The study was published online on October 28 in The BMJ.
“The study confirms increased risk of breast cancer in patients taking HRT but shows that the magnitude of risk depends on a number of factors,” first author Yana Vinogradova, PhD, said in an interview. Dr. Vinogradova is a medical statistician at the University of Nottingham (England).
The study also suggests the risk may be lower than was estimated in a large meta-analysis of 24 trials that was published in 2019 in The Lancet. In that study, researchers suggested the risk for breast cancer with HRT was higher and persisted longer than had been thought.
This conclusion from the meta-analysis was widely reported in the lay press and led to the UK Medicine and Healthcare Products Regulatory Agency issuing a safety alert for HRT regarding breast cancer. Experts in the field questioned the alert and said it caused undue anxiety. The European Medicines Agency also issued a safety alert because of the study.
This new study was begun before publication of the meta-analysis. Although the results are broadly similar in suggesting increased risk for breast cancer with HRT use, findings from the new study suggest the risk is lower than had been estimated in the meta-analysis and that the risk diminishes more rapidly after stopping HRT than was suggested by the meta-analysis.
“The publicity surrounding publication of the meta-analysis highlighted unexpectedly high risks and led to a heightened level of concern in some quarters,” Dr. Vinogradova commented. “Our study, based on general population data, has not confirmed any such findings. In general, it showed lower levels of risk and clarified the variability of magnitude within them.”
Dr. Vinogradova said the discrepancy could be related to the fact that the studies were designed differently. The meta-analysis relied on results from 24 studies that were conducted around the world at different periods and included women of different ages and backgrounds. The studies in the meta-analysis used different methods, including questionnaires that relied on women’s memories and therefore could have been biased, she said.
In contrast, the new study analyzed EMR data collected prospectively by general practices in the United Kingdom. The data came from the QResearch and from the Clinical Practice Research Datalink (CPRD) databases, the two largest primary care databases in the United Kingdom, which were linked to hospital, mortality, and cancer registries.
Because this study used a “consistent design” and “consistent data sources,” these new results “are likely to be more accurate and reliable for assessing risks among HRT users,” Dr. Vinogradova commented.
This study used an observational design, so it cannot prove that HRT causes breast cancer. These results may better represent women in the general U.K. population, compared with the earlier meta-analysis, she added.
Commenting on the new study, Michael Jones, PhD, senior staff scientist in genetics and epidemiology at the Institute of Cancer Research, London, also emphasized that it was large and its data came from general practitioner medical records, “so the strong statistical associations are unlikely to be due to chance.
“The results of this study generally confirm what has been seen before and is well established – that the use of combined estrogen plus progestogen HRT is associated with increased risk of breast cancer, and this risk increases with duration of use. But reassuringly, after stopping HRT, the raised risk of breast cancer mostly returns to that seen in nonusers of HRT,” he said.
“It’s important to note that no one study should be considered in isolation,” he added. “Even though some risks were found to be slightly smaller than those reported in another meta-analysis of the worldwide epidemiological evidence recently published in 2019, women considering use of HRT should still follow advice given to them by their [general practitioners].”