Purpose: The axillary arch (AA) is an anatomical variant of the axilla with a reported prevalence of 6% on MRI imaging. It comprises a musculotendinous slip extending from the latissimus dorsi to the pectoralis major, traversing the axilla. The AA is clinically significant in the setting of breast cancer, as it can obscure axillary lymph nodes and impede surgical clearance during axillary dissection. The appearance and prevalence of AA on breast MRI have not been described, and the sensitivity of ultrasound to detect abnormal axillary lymph nodes in breast cancer patients with this anatomic variant is unknown.
Materials and Methods: A cohort of 1,000 women with breast MRIs performed at our institution in the last two years were reviewed. Of these, 403 patients had a breast MRI for extent of disease for breast malignancy. The T1W axial breast MRI images were reviewed for the presence of an AA. Identified AAs were classified as unilateral or bilateral, and the anatomic relationship of the axillary lymph nodes to the AA as superficial, deep, or both was documented. The presence or absence of lymphadenopathy on MRI was documented and correlated to available pre-MRI axillary ultrasound to determine the proportion of nodal metastases detected by MRI that were missed with ultrasound. Breast cancer type, patient demographics, biopsy, and surgical outcomes were also recorded.
Results: Of 403 patients, 20 were excluded as identification of an AA could not be determined due to positioning. In total, 7.0% (27/383) of patients had an AA detected on breast MRI. Of these, 29.6% (8/27) were bilateral, 18.5% (5/27) were right-sided only, and 51.9% (14/27) were left-sided only. All 27 patients with AA had lymph nodes visible both superficial and deep to AA. Of the 403 patients, 327 had an axillary ultrasound prior to breast MRI. Of these, biopsy-proven malignant adenopathy was detected on MRI but missed on axillary US in 8.3% (2/24) of patients with an AA, but only 2.0% (6/303) patients without an AA.
Conclusion: Axillary arch variant anatomy was detected on breast MRI in 7.0% of patients, most of which were left-sided. Lymph nodes were both superficial and deep in all patients. Axillary nodal metastases were more frequently missed by ultrasound in patients with AA compared to those without AA.
Clinical Relevance Statement: The axillary arch is a common anatomic variant that may impede sonographic and surgical detection of lymph node metastases in patients with breast cancer.
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