Histological features useful in the distinction of phyllodes tumour and fibroadenoma on needle core biopsy of the breast

Aim: To identify features useful in distinguishing phyllodes tumours from fibroadenomas on core biopsy. Methods and results: Starting from the diagnosis made on the surgical specimen, 12 features in the previous core biopsy specimens were analysed. Thirty‐six phyllodes tumours had 44 previous core biopsy specimens, which were reported as fibroadenoma in 11 and spindle cell lesion of uncertain nature in one, and included phyllodes tumour in the differential diagnosis in 32. The lesions with a core diagnosis of fibroadenoma were excised largely because they were growing or exceeded 30 mm; review of the corresponding surgical specimen showed heterogeneous stromal cellularity. Thirty‐eight fibroadenomas had previous core biopsy specimens reported as fibroadenoma in 37, and one of which included phyllodes tumour in the differential diagnosis. The following four features were significantly more common in cores from phyllodes tumours and had a κ statistic of > 0.6 in a reproducibility study: stromal cellularity increased in at least 50% compared with typical fibroadenoma, stromal overgrowth (×10 field with no epithelium), fragmentation and adipose tissue within stroma. Conclusions: This study describes features useful in the diagnosis of phyllodes tumour on core biopsy. Some core biopsy specimens from phyllodes tumours show features of fibroadenoma on core biopsy because of tumour heterogeneity.