Aims This study was aimed to investigate the clinicopathological significance of immunohistochemical (IHC) Wilm’s tumour 1 (WT1) expression in invasive breast carcinoma with >90% mucinous components.

Methods One hundred specimens of invasive breast carcinoma with >90% mucinous component were collected. All H&E-stained slides were reviewed, and the clinicopathological data, including sex, age, tumour size, nuclear grade, histological grade, growth pattern and lymph node (LN) status, were collected. IHC staining of WT1, oestrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) and Ki-67 was performed. Fluorescence in situ hybridisation was used to verify the amplification of the HER2 gene. The relationship between WT1 expression and clinicopathological features was analysed statistically.

Results WT1 expression was detected in 67% (67/100) of invasive breast carcinoma with >90% mucinous components. WT1 expression was significantly associated with low-to-intermediate nuclear grade/histological grade, ER and PR positivity, HER2 negativity, Ki-67 proliferation index <30% and noLN metastasis (all p<0.001). Micropapillary architecture was observed in 80% of cases. WT1 expression was not significantly correlated with different percentage of micropapillary components (p=0.422). None of the histological grade 3 tumours, tumours with HER2 overexpression/amplification and triple-negative specimens showed WT1 expression.

Conclusions WT1 expression was significantly related with low-intermediate nuclear/histological grade, ER positivity, HER2 negativity, a lower Ki-67 proliferation index and no LN metastasis in invasive breast carcinoma with >90% mucinous component. The micropapillary growth pattern in this type of tumour did not show a specific relationship with WT1 expression.