The American Joint Committee on Cancer has designated staging by the three
criteria of tumor size (T), spread to regional lymph nodes (N), and the
presence of distant metastasis (M) to define breast cancer. If metastasis
exists (M1), then the stage is IV. Otherwise, the stage (I, II, or III) is
determined by the T and N scores.<sup>1</sup>

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<sup>1</sup> Stage Information for Breast Cancer; National Cancer
Institute; 2015. Available from:
<a href='http://www.cancer.gov/types/breast/hp/breast-treatment-pdq#section/_27' target='_blank'>
http://www.cancer.gov/types/breast/hp/breast-treatment-pdq#section/_27</a>