Malignant neoplasms infiltrating pseudoangiomatous' stromal hyperplasia of the breast: an unrecognized pathway of tumour spread

Aims: To describe five malignant tumours in the breast (two invasive carcinomas and three non‐Hodgkin's lymphomas) showing an unusual pattern of spread within anastomosing spaces consistent with those described in pseudoangiomatous stromal hyperplasia. Methods and results: The histological and immunohistochemical features of the cases are presented. In all cases, the neoplasms consisted of non‐cohesive E‐cadherin‐negative cells located within anastomosing vascular‐like channels. These channels showed morphological and immunohistochemical features consistent with those seen in pseudoangiomatous stromal hyperplasia: they were lined by flattened cells that expressed CD34, bcl2 and smooth muscle actin, while CD31 was consistently negative. Conclusions: The open channels of pseudoangiomatous stromal hyperplasia have been regarded as spaces due to loss of cohesiveness between stromal fibroblasts if not artefacts. The fact that neoplastic cells spread through these spaces suggests that the spaces are true open channels that may represent part of a complex network of stromal channels (pre‐lymphatics) connected to the main lymphatic system of the breast. This finding opens new insights into the routes of tumour spread.