Calcification and tumour‐infiltrating lymphocytes predict upgrade of human epidermal growth factor 2 ( <scp>HER2)</scp> ‐negative ductal carcinoma in situ diagnosed on core needle biopsy

Aims De‐escalated treatment for ductal carcinoma in situ (DCIS) aims to prevent overtreatment of patients. However, patients diagnosed preoperatively with pure DCIS via core needle biopsy (CNB) may be upgraded to invasive cancer upon excision. Therefore, it is essential to stratify patients at low risk for upgrade to invasive disease during the biopsy to inform management decisions. Methods and results In this study, we reviewed a cohort of patients with pure DCIS diagnosed by CNB and their corresponding excision specimens to identify predictive features for upgrading to microinvasion or invasive breast cancer (IBC). Of the 228 CNB cases, 82 (36%) were upgraded at excision, with 26.8% classified as microinvasion and 73.2% as frank invasion. Significant tumour intrinsic features associated with DCIS upgrade included larger tumour size, palpable mass, high grade and presence of necrosis ( P ≤ 0.026). Factors related to sampling adequacy, such as the number of cores, total core length and length of the longest core, were also predictive features ( P ≤ 0.027). Additionally, tumour microenvironment (TME) characteristics, including calcification and stromal tumour‐infiltrating lymphocytes (sTIL), were linked to upgrades, particularly in the human epidermal growth factor 2 (HER2)‐negative subgroup ( P ≤ 0.004). These features were independently associated with frank invasion in HER2‐negative cancers. Conclusions Our results provide valuable insights into additional predictive features for DCIS upgrade. When combined with adequate sampling and intrinsic tumour characteristics, TME features—namely tumour infiltrating lymphocytes (TILs) and calcification—are useful in predicting DCIS upgrades, especially in HER2‐negative cases.