Human epidermal growth factor receptor 2 (HER2) status is used for decision-making in breast carcinoma treatment. The status is obtained through immunohistochemistry or in situ hybridization. These two methods have the disadvantage of necessitating tissue sampling, which is prone to error due to tumor heterogeneity or interobserver variability. Whole-body imaging might be a solution to map HER2 expression throughout the body. Methods: Twenty patients with locally advanced or metastatic breast carcinoma (5 HER2-positive and 15 HER2-negative patients) were included in this phase II trial to assess the repeatability of uptake quantification and the extended safety of the [⁶⁸Ga]Ga-NOTA-anti-HER2 single-domain antibody (sdAb). The tracer was injected, followed by a PET/CT scan at 90 min. Within 8 d, the procedure was repeated. Blood samples were taken for antidrug antibody (ADA) assessment and liquid biopsies. On available tissues, immunohistochemistry, in situ hybridization, and mass spectrometry were performed to determine the correlation of HER2 status with uptake values measured on PET. If relevant preexisting [¹⁸F]FDG PET/CT images were available (performed as standard of care), a comparison was made. Results: With a repeatability coefficient of 21.8%, this imaging technique was repeatable. No clear correlation between PET/CT uptake values and pathology could be established, as even patients with low levels of HER2 expression showed moderate to high uptake. Comparison with [¹⁸F]FDG PET/CT in 16 patients demonstrated that in 7 patients, [⁶⁸Ga]Ga-NOTA-anti-HER2 shows interlesional heterogeneity within the same patient, and [¹⁸F]FDG uptake did not show the same heterogeneous uptake in all patients. In some patients, the extent of disease was clearer with the [⁶⁸Ga]Ga-NOTA-anti-HER2-sdAb. Sixteen adverse events were reported but all without a clear relationship to the tracer. Three patients with preexisting ADAs did not show adverse reactions. No new ADAs developed. Conclusion: [⁶⁸Ga]Ga-NOTA-anti-HER2-sdAb PET/CT imaging shows similar repeatability to [¹⁸F]FDG. It is safe for clinical use. There is tracer uptake in cancer lesions, even in patients previously determined to be HER2-low or -negative. The tracer shows potential in the assessment of interlesional heterogeneity of HER2 expression. In a subset of patients, [⁶⁸Ga]Ga-NOTA-anti-HER2-sdAb uptake was seen in lesions with no or low [¹⁸F]FDG uptake. These findings support further clinical development of [⁶⁸Ga]Ga-NOTA-anti-HER2-sdAb as a PET/CT tracer in breast cancer patients.

#	Concept	H-Index	Publications	Citations
1	Medicine	4	4	180
2	Health Sciences	3	3	98
3	Neurology	2	2	118
4	Neuroscience	2	2	118
5	Diagnostic Sciences	1	1	32

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