What is IGRT? Image-guided radiation therapy explained

A guide for radiation therapy students · ~8 min read

Image-guided radiation therapy (IGRT) is the everyday practice of imaging the patient at the treatment machine to confirm they are in the right place before the beam turns on. This guide explains what IGRT is, why it matters, the imaging modalities you'll meet, the daily verification workflow, and the difference between online and offline correction.

What IGRT means

IGRT — image-guided radiation therapy — is the use of imaging immediately before (and sometimes during) each treatment fraction to verify and correct the patient's position relative to the plan. A course of radiotherapy is delivered over many fractions, and the patient is set up fresh each day. Small day-to-day differences in position are unavoidable; IGRT measures them and removes them before treatment, so the dose lands where the plan intended.

In practice, IGRT means acquiring a setup image at the machine, matching it to a reference derived from the planning CT, reading off the shift, and moving the couch to correct it. The matching step — lining up today's anatomy with the plan — is the skill at the heart of the job.

Why IGRT matters: margins and dose

Every plan adds a margin around the target to account for setup uncertainty and motion. The less certain you are about position, the larger that margin must be — and a larger margin means more healthy tissue inside the high-dose region. By measuring and correcting position each day, IGRT shrinks geometric uncertainty, which lets planners use tighter margins.

Tighter margins are what make modern high-dose techniques possible. SBRT (stereotactic body radiotherapy) and SRS (stereotactic radiosurgery) deliver large doses in a few fractions to small targets, and they depend on millimetre-level position confidence — which only daily image guidance can provide while still protecting nearby organs at risk.

IGRT imaging modalities

"IGRT" is an umbrella term — several imaging technologies sit under it, often used together:

• Planar kV imaging (2D/2D): a pair of orthogonal kV X-rays matched to DRRs. Fast, low-dose, and excellent for bony verification. See our guide on 2D/2D portal vs DRR matching.
• MV portal imaging: images formed with the treatment beam itself. Lower soft-tissue contrast than kV, but useful for field verification.
• kV cone-beam CT (CBCT): the gantry rotates the kV source and panel around the patient to reconstruct a 3D volume, enabling soft-tissue matching and full rotational assessment. See CBCT 6DOF registration.
• Implanted markers: small radio-opaque gold fiducial seeds (or electromagnetic transponders) give a precise, high-contrast surrogate for a soft-tissue target such as the prostate.
• Surface-guided RT (SGRT): optical cameras track the skin surface in real time for setup and intra-fraction monitoring, with no extra imaging dose.
• Ultrasound: used at some centres for soft-tissue localisation, notably in the pelvis.

The daily IGRT workflow

1. Initial setup to skin marks, tattoos, and lasers, with any immobilisation devices in place.
2. Acquire the verification image — a 2D/2D pair, a CBCT, or a surface capture, depending on the protocol.
3. Register the image to the reference (DRR or planning CT) by aligning anatomy or markers.
4. Read the correction — the residual translation and rotation needed to match the plan.
5. Evaluate against tolerance. Inside the action level, apply the couch shift; outside it, investigate before proceeding.
6. Apply the shift and treat, sometimes re-imaging to confirm for high-precision cases.

Online vs offline correction

There are two broad strategies for acting on what the images show:

Online correction is the default where volumetric or fast planar imaging is available, because it catches both systematic and random error on the day. Offline protocols still appear where imaging is acquired less frequently.

Systematic vs random error

Setup error splits into two components, and IGRT handles them differently:

• Systematic error is a consistent offset present from the start (for example a planning-to-treatment difference). It biases every fraction the same way and has the largest dosimetric impact, so it is the priority to detect and remove.
• Random error is the day-to-day scatter around the mean. It blurs the delivered dose rather than shifting it, and online imaging is what keeps it in check fraction by fraction.

How to practice IGRT matching

The judgement at the centre of IGRT — is the anatomy actually aligned, and what shift corrects it? — is built by repetition with feedback. The RT Image Matching Trainer lets you practice both planar and volumetric guidance: 2D/2D portal-to-DRR matching and CBCT 6DOF fusion across brain, pelvis, thorax, breast, spine SBRT, lung SBRT, acoustic neuroma, and prostate fiducial cases. Each applies a hidden setup error you work back to within tolerance, with a live residual-error readout.

Frequently asked questions