Treatment delivery
Treatment machines
From kilovoltage skin units to proton cyclotrons, robotic radiosurgery, MR-guided and AI-driven adaptive platforms — how each treatment machine makes its beam, where it is used in the clinic, and what distinguishes it on the registry exam.
The organizing question for every machine is beam energy and geometry: kilovoltage beams deposit their maximum dose at the skin, megavoltage beams spare the skin and penetrate deeply, protons stop at a chosen depth, and stereotactic platforms converge many beams on one small target. Master that logic and the machines sort themselves.
Low-energy kilovoltage unit that treats lesions at or just below the skin surface.
A low-energy X-ray tube treats through open cones or applicators at a short source-to-surface distance (typically 15–25 cm). Because the beam is kilovoltage, the maximum dose sits AT the skin surface — there is no skin sparing, which is exactly why it works for skin lesions.
Clinical uses
- Basal and squamous cell skin cancers of the face, scalp, ears, and nose
- Keloid scar treatment
- Lesions < 4–5 mm thick where surgery or electrons are not ideal
Key points
- Dose maximum at the skin surface — no build-up region (opposite of megavoltage beams)
- Small, well-defined fields delivered through attached cone applicators
- Short SSD and high dose rate make fractions fast; often 2–5 fraction schedules for skin cancer
- Bone and cartilage absorption is modest compared with orthovoltage, giving good cosmetic results on the face
Common linac faults in the field
Nobody sees the textbook accelerator — they see the one with a low dose rate at 7 a.m. These are the faults that fill service logs, how they present on the console, and what the physics and therapy team does about each one. Tap a fault to open it.
Beam generation & RF power
The beam starts at the electron gun and is accelerated by RF power through the waveguide. These are the parts that wear first.
Electron gun filament wear
Dose rate settles below the value the plan calls for, a "gun emission" or "dose rate low" fault interrupts beam-on, and the morning warm-up needs repeated retries.
Electron gun filament wear
Dose rate settles below the value the plan calls for, a "gun emission" or "dose rate low" fault interrupts beam-on, and the morning warm-up needs repeated retries.
Likely causes
The filament erodes with every beam-on hour. The control system keeps raising filament current to hold emission, until it reaches its ceiling and output can no longer be maintained.
What the team does
Fault frequency is tracked against accumulated filament hours; service fits a new filament or gun, then physics rechecks output and energy before the unit returns to patients.
Exam point
Filament life is counted in beam-on hours, not calendar age.
Magnetron or klystron RF output loss
Progressive low dose rate, "charge rate" or RF faults, flatness and symmetry drifting out, and energy out of tolerance early in the day.
Magnetron or klystron RF output loss
Progressive low dose rate, "charge rate" or RF faults, flatness and symmetry drifting out, and energy out of tolerance early in the day.
Likely causes
Magnetron aging (on the order of 1–3 years) and klystron cathode wear quietly reduce RF power; a failing RF driver or pulse-forming network produces the same picture.
What the team does
RF power and pulse shape are measured in service; the tube is replaced when tuning no longer restores output, and beam quality is re-measured after the swap.
Exam point
Low-energy linacs use a magnetron; high-energy machines use a klystron.
Waveguide gas or vacuum fault
Beam-on is blocked by a "waveguide pressure" or "vacuum" interlock, sometimes right after an audible arc.
Waveguide gas or vacuum fault
Beam-on is blocked by a "waveguide pressure" or "vacuum" interlock, sometimes right after an audible arc.
Likely causes
Loss of SF6 pressure in the pressurized waveguide, a vacuum leak at the gun or accelerator structure, or a failing vacuum pump.
What the team does
Gas pressure and pump readings are checked first; a vacuum leak takes the machine down for service and usually means gun or structure work.
Photon target erosion or cooling failure
Photon output falls while electron modes stay normal; the beam softens and percent depth dose or TPR drifts.
Photon target erosion or cooling failure
Photon output falls while electron modes stay normal; the beam softens and percent depth dose or TPR drifts.
Likely causes
Years of electron bombardment pit and thin the target, and degraded target cooling accelerates the damage.
What the team does
Output and energy are checked on service; the target is replaced once output cannot be restored, and depth-dose data is re-measured before clinical use.
