The expectation to find cancers in X-rays doesn't affect accuracy of expert radiologists, although it does influence how much time is spent staring at the image, according to a new study.

Radiologists looking for lung nodules on chest X-rays spent more time scrutinizing the image if told beforehand that the group of images it was part of had more cancer cases, according to a study released this month in Radiology. However, accuracy was unaffected.

"Overall, findings of this study showed no evidence that the accuracy of expert radiologists is altered due to changing prevalence expectation rates," wrote the authors, led by Warren M. Reed, a lecturer in diagnostic radiography with the University of Sydney in Australia.

The study upends long-held suspicions that the expectation to find an abnormality increases the chance it will be found, and that lack of expectations makes it more likely for the tumor to be missed.

In the study, researchers tested 22 experienced radiologists. The physicians were divided into three groups, with each asked to review 30 posteroanterior chest images twice. Half the images were digitally altered to make it seem that cancer was present. (The researchers said using digital alterations, rather than actual cancers, helped ensure the results were less ambiguous.)

The radiologists were then told that each group contained a specific number of abnormal images, either nine, 15 or 22, or they weren't told how many to expect.

No significant differences appeared in how many images were found, the researchers said. However, radiologists told to expect more nodules spent more time looking at the images, had more eye movements and lingered their gaze longer on specific spots.

Reed told ABC News that radiologists spend on average 12 seconds looking at an X-ray, and that, counterintuitively, the longer they spend looking at an image, the worse their accuracy.

"A lot of people think they'll sit down and look carefully for a long time. Actually the reverse works. The longer you spend looking at an image the more chance that confusion and uncertainty will set in," he said.

Reed said the study could prompt further research into anecdotal reports from radiologists, such as mammographers, that low rates of positives sometimes lead to "lapses in concentration and attention."

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