medical physicist, who checks it before it treats patients again. A LINAC is expected to last about 10 to 15 years; a department with only one cannot treat anyone while it is being repaired, which is why WHO suggests that a department being set up plan to start with two.
And many more: receptionists, social workers, dietitians, IT staff, and the managers who keep it all running.
General principle Sources: Professions in Radiation Therapy; The Radiation Therapy Treatment Team; A Handbook for the Education of Radiation Therapists (RTTs), Training Course Series No. 58; Setting Up a Radiotherapy Programme: Clinical, Medical Physics, Radiation Protection and Safety Aspects; Staffing in Radiotherapy: An Activity Based Approach, Human Health Reports No. 13; Accident prevention in case of radiotherapy equipment malfunction; Medical equipment maintenance programme overview; Technical specifications of radiotherapy equipment for cancer treatment
9 Check your understanding
Seven questions on what you have read. Take your time: there is no clock, and nothing is recorded.
10 Words and sources
Words in this tutorial
The words are listed in alphabetical order.
- Accelerating waveguide
- The long tube, emptied of air, in which microwaves speed the electrons up.
- Dosimetrist
- Designs the treatment plan with a planning program, with the medical physicist and the radiation oncologist.
- Electron
- A tiny particle with a negative charge. A LINAC speeds electrons up to make its beam.
- External beam radiotherapy
- Radiotherapy given from a machine outside the body.
- Gantry
- The large arm that carries the treatment head and turns around the patient.
- Image guidance
- Taking images just before a treatment, to check that the tumor is where the plan expects it.
- Interlock
- A link that stops the beam, or keeps it off, when something is not right: for example, when the door opens.
- Isocenter
- The point in the room that the gantry and the couch turn around. The center of the beam passes through it from every angle.
- Jaws
- Heavy movable blocks in the treatment head that set the size of the beam.
- LINACMain term
- Short for linear accelerator: a machine that speeds electrons up along a straight line, to make a beam that treats cancer.
- Medical physicist
- A scientist responsible for the machines’ quality checks and safety tests, who makes sure each plan will be given as prescribed.
- Multileaf collimator (MLC)
- Many narrow metal leaves in the treatment head that give the beam the outline of the tumor.
- Radiation oncologist
- A doctor specialized in treating cancer with radiation.
- Radiation therapist
- Gives the daily treatments: places the patient, runs the machine and watches over the patient.
- Simulation
- A planning scan, made with the patient in the exact position for treatment.
- Treatment couch
- The table the patient lies on; it moves so that the tumor sits where the beams meet.
- Treatment head
- The end of the gantry, where the beam is made and shaped and comes out.
- Treatment plan
- The design of one patient’s treatment: the directions and shapes of the beams and how much each gives, decided before the first session.
- X-rays
- Invisible rays of high energy that pass through the body. A LINAC makes them when its fast electrons strike the target.
Sources
Everything here is in my own words, from the pages below, which explain more. They were last checked on .
Linear Accelerator RadiologyInfo.org (RSNA and ACR), reviewed 29 March 2025
External Beam Therapy (EBT) RadiologyInfo.org (RSNA and ACR), reviewed 1 May 2024
Intensity-Modulated Radiation Therapy (IMRT) RadiologyInfo.org (RSNA and ACR), reviewed 15 June 2026
Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiotherapy (SBRT) RadiologyInfo.org (RSNA and ACR), reviewed 1 May 2023
Professions in Radiation Therapy RadiologyInfo.org (RSNA and ACR), reviewed 1 June 2026
External Beam Radiation Therapy for Cancer US National Cancer Institute, reviewed 15 May 2025
The Radiation Therapy Treatment Team American Society for Radiation Oncology (ASTRO), RT Answers, 2024
Radiotherapy: Cancer treatment with radiation International Atomic Energy Agency (IAEA)
Equipment (radiotherapy), Human Health Campus International Atomic Energy Agency (IAEA)
Radiation Protection in the Design of Radiotherapy Facilities, Safety Reports Series No. 47 International Atomic Energy Agency (IAEA), 2006
Radiotherapy Facilities: Master Planning and Concept Design Considerations, Human Health Reports No. 10 International Atomic Energy Agency (IAEA), 2014
Introduction of Image Guided Radiotherapy into Clinical Practice, Human Health Reports No. 16 International Atomic Energy Agency (IAEA), 2019
Selecting Megavoltage Treatment Technologies in External Beam Radiotherapy, Human Health Reports No. 17 International Atomic Energy Agency (IAEA), 2022
Setting Up a Radiotherapy Programme: Clinical, Medical Physics, Radiation Protection and Safety Aspects International Atomic Energy Agency (IAEA), 2008
Staffing in Radiotherapy: An Activity Based Approach, Human Health Reports No. 13 International Atomic Energy Agency (IAEA), 2015
A Handbook for the Education of Radiation Therapists (RTTs), Training Course Series No. 58 International Atomic Energy Agency (IAEA), 2014
Radiation Oncology Physics: A Handbook for Teachers and Students International Atomic Energy Agency (IAEA), 2005
Accident prevention in case of radiotherapy equipment malfunction International Atomic Energy Agency (IAEA), Radiation Protection of Patients
Medical equipment maintenance programme overview World Health Organization (WHO), 2011
Technical specifications of radiotherapy equipment for cancer treatment World Health Organization (WHO), 2021
Fundamental Principles of Radiological Protection, ICRPaedia International Commission on Radiological Protection (ICRP), edited 20 June 2019
Law on Protection against Radiation (1989), as published in FAOLEX Food and Agriculture Organization of the United Nations (FAO), FAOLEX
Country Nuclear Power Profiles: Iran, Islamic Republic of International Atomic Energy Agency (IAEA), updated 2022
Radiation Generators, Therapy Physics Review Course (P.J. Biggs) American Association of Physicists in Medicine (AAPM), 2014
Radiological Safety Aspects of the Operation of Electron Linear Accelerators, Technical Reports Series No. 188 International Atomic Energy Agency (IAEA), 1979
Electron therapy Radiopaedia, revised 23 March 2023
Basic Applications of Multileaf Collimators, Report No. 72 American Association of Physicists in Medicine (AAPM), 2001
General principles only: not operating instructions, and not medical advice. Real procedures are set by the makers, the hospitals and each country’s rules.
Last reviewed by Ali TizMaghz on .
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