Reproducible evaluation of tumor-infiltrating lymphocytes (TILs) using the recommendations of International TILs Working Group 2014.

Tumor infiltrating lymphocytes (TILs) in breast cancer play an important role in predicting the outcome of breast cancer. The goal of our current study is to investigate the consistency and reproducibility of the recommendations published by the International TILs Working Group 2014 among pathology trainees and pathologists. Hematoxylin & Eosin (H&E) slides from 129 breast cancer cases (one slide each) from 2009 to 2014 were evaluated. Each case was blindly and independently reviewed by two observers following the International TILs Working Group 2014 recommendations. Three pathology trainees (PGY2, PGY3 and PGY4) and three pathologists (2 general pathologists and 1 breast pathologist) were involved in this study. Of the 129 cases, 10 (10/129, 7.8%) cases had TILs >50%, 90 (90/129, 69.8%) cases had <10% of TILs, and 29 (29/129, 22.4%) cases had TILs ranging from 10 to 50%. Our results showed that in 104 cases (104/129, 80.6%) the TILs percentage was identical between the 2 observers. In 18 cases (18/129, 14%), the difference between the two observers was by 10% and in 7 cases (7/129, 5.4%) there was a difference of 20% or more. The inter-observer kappa value was 0.776 between two observers, and the kappa score improved to 0.86 if using the 3 categoric groups (<10%, 10-50%, and >50%). Our study showed that the recommendations and instructions for TILs evaluation by the International TILs Working Group 2014 were sufficiently detailed to be applied for TILs evaluation in breast cancer.