Dose monitoring & beam quality
The monitor chamber and steering servos are what physics checks every morning. Most dose faults surface in daily QA before a patient is affected.
Ion chamber or dose servo fault
Displayed MU or dose rate disagrees with the expected value, a "dose servo", "chamber" or "gas" fault holds the beam, or the beam aborts mid-fraction.
Ion chamber or dose servo fault
Displayed MU or dose rate disagrees with the expected value, a "dose servo", "chamber" or "gas" fault holds the beam, or the beam aborts mid-fraction.
Likely causes
Chamber sensitivity drift, a sealed chamber losing gas or drifting with temperature, cabling, or the dose-servo electronics.
What the team does
The unit comes off the schedule until physics confirms calibration; the chamber is replaced and gain re-baselined. An unexpected dose error is treated as a possible misadministration and the delivered dose is reconstructed.
Exam point
The monitor chamber sits in the beam path between the beam-generating components and the patient, and measures delivered output — not prescribed dose.
Beam steering (flatness and symmetry) drift
Morning QA shows flatness or symmetry out of tolerance, the radiation field center is off from the crosshair, or a "steering servo" interlock appears.
Beam steering (flatness and symmetry) drift
Morning QA shows flatness or symmetry out of tolerance, the radiation field center is off from the crosshair, or a "steering servo" interlock appears.
Likely causes
Quadrupole or bending-magnet current drift, a steering servo card, or a chamber gain change that only looks like a profile shift.
What the team does
Treatment waits; physics re-tunes steering and repeats profile measurements until QA is in tolerance. Treatment is never run and fixed later.
Exam point
Steering controls flatness and symmetry; energy is set separately by the bending magnet.
Energy drift
Percent depth dose or TPR at 10 cm falls out of tolerance on monthly QA, and the commissioned beam curve no longer matches measured data.
Energy drift
Percent depth dose or TPR at 10 cm falls out of tolerance on monthly QA, and the commissioned beam curve no longer matches measured data.
Likely causes
Bending-magnet current drift, an RF power change, or target and filter wear.
What the team does
Energy is retuned and profiles re-measured; any change to commissioned beam data is made by physics before planning resumes.
Beam shaping: jaws & MLC
Jaws and the multileaf collimator shape the field after the beam is made. Their faults show up as shape errors, aborted arcs, or a field that no longer matches the plan.
MLC leaf fault
"Leaf out of position", motor overload, or a leaf-bank collision aborts an IMRT or VMAT beam; log files show leaves arriving late or not at all.
MLC leaf fault
"Leaf out of position", motor overload, or a leaf-bank collision aborts an IMRT or VMAT beam; log files show leaves arriving late or not at all.
Likely causes
Leaf motor or encoder wear, gear and belt wear, calibration drift, a broken leaf tip or debris in the bank, brake failure, or low air pressure on pneumatic MLCs.
What the team does
Leaves are re-homed and recalibrated or replaced, then MLC QA (leaf position, picket fence, leaf speed) runs before the next IMRT patient.
Exam point
MLC QA checks leaf position accuracy, leaf speed and the picket-fence pattern.
Jaw and accessory interlock fault
Jaws stop matching the planned field size, an asymmetric-jaw fault appears, or the accessory tray or electron applicator is not recognized so beam-on is blocked.
Jaw and accessory interlock fault
Jaws stop matching the planned field size, an asymmetric-jaw fault appears, or the accessory tray or electron applicator is not recognized so beam-on is blocked.
Likely causes
Jaw encoder or potentiometer drift, a failed accessory-tray sensor, or an electron-applicator interlock switch.
What the team does
Jaws are recalibrated or the sensor replaced, then the light field and radiation field are checked for agreement.
Motion systems
Gantry, collimator and couch must land exactly where the plan expects. Motion faults are the ones most likely to stop a fraction half-delivered.
Gantry or collimator rotation fault
Arc therapy aborts partway through with a gantry speed, rotation or interlock fault, leaving the patient with an incomplete fraction.
Gantry or collimator rotation fault
Arc therapy aborts partway through with a gantry speed, rotation or interlock fault, leaving the patient with an incomplete fraction.
Likely causes
Encoder or potentiometer drift, drive motor or brake wear, the gantry speed servo, or cable-wrap problems.
