LAUSR

To evaluate the relationship of the signature index (S-index), an advanced diffusion MRI marker, and the immunohistochemical (IHC) status, proliferation rate, and molecular subtype of invasive breast cancers. A retrospective study of patients with invasive carcinoma was conducted between 2017 and 2021. All patients underwent dynamic contrast-enhanced MRI and DWI using a 3-T system. For DWI, three b values (0, 200, and 1500 s/mm2) were used to derive the S-index. Three-dimensional ROIs were manually placed over the whole tumor on DWI. Mean and 85th percentile S-index values were compared to the IHC status, proliferation rate, and molecular subtypes of lesions. The study included 153 patients (mean age, 60 ± 13 years) with 160 invasive carcinomas. S-index values were significantly higher in estrogen receptor–positive (mean, p = .005; 85th percentile, p < .001) and progesterone receptor–positive (mean, p = .003; 85th percentile, p < .001) tumors, and significantly lower in human epidermal growth factor receptor 2 (HER2) – positive tumors (mean, p = .023; 85th percentile, p < .001). Mean and 85th percentile S-index values were significantly different among breast cancer subtypes (mean, p = .015; 85th percentile, p = .002), and the AUC of these values for the prediction of IHC status were 0.64 and 0.66 for HER2, and 0.70 and 0.74 for hormone receptors, respectively. The DWI S-index showed significantly higher values in invasive carcinomas with immunohistochemical status associated with good prognosis, suggesting its usefulness as a noninvasive imaging biomarker to estimate IHC status and orient treatment. • The signature index, an advanced diffusion MRI marker, showed good discrimination of immunohistochemical status in invasive breast carcinomas. • The signature index has the potential to differentiate noninvasively invasive breast carcinoma subtypes and appears as an imaging biomarker of prognostic factors and molecular phenotypes