New York: CT scans to measure lung tumors can be unreliable, potentially leading patients and doctors to believe cancer is growing when it`s not, according to a study.
In principle, that could mean stopping a treatment that is actually keeping the tumor in check, researchers said in the study, which they said was the first to test how reliable lung cancer scans are, and appeared in the Journal of Clinical Oncology.
"The patient and the doctor both need to understand that small changes don`t necessarily mean much," said Gregory Riely, at Memorial Sloan-Kettering Cancer Center in New York.
"Changes of up to 10 percent can happen simply as a result of the inherent variability of CT imaging."
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 within minutes. Thirty-three patients said yes.
Doctors normally scan such patients every few months to see if their tumor is growing, which could be a signal to change drugs.
Then the researchers gave the images to three radiologists who had no idea the scans had been repeated before the tumors could have grown or shrunk appreciably.
But the radiologists reported many changes, ranging from a 23 percent shrinkage to a 31 percent growth.
Overall, three percent of the tumours appeared to have grown so much that doctors would diagnose disease progression according to common criteria. And the smaller the tumor, the bigger the variation.
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 study`s findings.
"We begin to put more and more stock in the data without really understanding the true variability of those measurements," he said.
"The changes are not clinically meaningful and we should not alter clinical care based on them."
Riely did say, though, that the findings did not mean that patients should get repeat scans, which would increase their radiation exposure.
Most likely, the results also apply outside of lung cancer, although patients` breathing could make the chest scans especially variable.
Michael Maitland, at the University of Chicago and who wrote an editorial accompanying the study, said it was surprising such a study had not been done up to now and it was likely to be useful.
"This is telling us scientifically how much noise is naturally there without any treatment or the cancer getting worse," he said.
"It`s an important thing to do whenever you are going to use any kind of marker for a disease.
He added that the findings will be helpful to drug developers, who look at increasingly small changes in tumor size during drug testing, forgetting that the scans might be unreliable at that scale.
In addition, the new data can help scientists build better models of cancer progression that might save both time and energy in clinical trials.