Background: Extended endocrine therapy beyond 5 years for postmenopausal breast cancer has been studied within multiple phase III trials. Treatment compliance in these trials is generally poor. In this analysis, we aimed to determine factors that were associated with participation in the phase III Investigation on the Duration of Extended Adjuvant Letrozole (IDEAL) trial and with early treatment discontinuation, and how this influenced survival outcome. Methods: In the IDEAL trial, postmenopausal patients were randomised between 2.5 or 5 years of extended letrozole, after completing 5 years of endocrine therapy for hormone receptor-positive early breast cancer. A subgroup of this population participated earlier in the Tamoxifen Exemestane Adjuvant Multinational trial (5 years of exemestane or 2.5 years of tamoxifen followed by exemestane as primary adjuvant therapy) in which we explored which factors were determinative for enrolment in the IDEAL study. In the IDEAL cohort, we evaluated which factors predicted for early treatment discontinuation and the effect of early treatment discontinuation on disease-free survival (DFS). Results: Nodal status, younger age and adjuvant chemotherapy were significantly associated with higher enrolment in the IDEAL trial. In the IDEAL cohort, adverse events (AEs), the type of primary endocrine therapy and the interval between primary and extended therapy were associated with early treatment discontinuation. Among the reported AEs, depressive feelings (56%) were most frequently associated with early treatment discontinuation. Early treatment discontinuation was not associated with worse DFS (hazard ratio [HR] = 1.02, 95% confidence interval = 0.76–1.37). Conclusions: In this analysis, we found that risk factors were most strongly associated enrolment in the IDEAL trial. In contrast, patient experiences were the most significant factors leading to early treatment discontinuation, with no effect on DFS.

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Keywords Adverse events, Aromatase inhibitor, Breast cancer, Clinical trial, Compliance, Early treatment discontinuation, Extended endocrine therapy, Survival, Tamoxifen
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Journal European Journal of Cancer
Blok, E.J. (Erik J.), Kroep, J.R, Meershoek-Klein Kranenbarg, E, Duijm-de Carpentier, M. (Marjolijn), Putter, H, Liefers, G.-J, … van de Velde, C.J.H. (Cornelis J.H.). (2018). Treatment decisions and the impact of adverse events before and during extended endocrine therapy in postmenopausal early breast cancer. European Journal of Cancer, 95, 59–67. doi:10.1016/j.ejca.2018.03.014