Radiologic evidence supporting pectoral nerve involvement in perineural spread of breast cancer to the brachial plexus

Abstract

Objective: 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.

Methods: 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.

Results: 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.

Conclusion: 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.

Keywords: Brachial plexus; Breast cancer; MRI; Pectoral nerves; Perineural spread.

Fig. 1

Patient #0. ¹⁸F-FDG PET MIP (maximum intensity projection) (A) and sequential axial fused ¹⁸F-FDG PET/CT images (B-D) demonstrate extensive perineural spread (PNS) along the right brachial plexus (arrows in A and arrow in D). Increased FDG is present between the pectoralis major and minor muscles, along the course of the lateral pectoral nerve (LPN, solid arrowheads, A-D), and diffusely within the pectoralis minor muscle (open arrowheads, A-C), consistent with involvement of both the medial pectoral nerve (MPN) and LPN, and tumor infiltration of the pectoralis minor muscle

Fig. 2

Patient #1. ¹⁸F-FDG PET MIP (A and D) and axial fused.¹⁸F-FDG PET/CT images (B, C, E–F) demonstrate progressive PNS along the right brachial plexus (arrows) over 15 months. On the baseline exam (A-C), FDG uptake is present between the pectoralis major and minor muscles, along the course of the LPN (solid arrowheads, A and B), and along the posterolateral aspect of the pectoralis minor muscle (open arrowheads, A and C), consistent with involvement of both the MPN and LPN. There is diffuse FDG uptake along the brachial plexus, extending from the spinal nerves to the terminal branches (arrows). On the follow-up exam (D-G), there is marked progression of PNS along the brachial plexus, extending from the cervical spinal canal to the proximal upper extremity (arrows). Note the increased FDG activity within the atrophic pectoralis major muscle (solid arrowheads, D-F), and increased uptake along the pectoralis minor muscle (open arrowhead, E)

Fig. 3

Patient #1. Review of a previous MRI obtained for right shoulder pain demonstrates atrophy and increased T2 signal of the clavicular head of the right pectoralis major muscle (open arrowhead, A). A subsequent MRI (prior to the first available PET/CT) demonstrates enlargement and enhancement of the proximal LPN (solid arrowhead, B) and the cords of the brachial plexus (arrow, B) consistent with PNS

Fig. 4

Patient #2. ¹⁸F-FDG PET MIP (A), axial fused (B), and axial CT (C) images from an ¹⁸F-FDG PET/CT exam demonstrate extensive PNS along the entire right brachial plexus (arrows, A), with increased FDG activity along the LPN (solid arrowheads) and pectoralis minor muscle (open arrowheads). Corresponding axial T1 (D) and post-contrast (E) MRI images demonstrate an enlarged, enhancing LPN (solid arrowheads), enhancement within and along the posterior pectoralis minor muscle (open arrowheads), and diffuse enhancement within the pectoralis major muscle (arrow, E) consistent with PNS and intramuscular metastasis

Fig. 5

Patient #3. ¹⁸F-FDG PET MIP (A) and axial fused (B) images from an.¹⁸F-FDG PET/CT exam demonstrate intense FDG activity along the upper chest, corresponding to the LPN (solid arrowheads) and adjacent pectoralis major muscle. There is lower level uptake diffusely along the right brachial plexus (arrows, A) consistent with PNS. Corresponding axial T1 (C) and post-contrast (D) MRI images demonstrate enlargement and enhancement of the LPN (solid arrowheads). Note the atrophy of the pectoralis major and minor muscles (ovals, B-D)

Fig. 6

Patient #5. ¹⁸F-FDG PET MIP (A) and axial fused (B and C) images from an.¹⁸F-FDG PET/CT exam demonstrate increased uptake along the LPN (solid arrowheads), and increased uptake along the brachial plexus consistent with PNS, extending from the level of the cords to the trunks (arrows, A)