The radioactivity patterns of MCF-7 tumors after vehicle treatment were similar HCC1806 tumors after CB-839 treatment.<h4>Conclusion</h4>Our studies on [<sup>11</sup>C]glutamine in breast cancer models show significant retention of <sup>11</sup>C-activity in the form of [<sup>11</sup>C]glutamate in tumors with high GLS activity that confounds non-invasive inference of GLS activity. Here, GLS is linked to breast cancer.