An association between breast and thyroid cancers has been repeatedly observed in epidemiological studies, though its nature and underlying causes remain unclear. In a recent systematic review and meta-analysis, Viola and colleagues synthesised data from 38 studies including nearly 3 million women to examine the reciprocal risk between these two common malignancies.

They reported a statistically significant bidirectional association. Women with thyroid cancer had a 40% higher risk of developing breast cancer (standardised incidence ratio (SIR) 1.4), while those with prior breast cancer had a 50% increased risk of subsequent thyroid cancer (SIR 1.5). The risk of secondary thyroid cancer appeared greater in women diagnosed with breast cancer before the age of 50 and in those who had received chemotherapy, whereas radiotherapy was not associated with a significant increase in risk.

While substantial heterogeneity and incomplete subgroup data preclude firm conclusions, this synthesis provides important quantitative evidence regarding a long-discussed association. The findings highlight the need for further research into shared hormonal and genetic pathways and advocate for an individualised approach to surveillance in potentially higher-risk patients.

Read the full article in Endocrine-Related Cancer https://doi.org/10.1530/ERC-24-0338