*2.1. Search Strategy*

Two authors (A.P. and G.B.) searched the available literature independently. The search and selection process consisted of four separate steps.

In the first step, so-called "sentinel" studies were identified in PubMed by entering various combinations of the following keywords: 18F-FES, 18F-FDG, PET/CT, breast cancer and oestrogen receptor. In the second step, the results were used to identify specific MeSH terms in PubMed. In the third step, PubMed, CENTRAL, Scopus, Web of Science and the web were searched using the selected MeSH terms. In the final step, we evaluated the studies that compared the sensitivity of 18F-FES PET/CT and 18F-FDG PET/CT in identifying disease localization in patients affected by ER+ BC. All initially eligible articles were screened, and those reporting a head-to-head comparison of 18F-FES PET/CT and 18F-FDG PET/CT in BC patients were included. Review articles, studies based on preclinical data, phantom studies, case reports, small case series and studies with overlapping data were excluded.

The references of the included studies were searched to identify further potential matches. The search process was concluded on 21 December 2021.
