What is a CT scan?

Computed tomography (CT) scans are one of the primary tools doctors use while diagnosing, treating, and following up with colorectal cancer patients. 

A CT scan uses a thin beam of radiation to create images of the inside of your body from various angles. The technician will take many different images, which are then combined by a computer to make one detailed 3D picture.

CT scans can be done much more quickly than MRI or PET scans — so they’re often a good choice when doctors need to get a general idea of what’s going on throughout the entire body. They’re also a good choice for people who are claustrophobic, as MRI scans take much longer to complete. They may also be a good choice for people that have metal implants in their bodies, as some of these implants may be incompatible with MRI machines. The disadvantage is that CT scans use radiation. Your care team will decide how frequently you should get CT scans.

Here’s what a CT machine looks like:

What do I need to do before the scan?

Some patients may be asked to drink an oral contrast, which is a type of dye that allows the radiologist to see your abdomen better. If you have oral contrast, you’ll be asked to stop eating for a specific time before the CT scan. If you have a cold sensitivity, you can ask for your oral contrast without ice! Some patients might feel some stomach discomfort and may need to use the restroom.

If you are having a non-contrast CT scan, or a scan with IV contrast, you will not need to prepare much. If you receive IV contrast, you may need bloodwork done before you receive it. This is to make sure your kidneys are working well, because the dye can affect kidney function.

What happens on scan day?

If you are getting oral contrast, you’ll be asked to arrive about 90 minutes early. When you get there, you’ll register with the front desk and receive your contrast. They’ll ask you to finish drinking it within a specific time frame.

Then, you’ll be taken to a dressing area. You should remove all your jewelry and clothing, and put on a hospital gown. You’ll also be asked to sign some consent forms, outlining the minimal risks of CT scans.

Then, the technician will explain the procedure. If you’re having a contrast CT, they will place an IV. The IV will typically be put in your arm, but in some cases they will use your port (if you have one!). The technician will likely ask you if you’ve ever had an allergic reaction to the contrast dye used.

What happens during the scan?

A CT scan takes less than 15 minutes, and is completely painless.

During the scan, patients lie flat on a narrow table sticking out of a doughnut-shaped machine. The table will move, sliding you through the machine.

The technician will run you through the procedure. If you receive IV contrast, you might feel a warm flush sensation in your groin area. Making sure you’re well hydrated can reduce this sensation! The technician will then give you various directions from a cubicle next to the scan room — such as asking you to hold your breath or stay completely still. You’ll be able to hear the technician, and they will be able to hear you, so if you need help with anything just ask.

What happens after the scan?

It’s a good idea to drink plenty of water following a scan with contrast, to help your body get rid of the dye.

Your scan report should be available within a few days. This will be sent to your doctor and uploaded to your online patient portal, if you have one. If you receive your results before you have an appointment with your care team, it’s a good idea to wait to hear from them before trying to interpret the results by yourself. 

You should also request a CD copy of your scan photos for your records. Place this copy in a safe space. Some hospitals have a $5 charge for them, but they are legally obligated to provide you with a copy.

How often do I need to get a CT scan?

This will depend on your individual circumstances. If you have metastatic CRC and are receiving chemo, immunotherapy, or targeted therapies, scans are usually ordered every 3 months in the US. This is to make sure you’re responding well to treatment. 

If you have finished treatment, and are NED (no evidence of disease), patients who had metastatic CRC might have scans every 3 to 4 months. If you had early stage CRC, scans might be more spread out — such as once every 6 to 12 months initially, then annually for 5 more years.

Oncologists will use different types of surveillance scans (CT, MRI or PET) depending on what they think is the best way to monitor your cancer. So you’ll likely get a variety of scans at different points in your treatment.

How useful are CT scans?

CT scans are very useful for identifying the status of your cancer. CT results — along with other imaging and blood tests — help your care team understand how you are responding to therapy or how you are doing during surveillance.

As with any other test, CT scans have detection limits. Results depend on the size, type, and extent of metastases — as well as many other factors. For example, having scar tissue from previous surgeries! 

It’s important to note that CT scans can miss peritoneal metastases, especially in people with mucionous tumors. CT has also been proven less effective at diagnosing cancer than a PET scan, but CT is often used over PET due to cost differences — and to avoid frequent exposure to the radioactive dye used in PET scans. 

But in general, CT scans are very reliable. If your doctor is unsure of the results of a CT scan, they’ll likely order another test (such as an MRI or PET scan) to get a better look.

Should I be worried about radiation exposure from frequent CT scans?

CT scans expose patients to ionizing radiation, which can damage DNA. In rare cases, this can cause a secondary cancer. 

Depending on the part of the body being scanned, a CT scan exposes you to 1 to 10 millisieverts (mSv), the unit that scientists use to measure radiation exposure.

Each year, the average person is exposed to approximately 3 mSv from the environment. If you are having regular CT scans, you’ll be exposed to more radiation than the average person.

But don’t worry! The amount of radiation exposure is relatively low, and the benefits of being able to accurately monitor your cancer far outweigh the risks. The chances of developing cancer as a result of your CT scans is very low (approximately 1 in 2000). The risks are higher in children and younger adults than for adults over 60 years old. 

With metastatic CRC, patients often get CT scans every 3-4 months. This can add up to a lot of scans over time! PET scans use radioactive tracers, so this also adds to your exposure.

If you have concerns about radiation exposure, it’s a good idea to keep track of the dates and types of scans you’ve had throughout your treatment. Talk to your oncologist about any concerns you have. They might be able to recommend an alternative screening method like an MRI, which does not use radiation. However, they are usually much more expensive than a CT scan.