The "aTTom" study, performed by Cancer Research UK and presented at the 2013 American Society of Clinical Oncology (ASCO) conference this weekend, indicates that women who have had estrogen-sensitive breast cancers had better survival rates if they were using tamoxifen for 10 years instead of the typical five-year regimen. The treatment reduced the chances of estrogen receptor(ER)-positive breast cancer returning and decreased mortality over the study period. However, the usage of tamoxifen carries its own risks such as blood clots, strokes, and the possibility of developing another type of cancer, endometrial (uterine) cancer.

The study looked at close to 7,000 women, from 1995 to 2001, who had breast cancer that was ER-positive, who had either continued to take the drug for 10 years or stopped after the usual five years, as per current medical recommendations. The women who continued to take tamoxifen had a 25 percent less chance of breast cancer reoccurrence and 23 percent fewer women died with the extended treatment. Given that around 75 percent of all breast cancers are ER positive and the drug is off of patent, making it inexpensive, such use of tamoxifen may be a future treatment option.

"These results are important as they establish that giving tamoxifen for longer than the current standard of five years significantly cuts the risk of breast cancer returning. Doctors are now likely to recommend continuing tamoxifen for an extra five years and this will result in many fewer breast cancer recurrences and breast cancer deaths worldwide. Tamoxifen is cheap and widely available so this could have an immediate impact." said Dr. Daniel Rea, clinical lead researcher based at the University of Birmingham.

Tamoxifen works by blocking estrogen attachment to the estrogen receptor and essentially preventing the cancer cells from receiving the growth signal that the estrogen would give. The current study did not find that extended use of the drug increased the risk of stroke, but that it did induce a slight increase in the chance for endometrial cancer. The researchers estimated that for each individual that was on the extended treatment who developed uterine cancer, 30 deaths from breast cancer had been prevented.

The risk for development of endometrial cancer was doubled by taking the extended treatment, so the benefits significantly outweigh the risks. Endometrial cancer can be treated if caught early and usually presents a low risk of death if there is surgical intervention such as a hysterectomy. It would make sense that women who are on the extended treatment would be monitored closely for the development of uterine cancer, because of the well-known risk of increased incidence. This should lower the risk of dying from the secondary cancer.

"Five years of tamoxifen is already an excellent treatment but there have been concerns that giving it for longer might not produce extra benefits and could even be harmful. The aTTom study establishes that the benefits of taking tamoxifen for longer greatly outweigh the risks." said Professor Richard Gray from the University of Oxford, who presented the aTTom results at ASCO.

This data closely mirrors another international trial, called ATLAS, that enrolled 21,000 women. The study recommended the same course of treatment and was conducted by the United States National Cancer Institute.