RT Journal Article SR Electronic T1 Initial Results of 68Ga-FAPI-46 PET/MRI to Assess Response to Neoadjuvant Chemotherapy in Breast Cancer JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 717 OP 723 DO 10.2967/jnumed.122.264871 VO 64 IS 5 A1 Backhaus, Philipp A1 Burg, Matthias C. A1 Asmus, Inga A1 Pixberg, Michaela A1 Büther, Florian A1 Breyholz, Hans-Jörg A1 Yeh, Randy A1 Weigel, Stefanie B. A1 Stichling, Patricia A1 Heindel, Walter A1 Bobe, Stefanie A1 Barth, Peter A1 Tio, Joke A1 Schäfers, Michael YR 2023 UL http://jnm.snmjournals.org/content/64/5/717.abstract AB Improving imaging-based response after neoadjuvant chemotherapy (NAC) in breast cancer assessment could obviate histologic confirmation of pathologic complete response (pCR) and facilitate deescalation of chemotherapy or surgery. Fibroblast activation protein inhibitor (FAPI) PET/MRI is a promising novel molecular imaging agent for the tumor microenvironment with intense uptake in breast cancer. We assessed the diagnostic performance of follow-up breast 68Ga-FAPI-46 (68Ga-FAPI) PET/MRI in classifying the response status of local breast cancer and lymph node metastases after completion of NAC and validated this approach immunohistochemically. Methods: In women who completed NAC for invasive breast cancer, follow-up 68Ga-FAPI PET/MRI and corresponding fibroblast activation protein (FAP) immunostainings were retrospectively analyzed. Metrics of 68Ga-FAPI uptake and FAP immunoreactivity in women with or without pCR were compared using the Mann–Whitney U test. Diagnostic performance to detect remnant invasive cancer was calculated for tracer uptake metrics using receiver-operating-characteristic curves and for masked readers’ visual assessment categories of PET/MRI and MRI alone. Results: Thirteen women (mean age ± SD, 47 ± 9 y) were evaluated. Seven of the 13 achieved pCR in the breast and 6 in the axilla. FAP immunoreactivity was significantly associated with response status. The 68Ga-FAPI PET/MRI mean breast tumor-to-background ratio was 0.9 (range, 0.6–1.2) for pCR and 2.1 (range, 1.4–3.1) for no pCR (P = 0.001). Integrated PET/MRI could classify breast response correctly in all 13 women based on readers’ visual assessment or tumor-to-background ratio. Evaluation of MRI alone resulted in at least 2 false-positives. For lymph nodes, PET/MRI readers had at least 2 false-negative classifications, whereas MRI alone resulted in 2 false-negatives and 1 false-positive. Conclusion: To our knowledge, this was the first analysis of 68Ga-FAPI PET/MRI for response assessment after NAC for breast cancer. The diagnostic performance of PET/MRI in a small study sample trended toward a gain over MRI alone, clearly supporting future prospective studies.