Objective To elucidate the cytopathological characteristics, molecular subtypes, and clinical prognoses of metastatic breast carcinoma with serosal effusions.

Methods Seventeen cases that included effusion cytology and clinical data were retrospectively analyzed. Two patients were diagnosed between April 2016 and September 2023 at the Suzhou Ninth Hospital Affiliated to Soochow University, and 15 patients were diagnosed at The First Affiliated Hospital of Soochow University. Cytopathological characteristics, molecular subtypes, and prognoses were analyzed.

Results The cytopathological features of metastatic breast carcinoma in serosal effusions were as follows: 1) Invasive ductal carcinoma: one cell type was relatively densely arranged in nests or colored spheres, which displayed an elevated nuclear/cytoplasmic ratio, irregular nuclear membrane, and cytoplasmic mucin vacuoles. The other cell type was tiled, single scattered, and varied in size and shape, with enlarged nuclei and elevated nuclear/cytoplasmic ratio. 2) Invasive lobular carcinoma: cells were scattered and uniform in size and shape. The nuclei had a relatively regular shape, but displayed an elevated nuclear-to-cytoplasmic ratio. Among the 17 breast cancer cases, 6 had transitioned in molecular subtyping, including 1 case from Luminal A to triple-negative type, 1 case from Luminal B to human epidermal growth factor receptor 2 (HER-2)-overexpressing type, and 4 cases from Luminal B to triple-negative type.

Conclusions The cytopathological characteristics of serous effusion cells combined with immunocytochemical staining and fluorescence in situ hybridization (FISH) suggest that it is important to determine the origin and molecular typing of tumor cells. This provides an important basis for precise clinical treatment and prognosis.