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CT scans to measure lung tumours can be unreliable, potentially leading patients and doctors to believe the cancer is growing when it's not, a new study suggests.
In principle, that could mean stopping a treatment that is actually keeping the tumour in check, researchers say.
"The patient and the doctor both need to understand that small changes don't necessarily mean much," co-author Dr Gregory Riely at Memorial Sloan-Kettering Cancer Centre in New York said.
"Changes of up to 10% can happen simply as a result of the inherent variability of CT imaging," he said.
The study
Dr Riely's study, published in the Journal of Clinical Oncology, is the first to test how reliable lung cancer scans are – work that's long overdue, experts say, because CT scans have already become the gold standard for monitoring cancer growth and treatment response.
"It's the sense of, really, is this only happening now?” said Dr Michael Maitland of the University of Chicago. "This is telling us scientifically how much noise is naturally there without any treatment or the cancer getting worse. It's an important thing to do whenever you are going to use any kind of marker for a disease."
For the study, the Sloan-Kettering team asked patients with late-stage lung cancer if they'd be willing to have two chest CT scans done within minutes, and 33 patients said yes.
The results
Then the researchers gave the images to three radiologists who had no idea that the scans had been repeated before the tumours could have grown or shrunk appreciably.
But the radiologists reported many changes, ranging from 23% shrinkage to a 31% growth.
Overall, 3% of the tumours appeared to have grown so much that doctors would diagnose disease progression according to common criteria, and the smaller the tumour, the bigger the variation.
Dr Riely said some doctors will make treatment decisions based on tiny changes seen on scans, although that might be a costly mistake, according to the new findings.
"We begin to put more and more stock in the data without really understanding the true variability of those measurements," he said. "Small changes are not clinically meaningful and we should not alter clinical care based on them."
Dr Riely stressed, however, that his results don't mean patients should get repeat scans, which would increase their radiation exposure.
Hope for the future
Most likely, the results also apply outside of lung cancer, although patients' breathing could make the chest scans particularly variable.
Dr Maitland said the findings would also help drug developers, who look at increasingly small changes in tumour size during drug tests, forgetting that the scans might be unreliable at that scale.
"Many of the individuals analysing data that way perhaps are not aware of that limitation," he said.
With the new data, scientists can build better models of cancer progression that might save both time and money in clinical trials.
"There is a real opportunity here to update our systems and take advantage of the new technology," said Dr Maitland.
(Reuters Health, July 2011)
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