Objective <p>Perineural spread (PNS) of breast carcinoma to the brachial plexus is rare. This study investigates the radiologic features supporting the medial and lateral pectoral nerves (MPN and LPN, respectively) as pathways for PNS of breast cancer to the brachial plexus.</p> Methods <p>We reviewed 19 patients with biopsy-proven PNS of breast carcinoma to the brachial plexus. All available MRI and ¹⁸F-FDG PET/CT studies were re-evaluated by a musculoskeletal radiologist with expertise in PNS. Imaging features of interest included pectoralis major and minor muscle MRI signal abnormality, abnormal FDG activity, and atrophy; FDG avidity within or along the course of the pectoral nerves; and extent of brachial plexus involvement on MRI and ¹⁸F-FDG PET/CT. Demographic and clinical data were also collected.</p> Results <p>All 19 patients had MRI and ¹⁸F-FDG PET/CT scans. Six patients showed clear radiologic evidence of PNS via the pectoral nerves. All six patients demonstrated abnormal MRI signal or enhancement in both the pectoralis major and minor muscles and increased FDG uptake was present in the pectoralis major in 4/6 patients and pectoralis minor in 5/6 patients. Five patients demonstrated atrophy of both the pectoralis major and minor muscles. Increased FDG uptake was noted along the LPN in five patients and the MPN in four. All exhibited brachial plexus enhancement on MRI and increased FDG uptake on PET/CT, supporting contiguous spread from the pectoral nerves.</p> Conclusion <p>This study provides radiologic support for the MPN and LPN as a potential pathway for PNS of breast cancer to the brachial plexus. Pectoralis major/minor muscle atrophy, abnormal MRI signal or enhancement, and increased FDG activity within the pectoral muscles and/or along the pectoral nerves may serve as early, non-invasive imaging markers of this process, with potential implications for diagnosis and management.</p>