Morphological evidence for field effect as a mechanism for tumour spread in mammary Paget's disease

The histogenesis of mammary Paget's disease is controversial. The purpose of this study was to investigate the mechanism of tumour spread in the nipple epidermis by examining 28 cases of mammary Paget's disease associated with underlying intraductal carcinoma.

The atypical cells in the epidermis displayed a spectrum of cytological changes ranging from small-sized atypical cells located in the basal cell layer to large-sized atypical cells characteristic of Paget's cells in the upper layer of the epidermis. Serial sectioning revealed the presence of isolated, scattered and small atypical cells in the basal cell layer at the periphery of the epidermal lesion. The atypical cells, including those in the basal cell layer showed positive immunostaining for cytokeratin 7 and Her2/neu oncoprotein. Electron microscopy examination demonstrated the presence of intercellular junctions of desmosomal-like or desmosomal types between tumour cells and adjacent squamous cells. Furthermore, examination of the intraductal carcinoma of the breast tissue in cases of Paget's disease as well as control cases of intraductal carcinoma also revealed areas of skip lesions of intraductal carcinoma.

In view of these changes, it is unlikely that tumour expansion or tumour cell motility are sufficient explanations to account for the pattern of tumour spread in both the epidermis and the duct epithelium with skip lesions. A 'field effect' in the duct system harbouring intraductal carcinoma and the adjacent epidermis may play an important role in the tumour cell spread in the epidermis as well as in the ductal epithelium.