What the team does
Delivered MU and arc position are read from record-and-verify, then the fraction is completed or re-planned; angle and speed accuracy are checked before arcs resume.
Exam point
Arc treatments need accurate constant gantry speed — a speed error becomes a dose error.
Couch drive or indexing fault
The couch will not drive or index to the planned position, a couch interlock appears, or the recorded position disagrees with the setup sheet.
Couch drive or indexing fault
The couch will not drive or index to the planned position, a couch interlock appears, or the recorded position disagrees with the setup sheet.
Likely causes
Drive motor or brake, encoder drift, pendant or control fault, and worn indexing-bar or couch-top hardware.
What the team does
Position is verified against the plan before treatment; the couch is re-indexed and service replaces the drive or sensor.
Collision and door interlock fault
Beam will not start, or a collision interlock stops a rotation or couch move that the plan expects to be clear.
Collision and door interlock fault
Beam will not start, or a collision interlock stops a rotation or couch move that the plan expects to be clear.
Likely causes
Mis-set collision zones, a faulty pressure sensor on the accessory holder, or a door-interlock switch that does not close cleanly.
What the team does
The unit stays out of service until the interlock chain is verified. Interlocks are never bypassed.
Support systems: cooling, air & power
A linac is also a plumbing and refrigeration machine. Many unexplained shutdowns trace back to cooling, compressed air or power.
Cooling water or chiller fault
An overtemperature or water-flow fault shuts the beam off after a few minutes, or the machine will not come up in the morning.
Cooling water or chiller fault
An overtemperature or water-flow fault shuts the beam off after a few minutes, or the machine will not come up in the morning.
Likely causes
Chiller failure, low coolant, a failing pump, a clogged filter, or an overheated treatment room.
What the team does
Facilities and service check the cooling loop and room temperature; the unit stays down until flow and temperature are stable.
Compressed air or power supply fault
Pneumatic MLCs or brakes lose pressure and fault intermittently; or the unit drops out with a high-voltage, contactor or power-supply fault.
Compressed air or power supply fault
Pneumatic MLCs or brakes lose pressure and fault intermittently; or the unit drops out with a high-voltage, contactor or power-supply fault.
Likely causes
Compressor or regulator failure and air-line leaks; aging power supplies, high-voltage tanks and contactors.
What the team does
Air pressure is checked at the machine before service is called; electrical faults are service-only repairs.
Imaging & informatics
Modern treatment depends as much on the imaging chain and the network as on the beam itself.
kV imager or EPID fault
Images are noisy, show bad pixels or rings, or the detector arm will not deploy, so image-guided alignment cannot be trusted.
kV imager or EPID fault
Images are noisy, show bad pixels or rings, or the detector arm will not deploy, so image-guided alignment cannot be trusted.
Likely causes
Panel or gain calibration drift, detector arm motor fault, kV source aging, or image-chain electronics.
What the team does
Image QA (uniformity, noise, resolution, geometric accuracy) runs before IGRT is used for alignment; the panel is recalibrated or replaced.
Record-and-verify / network fault
The machine cannot pull the plan, reports a parameter mismatch, or shows "R&V offline"; treatment will not start or stops mid-field.
Record-and-verify / network fault
The machine cannot pull the plan, reports a parameter mismatch, or shows "R&V offline"; treatment will not start or stops mid-field.
Likely causes
Network or cable fault, R&V server or interface engine down, a plan that was never approved, or a parameter table that does not match the machine.
What the team does
Parameters are never hand-entered to work around record-and-verify. The plan version is confirmed, IT or service restores the link, and delivered dose is reconciled in R&V before the fraction continues.
Exam point
Record-and-verify systems are the electronic backstop against wrong-patient, wrong-site treatment.
How faults get caught
- Daily QA catches output, energy, flatness and symmetry drift, lasers, door interlock and imaging problems before the first patient.
- Monthly and annual QA catch slower mechanical wear — MLC position, jaw calibration, gantry and couch accuracy, imaging quality.
- Fault logs and machine uptime are reviewed together: a fault that recurs weekly is a service escalation, not a routine nuisance.
- When a fault interrupts a fraction, the delivered dose is reconciled in record-and-verify before treatment resumes — never estimated by eye.