IGRT — Why They Scan You Before Every Single Session

IGRT is image-guided radiotherapy: a quick scan taken on the treatment machine immediately before each session to confirm your body is in exactly the position your plan was built around. It images your position, not your cancer. If anything has shifted, the treatment couch is adjusted by a few millimetres before the beam is switched on.

Most people arrive at this page for one reason. The scan happens every day, nobody explains it, and the mind fills the gap: they must be checking whether it's working. Something must have changed. That reading is understandable and it is wrong. The verification image is a set-up check performed for every patient on an image-guided plan, on every day of treatment, whether the course is going perfectly or not.

NCCN and ASTRO patient-education materials describe image guidance as a routine part of modern external beam radiotherapy for many treatment sites — a standard step in delivering the plan, not an investigation ordered because of a problem.

Your radiotherapy is delivered at an NABH-accredited partner centre; CION Cancer Clinics coordinates your treatment plan, your oncology team and your care throughout, so the explanation below applies whichever centre delivers your daily sessions.

Some forms of image guidance do use a small amount of additional radiation — and your team already knows that. X-ray-based verification, such as a cone-beam CT or a pair of flat X-ray images, delivers an imaging dose that is far lower than the treatment dose given in that same session. Your medical physicist accounts for it when the plan is written.

Not every form of image guidance uses ionising radiation at all. Surface-mapping camera systems, ultrasound guidance and MRI-based guidance carry no X-ray dose. Which method your plan uses depends on the treatment site and on the equipment at the centre delivering your sessions.

The honest framing is a trade-off, not a free lunch. The small imaging dose buys the confidence to treat a tighter volume, because the team is no longer guessing at day-to-day movement. In many patients that means less healthy tissue sits inside the high-dose region across the full course — which is the reason the imaging is considered worth doing in the first place.

If the imaging dose is on your mind, ask your radiation oncologist two things: which imaging method your plan uses, and how often it is repeated. Both are reasonable questions and both have a specific answer for your case.

Because you are not in an identical position every day, and neither are your organs. Your treatment plan was built on one planning scan, often taken a week or more before your first session. Daily imaging is how the team confirms that today's reality still matches that plan — before, not after, the beam is delivered.

Doing it once would not work. A check on session one cannot catch a shift that happens on session fourteen. These are the things that move between sessions:

• How you lie on the couch — small differences in shoulder position, chin angle or arm placement change where internal structures sit.
• Bladder and bowel filling — a major factor in pelvic treatment, which is why bladder and bowel preparation instructions are given so firmly.
• Breathing and chest movement — in lung and upper-abdominal treatment, the target moves with every breath you take.
• Swelling, or its resolution — post-surgical swelling settling over a course of treatment changes the shape of the area being treated.
• Weight change over several weeks — a mask or cast fitted at the start can become slightly loose as treatment progresses.
• Tumour or organ shrinkage — where a mass shrinks during treatment, the surrounding anatomy resettles around it.

None of these are mistakes. They are ordinary human variation, and image guidance exists precisely because they are expected.

Nothing dramatic happens, and nothing is your fault. The radiation therapists compare your scan against your planning images, measure the difference, and move the treatment couch until your position matches the plan again. Corrections of a few millimetres are routine. Detecting and fixing that shift is the entire reason the scan is taken.

Here is what actually happens in the room when the images do not line up:

1. The therapists overlay the images. Today's verification scan is matched against your reference planning images on screen, usually within a minute.
2. The offset is measured. The difference is calculated in three directions — up-down, left-right, in-out — and often rotation as well.
3. The couch moves, not you. The treatment couch is shifted remotely to correct the offset. You are generally asked to stay still rather than reposition yourself.
4. A re-check where the shift was large. If the correction was substantial, a second image may be taken to confirm you are now where the plan expects.
5. Treatment proceeds as planned. In the great majority of sessions the beam is then delivered exactly as scheduled, and your course continues unchanged.
6. Repeated or large shifts are escalated. Your radiation oncologist is told, and may adjust your immobilisation, your set-up or your preparation instructions. Occasionally a session is rescheduled — and if that happens, you will be told why.

Patients sometimes tell us they lie rigid on the couch, terrified of moving and "ruining" the treatment. You don't need to. Breathe normally, stay relaxed, and let the imaging do the job it was put there to do.

No. The verification image is a set-up check, not a response assessment. It is taken for every patient on an image-guided plan, on every day of treatment, regardless of how the course is going. It is not designed to judge whether a tumour is shrinking, and your team does not read it that way.

The confusion is easy to understand. Before your diagnosis, every scan you had was a diagnostic scan, and every diagnostic scan came with a result that mattered. The verification image belongs to a different category entirely — it sits closer to a carpenter checking a measurement before each cut than to a test with a verdict attached.

Response to treatment is assessed separately, using dedicated diagnostic imaging at points your oncologist plans in advance. Those scans are scheduled deliberately, you will know they are coming, and the results are discussed with you. If nobody has mentioned a response scan, that is not a sign of bad news — it usually means the assessment point has not arrived yet.

A reasonable thing to say to your therapists: "Is today's image just a position check, or is anyone reviewing it for anything else?" You will get a straight answer, and it will almost always be the first.

• Told your treatment machine will rotate around you while the beam is on? VMAT Explained: Rotational Radiotherapy in Plain Language covers the delivery technique that image guidance most often accompanies.
• Comparing an older machine against a newer one at another centre? Cobalt Machine vs Linear Accelerator: Is an Older Machine Unsafe? is the honest comparison to read before you switch centres over equipment alone.
• Facing a brain diagnosis and hearing the word "surgery" attached to a radiation device? Is Gamma Knife Actually Surgery? No Cuts, No Anaesthesia Explained clears up what that name really means.
• Recommended internal radiation instead of daily external sessions? HDR vs LDR Brachytherapy: Sessions, Stay and Safety explains how those two approaches differ in schedule and hospital stay.
• For the full picture of how radiotherapy is planned, verified and coordinated with NABH-accredited partner centres, see our radiation therapy hub.

Still uneasy about something in your own plan? Book a free consultation and a radiation oncologist will go through it with you directly.