Measuring serum oestrogen levels in breast cancer survivors using vaginal oestrogens: a systematic review

Abstract

Purpose Vaginal oestrogens can be used to treat genitourinary symptoms in women with early breast cancer. Studies evaluating vaginal oestrogens have commonly measured serum oestrogen levels as a surrogate marker of safety, but methods vary. We sought to summarise the data on serum oestrogen measurement in women with breast cancer using vaginal oestrogens to better understand the methods, levels and reliability. Methods We searched Medline, Embase, CENTRAL, SCOPUS and CINAHL from inception to October 2023 for clinical studies where serum oestrogen was measured in women with a history of early breast cancer using vaginal oestrogens. Studies with a reported testing methodology were included. Results Nine studies met the inclusion criteria for this systematic review. Methods used to measure oestradiol and oestriol in selected studies included mass spectrometry and immunoassays; several studies used more than one with variable concordance. Mass spectrometry detected oestradiol levels down to a lower limit between 1.0 pg/mL and 3.0 pg/mL. Immunoassays such as ELISA (enzyme-linked immunosorbent assay), ECLIA (enhanced chemiluminiscence immunoassay) and RIA (radioimmunoassay) had lower detection limits ranging between 0.8 pg/mL and 10 pg/mL. Studies were heterogeneous in testing techniques used, timing of testing, and the population including with subsequent varying results in the effect on oestrogens reported. Conclusions Adopting consistent and standardised methods of measuring oestrogens in clinical trials involving women with early breast cancer on vaginal oestrogens is critical. Serum oestrogens are used as a surrogate marker of safety in this population, and good-quality data are necessary to enable clinicians and patients to feel confident in prescribing and taking vaginal oestrogens. Mass spectrometry, although more expensive, gives more reliable results when dealing with very low levels of oestrogens often found in women on aromatase inhibitors, compared to immunoassays.