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"Our earlier analysis, based on a shorter follow-up, had shown a clear relapse advantage but until now, the magnitude and duration of the overall survival benefit had been uncertain. These updated results show that the relapse improvement does not seem to diminish over time and have clarified that the survival advantage is robust and enduring."

The study, which has the longest follow-up of any trial to date investigating the impact of switching from tamoxifen to an aromatase inhibitor, involved 4,724 postmenopausal women from 37 countries with oestrogen-receptor-positive or unknown receptor status breast cancer who had their tumours cut out and had remained disease free after two or three years on tamoxifen. About half continued with tamoxifen until they had completed a total of five years of treatment, while the other half were switched to exemestane for the remaining period of treatment. The women were followed for an average of 91 months.

The 18% improvement in disease-free survival is derived from a hazard ratio of 0.82, while the 14% improvement in overall survival is calculated from a hazard ratio of 0.86.

"Practice changed in many countries after our early findings were released in 2004, from using five years of tamoxifen to the current recommended treatment strategy of switching these patients to exemestane or another aromatase inhibitor after two or three years of tamoxifen. The issue that has yet to be clarified is whether starting with tamoxifen and then switching is better than starting with an aromatase inhibitor," Prof Coombes said.

Cancer Research UK and Pfizer Ltd., which makes exemestane, funded the study.