Aim: Metastases from ovarian serous papillary carcinoma to the breast and primary invasive micropapillary carcinoma of the breast are histologically similar. The distinction is clinically important to ensure appropriate management. Wilms' tumour-1 (WT1) and Ca125 are frequently expressed in serous papillary carcinomas, and uncommonly in unselected mammary carcinomas. One previous study found Ca125 expression in 69% of invasive micropapillary carcinomas. The aim was to assess the frequency of expression of WT1 and Ca125 in invasive micropapillary carcinoma.
Methods and results: Twenty-five of 34 invasive micropapillary carcinomas showed no nuclear expression of WT1. The remaining nine tumours showed weak to moderate immunoreactivity in 1-10% of nuclei. Six of these nine tumours also contained ductal carcinoma in situ, which expressed WT1 in five of the six. Membranous or cytoplasmic expression of Ca125 was found in seven tumours.
Conclusion: Nuclear WT1 expression is present in a minority of invasive micropapillary carcinomas and, when present, expression is focal. The frequency of expression of Ca125 was similar to the results in unselected mammary carcinoma. Thus, these markers are useful members of the immunohistochemical panel for the distinction of mammary invasive micropapillary carcinoma from ovarian serous papillary carcinoma.