After Immunotherapy, Imaging Test Results Can Be Misleading

Imaging tests such as PET/CT scans are an important tool to guide next steps after you’ve had treatment. These scans can show your doctor how well your cancer has responded to the treatment — whether it has gotten smaller, or if it’s time to switch to a different type of therapy.

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These tests are pretty accurate after chemotherapy, because areas where the cancer cells have died off appear visibly smaller. But after newer treatments like immunotherapy, PET/CT scans “may be a little confusing,” Dr. Jakub Svoboda, medical oncologist at Penn Medicine, tells SurvivorNet.

What Your Scan Might Show

Positron emission tomography (PET)/computed tomography (CT) is an imaging technique that combines two tests in one. PET uses a radioactive sugar, which travels through your body to areas of cancer. Because cancer cells use up more energy than healthy cells, they absorb more of the tracer. Areas where the tracer collects show up as hot spots on the imaging scan.

A CT scan takes x-ray images from many different angles. Together, these tests can show whether any lymph nodes or organs are enlarged — a sign that your cancer is growing — and whether your cancer is metabolically active.

PET/CT results are pretty straightforward after you’ve had chemotherapy. If the chemo works, the areas of cancer should appear visibly smaller on the scan. Yet the results are much less clear after newer treatments such as immunotherapy.

“When you think about how immunotherapies work, they actually don’t kill the fast dividing cells. What they do is they stimulate your own immune system to attack the cancer,” Dr. Svoboda explains. As the immune cells attack abnormal white blood cells called lymphocytes, “you will likely get some sort of inflammation in the sites of the tumor.”

That inflammation can make it appear as though your cancer is growing, when it has actually improved. Doctors call this false-positive result a “flare” or “pseudo-progression.” “You can actually see progression on a PET/CT, meaning that lymph nodes that were not metabolically active previously are now active, or some of the lymph nodes may be a little bit larger, while the patient may be doing really well,” Dr. Svoboda says.

If doctors read PET/CT scans after immunotherapy in the same way as they would read them after chemotherapy, they might mistakenly label a patient’s cancer as progressing when they are actually having a response, he adds.

The Search for a More Sensitive Imaging Test

Accurate test results are important, because they can help determine which course of action your doctor recommends next. Researchers want to ensure that PET/CT scans provide accurate results for every type of cancer treatment. “There’s definitely a lot of potential for improvement,” Dr. Svoboda tells SurvivorNet.

One way to fine-tune the PET/CT scan is to use a contrast material other than sugar. Another is to add different measurements. “There’s a lot of interest in what we refer to as circulating tumor DNA, looking in the bloodstream for any pieces of the tumor,” he says. As cancer cells grow and then die off, they release pieces of their DNA into the bloodstream. A decrease in the amount of circulating tumor DNA can be a sign that the cancer is shrinking and treatment is working.

One day, imaging tests may be fine-tuned to accommodate new cancer treatments. For now, it’s important to keep in mind that these tests aren’t perfect. They can sometimes pick up cancer that is no longer there, or miss cancer cells that remain.

Your oncologist can help you make sense of your test results. Don’t be afraid to ask questions if anything isn’t clear to you. It’s also ok to seek out a second opinion if you think your test results don’t accurately reflect the status of your cancer.

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