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

Objective: 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.

Methods: 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.

Results: 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.

Conclusion: 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.