Background: Assessment of pectoralis muscle invasion is important for treatment planning for breast cancer. We evaluated the usefulness of breast magnetic resonance (MR) imaging for the detection of tumor invasion of the pectoralis muscle in breast cancer patients.
Materials and Method: A total of 306 breast MR examinations were performed preoperatively. Three-dimensional gradient echo sequences, at a section thickness of 1.5 or 2 mm were obtained with administration of gadolinium-DTPA. All patients underwent surgery.
Results: In 33 breasts, disruption of the fat plane between tumor and muscle was noted. Seven of 33 cases showed muscle enhancement contiguous to enhanced tumors. Pathology reports indicated that 5 of 7 of the tumors involved muscle invasion. Of the 2 false positive cases, one showed muscle enhancement because of a previous biopsy, and the other was incorrectly interpreted as showing muscle enhancement. Of the 26 breasts which did not demonstrate muscle enhancement, none were found at surgery to have tumor involvement.
Conclusion: Enhancement of the pectoralis muscle correlates well with muscle invasion, but there are a few potential pitfalls. Disruption of the fat plane between tumor and muscle, without muscle enhancement, might not indicate tumor involvement of the pectoralis muscle.

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