[⁶⁸Ga]Ga-ABY-025 PET in HER2-positive breast cancer: Benefits and pitfalls in staging of axillary disease.

1035 Background: [ 68 Ga]Ga-ABY-025 PET showed promising results in quantifying human epidermal growth factor receptor Type 2 (HER2) expression in breast cancer patients [1]. In this study we extend the findings from [ 68 Ga]Ga-ABY-025 PET with data from both treatment-naïve patients and previously treated patients with recurrences, mimicking the clinical situation other sites might encounter when HER2-targeting imaging is made more widely available. The focus of this additional data analysis is on staging, potential pitfalls and changes in uptake patterns associated with previous treatments. Methods: [ 68 Ga]Ga-ABY-025 PET and [ 18 F]-FDG PET images from 50 women with biopsy confirmed HER2 positive breast cancer were examined. Each patient underwent both scans within 1 week. A follow-up [ 18 F]-FDG PET was performed after 2 courses of HER2-targeted treatment to assess metabolic response. Results: Twelve out of 50 patients showed heterogeneous [ 68 Ga]Ga-ABY-025 uptake. Three out of these 12 patients presented with a wider local spread that was only detectable using [ 68 Ga]Ga-ABY-025 PET as distinct uptake in lymph nodes of 2-5 mm size. Based on the additional information from [ 68 Ga]Ga-ABY-025 PET, restaging was evaluated in two of the three cases, both of which had lobular carcinoma, and resulted in modifications in treatment plans. Two patients who received COVID-19 vaccine closely before undergoing PET scans showed high uptake on [ 18 F]-FDG PET in lymph nodes ipsilateral to the vaccine injection site, but not on [ 68 Ga]Ga-ABY-025 PET. Additionally, the physiological [ 68 Ga]Ga-ABY-025 uptake in thyroid gland and axillary sweat glands was inversely correlated with the number of previous treatments and was significantly more pronounced in treatment-naïve patients. Conclusions: [ 68 Ga]Ga-ABY-025 PET provided additional information that, in conjunction with [ 18 F]-FDG PET findings, might help in accurate staging, particularly in cases with very small lymph node metastases. It also helped reduce false positive results as [ 68 Ga]Ga-ABY-025 uptake was so far not seen in any benign inflammatory lesions. Physiological uptake can be seen in thyroid and apocrine glandular tissues but gradually disappear with subsequent HER2-targeted therapies. 1,Alhuseinalkhudhur, A. et al. J Nucl Med, 2023 64(9): p. 1364-1370. Clinical trial information: NCT03655353 .