Purpose: The immune microenvironment of breast ductal carcinoma in situ (DCIS) has yet to
be fully explored, and the relationship of immune cells to genetic features of DCIS is
unknown. Experimental Design: We quantified tumor associated lymphocytes (TIL) and
evaluated PD-L1 protein levels by immunohistochemistry in a cohort of pure DCIS (138 and
79 cases, respectively), some of which had copy number (n= 55) and mutation data (n= 20).
Results: TILs were identified in the stroma surrounding DCIS (119/138, 86%) and present at …

e12565 Background: DCIS is usually treated with resection followed by 5 years of adjuvant
endocrine therapy for hormone receptor (HR)+ DCIS. Endocrine therapy is not used in HR-
DCIS. Although DCIS is considered a precursor lesion to invasive breast cancer, the
different molecular subtypes confer variable clinical outcomes. The host immune response
plays a key role in breast cancer progression and response to therapy. However, relative to
invasive breast cancer, the immune milieu of DCIS is less understood. This retrospective …