Although the association between tamoxifen and thrombosis is well reported, it was unclear if “the risk changes with the amount of time elapsed since the initial tamoxifen prescription,” Dr. Rohini K. Hernandez, from Boston University School of Public Health, and colleagues note.
To investigate, the researchers analyzed data from 16,289 women who were identified in the clinical database of the Danish Breast Cancer Cooperative Group. The women had been diagnosed with stage I or II ER-positive breast cancer from 1990 to 2004 at the ages of 45 to 69 years. About half of the women had received tamoxifen and half did not.
As reported in the October 1st issue of Cancer, the 5-year risk of DVT/PE was significantly higher in the women treated with tamoxifen: 1.2% vs. 0.50%.
Further analysis, however, showed that the elevated risk was confined to the first 2 years of treatment with a relative risk of 3.5. Beyond that point, no significant difference in risk was seen between the groups.
The DVT/PE risk seen in the first 2 years was higher for tamoxifen users over 50 years of age than for younger users. In the older group, 8.8 cases of DVT/PE per 1000 were seen compared with 2.6 per 1000 in the younger group.
“The results of the current study indicate that women may be most susceptible to DVT/PE during the first 2 years of tamoxifen therapy—the time when targeted monitoring for venous thromboembolism is most needed,” the authors conclude.