Defining lymphocyte‐predominant breast cancer by the proportion of lymphocyte‐rich stroma and its significance in routine histopathological diagnosis

Lymphocyte‐predominant breast cancer (LPBC) defined by the density of stromal lymphocytes shows favorable behavior. However, considerable distribution heterogeneity of lymphocytes is a major problem. The present study defined LPBC by the proportion of lymphocyte‐rich stroma with the cut‐off values of 30, 50, and 75%, and clinicopathologically analyzed mainly LPBC (area > 30%) defined by the cut‐off value of 30%. LPBCs (area > 30%), 39 cases in total, were composed mainly of triple‐negative and HER2(+)/ER (‐) subtypes, without any luminal A‐like subtype. LPBCs were composed predominantly of histological grade 3 tumors, without any grade 1 lesions. Multivariate analyses on 477 consecutive tumors revealed that ER‐negativity and grade 3 status associated significantly with LPBC. LPBC (area > 30%) showed better disease‐free survival than grade‐matched controls, and it was a good indicator of complete pathological remission after pre‐operative chemotherapy. Patients with LPBC with the cut‐off value of 50% and that of 75% showed 100% disease‐free survival. These results demonstrated the validity of our definition of LPBC. Our data also suggest that de‐differentiated cancers without TILs could be regarded as high‐grade cancer without lymphocyte‐mediated responses. In conclusion, the definition of LPBC by the proportion of lymphoid stroma is useful for prognostication of high grade breast cancer in routine diagnosis.