Despite the utility of certain biomarkers in predicting triple negative breast cancer outcomes after chemotherapy treatment, it appears as though the level of tumor-infiltrating lymphocytes (S-TILs) cannot predict the outcome when trastuzumab, an antibody targeted against HER2 protein, is used. These results, produced by the laboratory of Edith A. Perez, MD, at the Mayo Clinic Cancer Center, are insightful to clinicians who want to choose the correct therapy for their breast cancer patients.

“This study is part of a series we are conducting to determine which patients should use trastuzumab,” stated Dr. Perez in a news release from the Mayo Clinic. “We are now giving it to everyone whose tumors are HER2-positive, but not every patient benefits.” As is true for any drug that can have adverse side effects, it is best not to treat patients with trastuzumab if there are no positive effects.

Yet these findings are far from dismal. Although S-TILs do not predict outcome for this subset of patients, it does not mean immune function of these patients is unimportant. Rather, the way in which S-TILs are involved may include ways other than number.

“Researchers are really interested in the different components of the immune system as a predictor of the natural history of breast cancer and benefit from different therapies, but it may be that when trastuzumab is used, it is the function of the immune system and not the number of lymphocytes that is important,” said Dr. Perez.

Dr. Perez presented the findings at the 2014 San Antonio Breast Cancer Symposium. Approximately 1,000 patients with early stage HER2-positive breast cancer enrolled in the study. S-TILs were counted in patients undergoing chemotherapy or chemotherapy with trastuzumab. Only those individuals treated with chemotherapy alone showed a positive association between S-TIL level and recurrence-free survival. This starkly contrasts a separate study that indicated an increased benefit of trastuzumab in patients with HER2-positive breast cancer tumors.

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These results have impact in the clinical community, as trastuzumab is the standard treatment option for HER2-positive breast cancer, rather than chemotherapy alone. Due to the poor predictability of S-TILs in this context, it is unnecessary to test for S-TILs in HER2-positive breast cancer patients.

“Ours is a much more definitive study, showing this testing is not necessary in this subset of patients with HER2-positive breast cancer,” said Dr. Perez.