Background: The level of tumor-infiltrating lymphocytes (TILs) and the presence of tertiary lymphoid structures (TLSs) are significant prognostic and predictive factors in primary breast cancer. However, understanding about the differences of TILs and TLSs among different metastatic sites or between primary breast tumors and metastatic sites is limited.

Methods: A total of 337 cases of metastatic breast cancer with available hematoxylin and eosin slides of metastatic sites (biopsy, n=185; operation, n=152) were included. We analyzed the percentage of TILs (defined as the percentage of invasive carcinoma stroma infiltrated by lymphocytes in 10% increments; if less than 10% of stroma was infiltrated by TILs, 1% or 5% criteria were used; all available full sections were evaluated) and the presence of TLSs (defined as the lymphoid aggregation with high endothelial venules) in primary and metastatic sites.

Results: The mean percentage of TILs in the lung (19.5%) was significantly higher than those in the liver (4.2%), brain (8.3%), and ovary (3.4%, p<0.001). The level of TILs in metastatic tumors of the lung was higher than those in primary tumors (Wilcoxon signed-rank test, p<0.001), while metastatic tumors of the liver (p=0.024) and brain (p=0.095) showed lower level of TILs than in primary tumors. TLSs were found only in the lung (27%) and liver (13%). Patients with lung metastasis (p<0.001), higher level of TILs (p=0.001), and presence of TLS (p=0.002) showed better overall survival. In patients with lung metastasis, who showed best clinical outcome, those with 10% or more of TIL had better overall survival than those with less than 10% (p=0.008).

Conclusion:. Metastatic breast tumors in the lung had more TILs than other sites and matched primary tumors. Underlying mechanism of differences of TILs among metastatic sites should be investigated.

Citation Format: Miseon Lee, Hee Jin Lee, In Hye Song, In Ah Park, Sun-Hee Heo, Young-Ae Kim, Won Seon Bang, AReum Lee, Hyeonjin Lee, Heejae Lee, Gyungyub Gong. Comparison of tumor-infiltrating lymphocytes between primary and metastatic tumors in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 697.