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
