Pseudoangiomatous Stromal Hyperplasia (PASH) is a benign lesion of the breast that may present as a clinical nodule or be found as an incidental finding. When found as an incidental finding it may be referred to as the PASH phenomenon.


Histologic features:

(1) There is a stromal proliferation with anastomosing channels resembling small blood vessels that may range from slit-like spaces to elongated, open channels.

(2) The channels are lined by a discontinuous layer flat cells with thin, elongated nuclei.

(3) There are dense bands of collagen separating the channels.

(4) Elongated strap-like cells may be present in the stroma that are CD34 positive.

(5) Gynecomastia-like changes may be present.


Lesions with a proliferation of bland spindle cells without nuclear atypia are referred to as fascicular/proliferative. Lesions without this change are termed simple.


The spaces are real and not artifacts of tissue processing.


Features distinguishing PASH from a vascular tumor:

(1) absence of red blood cells within the vascular slit-like spaces

(2) immunostains for CD31 and Factor VIII are negative


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