MRI-only breast cancers are less aggressive than cancers identifiable on conventional imaging.

Background: Magnetic resonance imaging (MRI) has a superior sensitivity for the diagnosis of breast cancer, leading to lesions primarily detected by MRI. Some of these lesions cannot be identified by targeted second-look ultrasound (SLUS) examinations and are thus referred to as MRI-only lesions. We hypothesize that biologically more aggressive cancers lead to more distinct tissue damage improving visibility on SLUS.

Objective: To investigate whether there are differences in cancer subtypes between MRI-only and SLUS-detected malignant lesions.

Methods: This retrospective single-center observational study evaluated 435 patients who received breast MRI examinations between January 2017 and December 2022, with at least one lesion primarily detected on MRI and histologically confirmed as malignant. Demographic characteristics, lesion type (mass or non-mass), MRI-assessed lesion size (mm), histological diagnosis, stage, immunohistochemical analysis (ER, PR, HER-2, Ki-67), and lymph node status were assessed and compared between MRI-only and SLUS-detected.

Results: Among 435 patients (mean age of 57.4 ± 13.3), 34.02 % (n = 148) were in the MRI-only group, while the remaining 65.98 % (n = 287) were identified by SLUS. MRI-only cases were significantly smaller in size (10 mm vs 20 mm), mostly staged as T1 (66.9 %) and showed features associated with less biological aggressiveness (higher pure ductal carcinoma in situ rates: 30.4 % vs 5.2 %; lower Ki-67, median values: 10 vs 20) compared to SLUS-detected cases (P < 0.001). SLUS-detected cancers had higher ratios of microscopic (4.9 % vs 3.4 %) and macroscopic axillary metastasis (26.8 % vs 7.4 %) compared to MRI-only lesions (P < 0.001).

Conclusion: MRI-only lesions presented histologically and immunohistochemically with less aggressive patterns compared to those detected via SLUS. Clinic Impact: Our data provide evidence that MRI-only lesions are biologically less aggressive and of lower stage, offering the potential of earlier treatment chance since they are visible on MRI before becoming more aggressive and destructive.