As many as 1 in 5 people risk developing a venous thromboembolism (VTE) within a year after undergoing treatment for some types of cancers, researchers reported at the 2011 European Multidisciplinary Cancer Congress, held in Stockholm, Sweden, September 23-27, 2011.

Gary Lyman, MD, MPH, and colleagues also noted that in up to 2% of cases, the complication is deadly.

A professor of medicine and the director of the Comparative Effectiveness and Outcomes Research Program at Duke University School of Medicine in Durham, North Carolina, Lyman spearheaded an analysis of 30,552 records of people in the United States who had begun chemotherapy between 2004 and 2008 as treatment for cancer of the lung, pancreas, stomach, colon/rectum, bladder, or ovaries. None of these patients were participants in clinical trials.

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The risk of developing VTE within 3.5 months of starting treatment was highest for persons with pancreatic cancer at 11.9%, and lowest for those with bladder cancer, at 4.8%. People with lung cancer and stomach cancer each ran an 8.5% risk of VTE; women with ovarian cancer had a 6.2% risk; and those with colon/rectum cancer had a 6.1% risk.

After a year of treatment, however, VTE risks nearly doubled across many of the six cancer types, rising to 21.5% for pancreatic cancer patients; 9.9% for those with bladder cancer; 14.8% for those with lung cancer; 16.7% for those with stomach cancer; 11.4% for those with ovarian cancer, and 11.9% for those with colon/rectum cancer.

Lyman noted in a statement describing his group’s findings that although scientists don’t fully understand why VTEs form during cancer treatment, they have identified certain contributing factors, such as blood-clotting agents released by tumors, side effects of chemotherapy, and pre-existing health conditions such as obesity and anemia.

Knowing which patients are at highest risk for VTE could lead to better preventive use of blood thinners, but these agents carry their own risk of potentially dangerous bleeding. Lyman called for more data from randomized controlled trials in higher-risk populations that would demonstrate a favorable benefit-to-harm ratio for such preventive measures.

Lyman’s team also found that clot-related medications and hospitalizations can raise the average care bill to $110,362, compared with $77,984 for patients who do not develop VTEs.