Associations Between Apparent Diffusion Coefficient Values of Breast Tumors Presenting as Non-Mass Enhancement and Ki-67 Index

Objectives: This study aimed to investigate whether apparent diffusion coefficient (ADC) values of malignant breast lesions presenting as non-mass enhancement (NME) are associated with the Ki-67 proliferation index. Materials and Methods: This retrospective, single-center study included 70 patients with malignant breast lesions presenting as NME on MRI. Patients were divided into two groups according to their Ki-67 index; Group KL (Ki-67 < 20%) and KH (Ki-67 ≥ 20%). All examinations were performed on a 1.5T scanner. Two radiologists, blinded to histopathology, independently outlined the whole NME on every subtraction slice on which it was visible and transferred these regions of interest to the ADC maps; the mean ADC value was used for subsequent analysis. Results: Mean ADC values were lower in Group KH than in Group KL (0.897 ± 0.158 vs. 1.116 ± 0.155 × 103 mm2/s, p < 0.001) and were inversely correlated with the Ki-67 index (Spearman’s ρ = −0.458, 95% CI −0.626 to −0.250, p < 0.001). A high Ki-67 index remained associated with lower ADC values after adjustment for molecular subtype (b = −0.228 × 10−3 mm2/s; p < 0.001). ROC curve analysis yielded an AUC of 0.833 (95% CI 0.725 to 0.912). The exploratory Youden threshold of 1.045 × 10−3 mm2/s yielded a sensitivity of 85.7% and a specificity of 66.7%. Bootstrap resampling confirmed that the AUC was internally stable (mean 0.832, 95% CI 0.728 to 0.918). Conclusions: In malignant NME, lower ADC values are associated with a higher Ki-67 index. The ADC may add noninvasive information on tumor proliferation but cannot replace histopathology. The proposed threshold is exploratory and needs external validation before clinical use.