Integrated [18F]FAPI-42 PET/MR improves diagnostic accuracy in breast cancer: comparison with simultaneously acquired breast MRI.

PURPOSE This study assessed the diagnostic efficacy of fluorine-18-labeled fibroblast activation protein inhibitor 42 ([18F]FAPI-42) PET/MR versus simultaneously acquired breast MRI in identifying primary breast lesions and axillary lymph node metastases. METHODS A prospective study enrolled 64 women with BI-RADS 4 or 5 lesions identified through mammography or ultrasound. All participants underwent contrast-enhanced [18F]FAPI PET/MRI scans of the breast. Histology and imaging follow-up (median 11.5 months) were used as the gold standard. Primary lesions and lymph nodes were assessed using three imaging modalities: breast MRI, qualitative/quantitative [18F]FAPI PET, and integrated [18F]FAPI PET/MR. Quantitative PET parameters comprised the maximum standardized uptake value (SUVmax) and the tumor-to-background ratio (TBR). Receiver operating characteristic analysis assessed diagnostic performance, while net reclassification improvement (NRI) evaluated the diagnostic enhancement of PET/MR compared to breast MRI. RESULTS The study included 114 breast lesions (89 malignant, 25 benign) and 114 lymph node groups (82 malignant, 32 benign). In detecting primary breast lesions, the quantitative PET/MR based on TBR (PET/MR-TBR) demonstrated superior specificity over breast MRI (96% vs. 68%, P = 0.016), corresponding to a marked reduction in false positive rate from 32% (8/25) to 4.0% (1/25; P = 0.027), while maintaining comparable sensitivity (94% vs. 97%, P = 1.00). For BI-RADS 3/4 lesions on breast MRI, PET/MR-TBR achieved an AUC of 0.90, with a significant NRI of 90.5% (P = 0.018) for BI-RADS 4 lesions. For breast lesions smaller than 10 mm, PET/MR-TBR increased specificity to 94% versus 75% for breast MRI (P > 0.05). For axillary lymph node evaluation, the quantitative PET/MR based on SUVmax (PET/MR-SUVmax) showed improved sensitivity (95% vs. 62%, P < 0.001) and a nonsignificant decrease in specificity (94% vs. 97%, P = 1.00) compared with breast MRI. CONCLUSION [18F]FAPI PET/MR significantly improves diagnostic accuracy over breast MRI, particularly in reducing false positives and improving the detection of axillary lymph node metastases. This modality holds potential for refining breast cancer diagnostics, especially in challenging BI-RADS 4 lesions and improving more accurate staging for smaller lesions.