Couch shifts and 6DOF corrections explained
Once you've matched today's image to the plan, the result is a couch correction — the shift that moves the patient into the planned position. This guide breaks down the six degrees of freedom, the couch coordinate names you'll see on the read-out, the difference between 5DOF and 6DOF, and how action levels decide whether you treat or intervene.
Educational overview. General background for radiation-therapy learners — not clinical advice or protocol.
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The six degrees of freedom Couch axes: Lat, Lng, Vrt The rotations: pitch, roll, yaw 5DOF vs 6DOF couches Reading and applying a correction Tolerances and action levels Common mistakes How to practice reading shifts FAQThe six degrees of freedom
Any rigid object — including a patient lying on the couch — can be displaced in exactly six independent ways: three translations (sliding along each axis) and three rotations (turning about each axis). Together these are the six degrees of freedom (6DOF), and they completely describe how today's setup differs from the plan. A correction is simply the inverse displacement that cancels the error.
Couch axes: Lat, Lng, Vrt
The three translations are usually labelled with IEC-style couch names:
Sign conventions (which direction is positive) vary between systems and are often expressed relative to the patient or the isocenter, so always confirm the convention on the machine you're using rather than assuming.
The rotations: pitch, roll, yaw
The three rotations are named for how they tip and turn the patient:
5DOF vs 6DOF couches
How many of these you can actually correct depends on the couch and the imaging:
- Standard (5DOF) couch: applies the three translations plus yaw (couch rotation), but cannot apply pitch and roll mechanically. With planar 2D/2D imaging, yaw is also hard to measure, so corrections are often described as 5DOF — three translations and two rotations read from the images.
- Robotic 6DOF couch: adds motorised pitch and roll, so a full 6DOF correction can be applied. This is paired with volumetric imaging (CBCT) that can measure all three rotations, and is common for SRS and SBRT.
For how each imaging method resolves these axes, see CBCT 6DOF registration and 2D/2D portal vs DRR matching.
Reading and applying a correction
- Match the setup image to the reference until the anatomy or markers line up.
- Read the residual — the remaining shift and rotation on each axis after your match.
- Check it against tolerance before doing anything.
- Apply the couch move (automatically or manually) to take out the translations, and the rotations the couch supports.
- Re-image to confirm for high-precision treatments, or where the shift was large.
Tip: a large rotation can masquerade as a translation near the edges of the field. If a translation won't resolve on both views at once, suspect an uncorrected pitch, roll, or yaw.
Tolerances and action levels
An action level is the residual-error threshold that separates "treat" from "intervene". Inside it, the remaining error is clinically acceptable and you proceed; outside it, you re-image, reposition, or escalate. Thresholds depend entirely on the technique:
- Routine conformal / IMRT: typically a few millimetres and a degree or two.
- SRS / SBRT: sub-millimetre and a fraction of a degree, which is why these treatments need a 6DOF couch and volumetric guidance.
The exact numbers are set by your department's protocol — the principle to learn is that the tolerance scales with how conformal and how high-dose the treatment is.
Common mistakes
- Correcting to the bone when the target moves off it — for example aligning the pelvis on a prostate case instead of the implanted fiducial markers.
- Ignoring rotation because the couch can't fully apply it — note it, because it still affects coverage.
- Assuming a sign convention instead of confirming which direction is positive on the system in front of you.
- Over-correcting random error — chasing a single day's noise rather than the systematic trend.
How to practice reading shifts
Reading a couch correction quickly and correctly is a trainable skill. The RT Image Matching Trainer applies a hidden 6DOF (or 5DOF) setup error to each case and shows a live read-out of the residual translation and rotation as you work, graded against tolerance. You can practice on 2D/2D and CBCT cases across multiple sites until reading and applying a shift becomes second nature.
Practice reading couch shiftsHidden 6DOF errors, live residual read-out, graded to tolerance.
Open the trainerFrequently asked questions
What are the six degrees of freedom in radiation therapy?
They are three translations and three rotations of the patient relative to the plan: lateral, longitudinal and vertical translation, plus pitch, roll and yaw rotation. Together they fully describe any rigid displacement on the couch.
What do Lat, Lng and Vrt mean on the couch read-out?
They are the three translation axes: Lat (lateral) is left–right, Lng (longitudinal) is superior–inferior along the couch, and Vrt (vertical) is up–down. A 6DOF read-out adds pitch, roll and yaw.
What is the difference between a 5DOF and a 6DOF correction?
A 5DOF correction resolves three translations plus two rotations; yaw (couch rotation) is not corrected — common with planar imaging and standard couches. A 6DOF correction also applies pitch and roll, needing a robotic 6DOF couch and volumetric imaging such as CBCT.
What is a tolerance or action level for couch shifts?
It is the residual-error threshold below which you treat and above which you intervene — re-image, reposition, or escalate. Routine thresholds are a few millimetres; SRS and SBRT are much tighter. Always follow your department's protocol.
Educational use only. This guide is general background for learning, not clinical protocol — always follow your department's procedures. The trainer is not a medical device; its patient offsets and values are fictional, and its imaging derives from de-identified, openly licensed research datasets (see image credits).