Prevalence of Axillary Arch Variant Anatomy on Breast MRI and Impact on Axillary Lymph Node Assessment

To assess the prevalence of the axillary arch (AA) variant and its impact on the sensitivity of US vs MRI for detecting axillary nodal metastases in patients with breast cancer.

The IRB waved informed consent for this retrospective study. Three hundred, eighty-two breast MRIs performed for the extent of disease of breast cancer between 2012 and 2023 were reviewed for the presence of AA. Pre-MRI axillary US was available in 322 of these cases. The presence of axillary adenopathy was documented and correlated with pathology. A paired sample comparison was used to calculate sensitivities of US and MRI for detection of the AA.

The AA was detected on breast MRI in 6.8% (26/382) of patients. Of these, 30.8% (8/26) were bilateral, 50% (13/26) were unilateral left, and 19.2% (5/26) were unilateral right. All had lymph nodes both superficial and deep to the AA. Of the 26 patients with AA, 19 patients underwent pre-MRI axillary US. Biopsy-proven malignant adenopathy was detected on MRI but missed on US in 10.5% (2/19) of patients with AA but only 2.5% (7/279) of patients without AA. Sensitivity for detection of lymphadenopathy on MRI and US was 69.7% (82/122)% and 67.5% (77/114) for all patients (P = .17), 72.2% (78/108) and 68.5% (74/108) for patients without AA (P = .21), and 66.2% (4/6) and 50.0% (3/6) for patients with AA. The small sample size of patients with AA precluded statistical comparison.

The AA is a common variant detectable on breast MRI. Axillary nodal metastases may reduce US sensitivity for identifying nodal metastases. Further investigation is required to establish statistical significance.