Patients' understanding of how genotype variation affects benefits of tamoxifen therapy for breast cancer.

6029 Background: New genomic markers will increasingly inform clinical care, but some may become available to patients prior to clinical validation. Studies have shown that CYP2D6 is a critical enzyme in the metabolism of tamoxifen and potentially a key determinant in breast cancer outcomes. Our study sought to examine patients’ beliefs about how CYP2D6 genotype would affect their prognosis. Methods: In LCCC 0801, women on tamoxifen for prevention or treatment of breast cancer underwent CYP2D6 genotyping; we increased tamoxifen dose in patients with any intermediate or poor metabolizing (IM or PM) alleles, but not in patients homozygous for extensive metabolizing (EM) alleles. The informed consent said that the purpose of the study was to see if dose adjustment could raise endoxifen concentrations in reduced metabolism patients, but that it is not clear whether this would provide clinical benefit. In an embedded sub-study, 321 patients (84% of possible responders) completed a survey (6 hypothetical vignet...