**5. Conclusions**

The data presented in this study show that FDG PET imaging inadequately staged 22.9% of grade 1–2, ER+ BC cases. This can lead to incorrect staging and subsequently to inappropriate treatment choices, potentially affecting survival and quality of life. Prospective studies investigating novel radiotracers are urgently needed to improve the current imaging staging procedures.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2075-441 8/11/11/1954/s1. Please see 'Supplemental data'.

**Author Contributions:** Conceptualization: R.I., C.W.M.-v.d.H.v.O.; methodology: R.I., L.H.M., C.W.M.-v.d.H.v.O.; data acquisition: R.I., L.H.M., W.V.V., C.W.M.-v.d.H.v.O.; data analysis: R.I., T.A., T.v.d.B.; data interpretation and revision: R.I., L.H.M., T.A., W.V.V., T.v.d.B., D.E.O.-L., H.M.W.V., O.S.H., R.B., C.W.M.-v.d.H.v.O.; writing—original draft preparation: R.I.; supervision: H.M.W.V., O.S.H., R.B., C.W.M.-v.d.H.v.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Cancer Center Amsterdam.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of the Amsterdam UMC—location VUmc (protocol number: 2017.382; date of approval: 23 February 2018).

**Informed Consent Statement:** Informed consent was obtained from subjects involved in the study except when it was not possible to approach them for consent due to various reasons (e.g., death or no contact details available).

**Data Availability Statement:** The data supporting the conclusions of this article are included within the article.

**Acknowledgments:** The authors would like to thank the patients for participating in this study and the members of the associated departments of the VUmc and NKI-AvL for tracer production and data acquisition.

**Conflicts of Interest:** The authors declare no conflict of interest.
