Complete disappearance of ductal carcinoma in situ after neoadjuvant therapy in HER2-positive breast cancer: a case report

Introduction and importance: Ductal carcinoma in situ (DCIS) is traditionally considered poorly responsive to neoadjuvant therapy (NAT) because the intact myoepithelial layer and basement membrane may limit drug penetration. Recent studies, however, suggest that some DCIS components accompanying invasive breast cancer may regress after NAT, particularly in biologically aggressive subtypes. Case presentation: A 51-year-old woman presented with a right breast mass, clustered microcalcifications, biopsy-proven axillary nodal metastasis, and a suspicious subclavicular node. Core needle biopsy of the breast showed high-grade DCIS with focal suspicious microinvasion and an HR-negative/HER2-positive phenotype. She received anthracycline- and taxane-based NAT with anti-HER2 therapy, followed by a right modified radical mastectomy and nodal dissection. Final pathology demonstrated no residual invasive carcinoma, no residual DCIS in the breast, and no residual nodal disease, consistent with a pathological complete response. Clinical discussion: This finding should not be interpreted as definitive evidence that DCIS itself is chemosensitive. In the present case, the disappearance of the DCIS component may have resulted from a bystander effect after the disruption of the ductal barrier by microinvasive or unsampled invasive disease. In addition, persistent post-NAT microcalcifications on mammography did not indicate viable residual tumor, highlighting the risk of overtreatment when the surgical extent is determined primarily by residual calcifications. Conclusion: This case underscores three clinically relevant points: nodal metastasis in biopsy-diagnosed DCIS/ductal carcinoma in situ with microinvasion should prompt careful reassessment for occult invasion; disappearance of DCIS after NAT is hypothesis-generating rather than proof of direct chemosensitivity; and residual microcalcifications alone should not determine the surgical extent.