What is a CT scan used for in cancer?

A CT (computed tomography) scan uses special X-ray equipment and a computer to create detailed pictures of the inside of your body, slice by slice.

According to the National Cancer Institute, CT has many uses in cancer care. It can:

• Screen for cancer
• Help diagnose a tumor
• Give information about the stage of a cancer
• Show exactly where to perform a biopsy
• Guide certain local treatments
• Help plan radiation therapy or surgery
• Check whether a cancer is responding to treatment
• Detect whether a tumor has come back

That list spans the whole arc of care. It is why you may have many CT scans over time, each with a different purpose. The scan before treatment sets a baseline. The one during treatment measures change against it.

Contrast, and why they ask about your kidneys

Many cancer CT scans use contrast material, a dye based on iodine or barium. You may swallow it, get it in a vein, or receive it as an enema. It makes some tissues stand out that would otherwise blend together.

Two risks come with it. NCI says that in very rare cases contrast can cause an allergic reaction. These range from mild itching to serious trouble breathing. Contrast can also cause kidney problems in some patients. That is why you may have a blood test to check your kidneys first.

Tell the staff before the scan if you have ever reacted to contrast, or if you have kidney disease.

The radiation question, with real numbers

CT uses more radiation than a plain X-ray, and people are right to ask about it. NCI gives figures that make the scale concrete:

• Natural background radiation: about 3 mSv per year, just from living
• A low-dose CT of the lungs: 1.5 mSv, about 6 months of background
• A typical chest CT: 6.1 mSv, about 2 years of background
• CT colonography: about 6 mSv

NCI's conclusion is that the added cancer risk from one CT scan is still small. For diagnosis, the benefit outweighs the risk. So the sensible move is not to refuse scans. It is to ask what a given scan will change about your plan.

Two screening uses worth knowing

Low-dose CT of the lungs is used to screen for lung cancer. NCI relays the US Preventive Services Task Force recommendation: annual screening for people aged 50 to 80 with a 20 pack-year or more smoking history who currently smoke or quit within the last 15 years. The National Lung Screening Trial found it cut lung cancer deaths by 20%. It also turns up false alarms. And it can find cancers that would never have caused harm.

CT colonography is sometimes called virtual colonoscopy. It can find polyps and tumors in the colon. There is one catch. If something turns up, you still need a standard colonoscopy to remove it.

Reading your results

No single scan diagnoses cancer on its own. Your doctor reads a CT scan alongside your other tests and is the best person to explain what it means for you.

When a result comes back, the useful question is a comparison. Ask how this scan differs from the last one. Then ask what the team will do differently because of it.