Breast cancer abutting the pectoralis major muscle on breast MRI: what are the clinical implications?

Objectives: Defining the posterior extent of breast cancer prior to surgery has clinical implications. However, there are limited data available to guide the interpretation of breast cancers seen on MRI that abut the pectoralis muscle but lack associated muscle enhancement. Methods: In this retrospective study of breast MRIs performed between May 2008 and July 2019, 43 female patients demonstrated breast cancers abutting the pectoralis muscle without enhancement of the muscle itself. Imaging features of the cancers as well as pathologic and clinical outcomes were recorded. Statistical analyses of associations between imaging findings and clinical outcomes were performed using Fisher's exact test, logistic regression, a Mann-Whitney U test and/or Student's t-test. Results: The pectoralis major muscle was pathologically invaded by carcinoma in 4/43 (9.3%). There was no significant association between pectoralis muscle invasion and any MR imaging feature of the breast cancer. Tumors causing deformation of the muscle contour by MRI, tumors larger in size, tumors with a larger extent abutting the muscle and tumors in which the imaging feature abutting the muscle was a mass or non-mass enhancement (rather than a spicule) were more commonly seen in patients with muscle invasion, although these did not reach statistical significance (p > 0.05). Conclusion: In this study, a lack of pectoralis muscle enhancement by MRI did not exclude pathologic muscle invasion by breast cancers abutting the muscle. Advances in knowledge: Knowledge of the likelihood of pectoralis muscle involvement for breast cancers abutting the pectoralis muscle on MRI may guide accurate interpretation and definition of the posterior extent of disease.