My CT Scan Is Already Done—Why Do I Need Another Scan for Radiotherapy?

Dr Vidhi Vaghasiya | MD Radiation Oncologist | Bhavnagar


You may have already undergone a CT scan, MRI or PET-CT. Then your radiation oncologist asks you to undergo another CT scan before radiotherapy.

Naturally, you may wonder:

“Why repeat it? Can’t you use my previous scan?”

This new scan is usually not being performed because we doubt the earlier report. It has a different purpose. A diagnostic scan helps us find and understand the disease. A radiotherapy planning CT helps us design and accurately deliver your treatment.

Let me take you through the process.

A diagnostic scan finds the problem

A diagnostic CT, MRI or PET-CT helps answer questions such as:

These scans are extremely important. However, they are usually performed with the patient lying in a position that is convenient for diagnosis—not necessarily in the exact position required for daily radiotherapy.

Your arms, neck, legs or pelvis may be positioned differently. The scanning table may be curved. Your bladder or rectum may not have been prepared in the required way. Immobilisation equipment used during radiotherapy may not be present.

A scan can therefore be excellent for diagnosis but unsuitable for accurate radiation-dose calculation.

The planning CT creates your treatment map

Radiotherapy planning CT is also called CT simulation.

The word “simulation” does not mean that treatment is being given. No therapeutic radiation is delivered during this appointment. The team is reproducing—or simulating—the position in which you will receive treatment later.

The planning scan allows us to create a three-dimensional map of:

The scan also provides information needed by the planning computer to calculate how radiation will travel through different tissues.

Bone, lung, muscle and air do not interact with radiation identically. Accurate CT information helps the medical physicist and radiation oncologist calculate where the prescribed dose will go and how much radiation nearby organs may receive.

Why is my position so specific?

Radiotherapy is generally delivered over several sessions. Your position should be reproduced as closely as possible every day.

Depending on the treatment site, you may require:

These devices are not restraints or punishments. They help you remain comfortable and reduce unnecessary movement.

For head-and-neck or brain radiotherapy, a warm thermoplastic sheet may be moulded over your face. Once it cools, it becomes firm and keeps your head and neck in the planned position. The mask has openings and does not stop you from breathing.

Please tell the team beforehand if you have severe anxiety or claustrophobia. Most patients manage well after an explanation and a practice session. We can use reassurance, breathing techniques, minor mask adjustments or medication when genuinely required.

Why are marks placed on my skin or mask?

Reference marks help the radiotherapy team align you with the treatment-room lasers.

Depending on the centre and treatment site, marks may be placed on the mask, on temporary stickers or directly on the skin. Some departments use tiny permanent tattoo dots.

These marks do not show the complete tumour boundary. They are reference points that help us reproduce your position and connect your body to the virtual coordinates of the treatment plan.

Do not scrub off skin marks unless your team tells you that they are no longer needed. If a mark fades, do not redraw it yourself. Ask the radiotherapy staff to check it.

Why do I need a full bladder or empty rectum?

For some pelvic cancers, the position of the bladder and rectum can change the location of the uterus, cervix, prostate, bowel and other structures.

A comfortably full bladder may move parts of the small bowel away from the treatment area and create a reproducible internal position. An excessively full bladder, however, may cause pain and be difficult to reproduce every day.

Similarly, excessive rectal gas or stool can alter internal anatomy, particularly during prostate radiotherapy.

Your centre may therefore give you specific instructions about drinking water, passing urine, diet, bowel preparation or timing. These instructions vary according to the cancer, technique and departmental protocol.

Please follow your own team’s instructions rather than copying another patient’s preparation.

Why isn’t treatment started immediately after the planning scan?

Once the scan is completed, a substantial amount of work happens before the first treatment.

The radiation oncologist reviews the scan and outlines the areas requiring treatment. Nearby organs that should be protected are also marked. Depending on the site, MRI, PET-CT or previous scans may be fused with the planning CT to improve accuracy.

The medical physicist then designs a treatment plan. The aim is to cover the required target while keeping the dose to healthy organs within safe limits.

The radiation oncologist evaluates:

Some plans are straightforward. Others—particularly IMRT, VMAT, stereotactic radiotherapy and re-irradiation—require complex calculations and patient-specific quality checks.

This planning interval is not idle waiting. 

It is a safety step.

Will I receive radiation during the planning CT?

You receive only the radiation associated with the CT scan itself, similar in principle to other medical CT imaging. You do not receive the therapeutic radiotherapy dose at simulation.

Contrast dye may be used when it helps distinguish the tumour or blood vessels. Tell the team if you have had a previous contrast reaction, significant kidney disease, asthma, diabetes medication requiring special instructions or possible pregnancy.

What if my body changes after planning?

Significant weight loss, swelling reduction, tumour shrinkage or a change in body shape may affect how well the original plan fits.

This is particularly relevant during some head-and-neck treatments. If your mask becomes loose or your anatomy changes substantially, your radiation oncologist may order a new planning scan and create an adapted plan.

This does not automatically mean that treatment has failed. It may simply mean that the plan needs to be adjusted to your current anatomy.

Why is imaging sometimes performed again on the treatment machine?

Many modern radiotherapy systems use X-rays or cone-beam CT before treatment. These images verify that your external and internal position matches the planned position.

This is called image-guided radiotherapy. It does not replace the original planning CT. It acts as a daily or periodic safety check.

The team may shift the treatment couch by a few millimetres after reviewing these images. Such corrections are a normal part of precise treatment.

What should you do on planning day?

Most importantly, tell us if the position is painful or impossible to maintain. The position must be accurate, but it also needs to be reasonably reproducible throughout your course.

Your diagnostic scans told us where the cancer may be. The planning CT helps us turn that information into a safe, personalised and deliverable treatment.

It is not “just another scan.” It is the foundation upon which your radiotherapy plan is built.