**5. Conclusions**

The main finding of this study is the relatively high day-to-day variability of tumor SUVmax with repeatability levels of agreemen<sup>t</sup> varying between +43% and −46% for all lesions taken together. Small lesions tend to have larger day-to-day variability of tumor SUVmax when compared to larger lesions. With respect to response monitoring, minimum response of 50% for Ga-68 PSMA PET might be more appropriate.

With respect to our second aim no significant differences in repeatability between standard OSEM reconstruction and BPL reconstruction could be shown in this pilot study. There was a small but not significant difference in favor of the standard reconstruction, however. These results do not support the use of BPL with respect to our hypothesis that it might lower signal variability.

**Author Contributions:** M.J.R. was the leading contributor to study design, data analysis and interpretation as well as to writing the manuscript. S.R. and A.J.A. were major contributors to study design and to writing the manuscript. C.B. and D.N.W. were major contributors to data analysis and interpretation. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was funded by General Electrics Health Care (investigator-sponsored research support for principal investigator Mark J. Roef). The study was investigator driven. General Electrics Health Care played no role in study design, data analysis or in writing the manuscript.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the local institutional ethics review board (NL52809.100.16/ R16.058/Ga-68 PSMA test–retest study, 27 March 2017).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** First: we thank all the patients who agreed to participate in the study. Furthermore, we want to thank our PET/CT team for their excellent support. We would also like to thank Marcel van 't Veer for his statistical help.

**Conflicts of Interest:** The authors declare no conflict of interest. The sponsors had no role in the design, execution, interpretation or writing of the study.
