Clinical and Biological Significance of HER2-Low in Ductal Carcinoma In Situ of the Breast

Ductal carcinoma in situ (DCIS) is the most common form of preinvasive breast cancer, with 5-10% of cases progressing into invasive disease. Herein, we investigated the association between HER2-low and clinico-pathological characteristics in DCIS and subsequent ipsilateral loco-regional relapse (LRR). We accessed our prospectively maintained institutional database. HER2 status was determined by immunohistochemistry and classified as null (score 0), over-expressed (3+), and low (1+ or 2+); in situ hybridization was not considered since it is not used for routine DCIS diagnostics. Among 375 patients with DCIS, median age was 54 (27-88) years, with a primary tumor size < 2.5 cm in 63%, grade III in 33%, and positive hormone receptor status (HR) in 81% of cases; 71% underwent breast-conserving surgery, 34% received adjuvant endocrine and 39% radiotherapy. A total of 197 (52%) had tumors with low HER2 expression, which resulted significantly associated with grade I/II (P < .001), Ki67< 20% (P < .001), and HR-positive status (P < .001). HER2-low distribution varied from 19.61% and 50% in ER negative and ER-low ( 95%) (P < .001). After a median 39-month follow-up (IQR 16-65), cumulative incidences of LRR was 0.054. Among 17 patients with paired primary tumor and LRR, 5 had discordant HER2 status, with an even distribution of increased and decreased HER2 expression. Low HER2 expression in DCIS is associated with features of reduced aggressiveness. Importantly, changes in HER2 expression may occur prompting retesting in recurrent cases, in line with observations in invasive breast cancer. Among 375 patients with DCIS, those with low HER2 expression (52%) also exhibited less aggressive clinico-pathological features. Importantly, HER2 expression changed in up to one-third of patients experiencing ipsilateral loco-regional relapse, prompting retesting in recurrent cases. Findings align with observations in invasive breast cancer, emphasizing the potential clinical relevance of HER2 status in DCIS.