Contrary to expectations, a CT scan of the abdomen and pelvis does not improve cancer detection in people with unexplained blood clots in their legs and lungs, according to a large clinical trial. The results, published in of the New England Journal of Medicine (2015; doi:10.1056/NEJMoa1506623) , are expected to improve patient care and reduce screening costs around the world.

More than 500,000 Canadians and Americans are diagnosed with blood clots in the lungs and legs each year (referred to as venous thromboembolism). In some cases, the clots are caused by trauma, surgery-prolonged immobility, or a known cancer, but in about half of cases, the cause of the blood clots is unknown.

“Unexplained blood clots have long been thought of as a possible early warning sign of cancer, with previous studies suggesting that up to 10% of patients with unexplained clots will be diagnosed with cancer within the year,” explained lead author Marc Carrier, MD, a hematologist and senior scientist at The Ottawa Hospital in Ottawa, Canada.

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“Some clinical guidelines recommend a CT scan of the abdomen and pelvis in these patients, in addition to other cancer screening, but there has been very little evidence to know if the added CT scan is helpful. We did this study to find out.”

The trial involved 854 patients in nine Canadian centers who had unexplained blood clots in the legs, lungs, or both. Participants were randomly assigned to receive basic cancer screening or basic cancer screening plus a CT scan of the abdomen and pelvis. Basic cancer screening included blood work and a chest X-ray, in addition to gender-specific screening (such as a breast exam, pap smear, and prostate exam) if it had not been conducted in the last year.

The study showed that there was no difference in the number of new cancers detected in the two groups, with approximately 4% of patients from each group being diagnosed with cancer within the next year. There was also no difference in the number of cancer-associated deaths.

“Although it is tempting to believe that more cancer screening is always better, our study shows that this is not necessarily the case,” said Dr. Carrier. “In fact, unnecessary CT scanning has real risks. It can cause stress and anxiety in patients, as well as radiation exposure, and it can lead to over-investigation of false–positive findings. Our study means many patients will now be able to avoid this.”

The results could also lead to significant savings for the health care system.

The study’s senior author, Marc Rodger, MD, a hematologist and senior scientist at The Ottawa Hospital and professor at the University of Ottawa in Ottawa, Canada, noted that the 4% incidence of cancer observed in this study was lower than the 10% found in previous studies, possibly because of improvements in other kinds of cancer screening in the general population.

“It is very reassuring to know that the risk of cancer is less than we thought in these patients,” said Dr. Rodger. “It means I can have a very different kind of conversation with my newly diagnosed patients.”