Mass-Forming Ductal Carcinoma In Situ >10 mm on Ultrasonography: Sonographic-Pathological Correlation Underlying Mass Formation and Implications for Invasion Assessment.

OBJECTIVE: To clarify the sonographic-pathological correlation of DCIS presenting as masses >10 mm on ultrasonography (US) and identify pathological factors complicating differentiation from invasive carcinoma. METHODS: This retrospective single-center study included 52 patients with surgically confirmed conventional DCIS presenting as mass-forming lesions >10 mm on preoperative US between 2020 and 2023. Non-mass abnormalities, masses ≤10 mm, neoadjuvant therapy, and vacuum-assisted biopsy were excluded. Sonographic features were evaluated according to the Japan Association of Breast and Thyroid Sonology (JABTS) guidelines. Comprehensive sonographic-pathological correlation was performed by multidisciplinary consensus review to clarify the histopathological architectures underlying mass-forming appearance on US and identify pathological factors associated with US findings suspicious for invasion. RESULTS: Through sonographic-pathological correlation, mass-forming DCIS were found to fall into three major sonographic-pathological patterns: intracystic (9/52, 17.3%), clustered outline (34/52, 65.4%), and sclerosing stromal (7/52, 13.5%). On preoperative US, invasion was suspected in 26/52 cases (50.0%). Among the three major patterns, suspected invasion was most frequent in the sclerosing stromal pattern (6/7, 85.7%), followed by the clustered outline pattern (17/34, 50.0%) and the intracystic patterns (2/9, 22.2%). Suspicious US findings were frequently related to interruption of the mammary gland interface and echogenic halo. Histologically, background sclerosing adenosis, radial sclerosing lesion, inflammatory changes, and fibrosis contributed to invasion-mimicking findings. CONCLUSION: The mass-forming appearance of DCIS >10 mm on US could be explained by three major sonographic-pathological patterns: intracystic, clustered outline, and sclerosing stromal. Background sclerosis, particularly in the sclerosing stromal pattern, can mimic invasion and complicate preoperative differentiation from invasive carcinoma.