Breast phyllodes tumour with epithelioid feature predisposes to malignant transformation.

Aims: Phyllodes tumours (PTs) are relatively common fibroepithelial tumours comprising epithelial and stromal component. Usually, PTs show a spindle cell morphology with a fibroblast phenotype, while some tumour cells exhibit epithelioid morphological features and sarcomatoid transformation. However, the molecular characteristics of this morphology subset remain unclear. This study aimed to summarise the clinicopathological, morphological and molecular characteristics of seven cases of PT with epithelioid features.

Methods: Morphological and clinicopathological characteristics were observed and retrieved. Immunohistochemistry, immunofluorescence and electron microscope were performed on seven cases of epithelioid PT to explore immunophenotypic and ultrastructural characteristics. Transcriptomic and proteomic analyses were conducted to compare differentially expressed genes and proteins between epithelioid PT and classical PT.

Results: Patients with epithelioid PT exhibit a high recurrence rate (42.8%). Morphologically, in addition to having epithelioid cytological features, neoplastic stromal cells exhibit moderate to marked atypia and often exhibit sarcomatoid transformation, similar to the characteristics of borderline PT. Transcriptomic and proteomic analyses demonstrated that epithelioid PTs are distinct from classical PTs in gene expression and protein abundance levels. Immunohistochemical analysis showed that among all differentially expressed proteins, epithelioid PT showed abnormal p16/retinoblastoma expression patterns, similar to those of malignant PT.

Conclusions: Epithelioid PT has unique morphological characteristics, biological behaviour and protein expression profile, which meets the diagnostic criteria of borderline PT and is prone to sarcomatoid transformation. It may be a special morphological subgroup of borderline PT and has partial characteristics of malignant PT, which should be taken seriously in pathological diagnosis and clinical management.