Breast - nonmalignant
Benign tumors / changes
Pseudoangiomatous stromal hyperplasia (PASH) of breast

Author: Hind Nassar, M.D. (see Authors page)

Revised: 11 August 2017, last major update April 2010

Copyright: (c) 2002-2017,, Inc.

PubMed Search: Pseudoangiomatous stromal hyperplasia [title] breast

Cite this page: Nassar, H. Pseudoangiomatous stromal hyperplasia (PASH) of breast. website. Accessed July 21st, 2018.
Definition / general
  • Benign lesion of complex, anastomosing, slit-like spaces, that are either acellular or composed of spindled cells
  • First described in 1986 (Hum Pathol 1986;17:185)
  • Also called pseudoangiomatous hyperplasia of mammary stroma
  • PASH phenomenon: cases in which PASH is an incidental microscopic finding (up to 23% of breast surgicals)
  • Myofibroblastic origin
Clinical features
  • Usually an incidental finding but may produce nodular lesions or have rapid growth
Radiology images

Images hosted on PathOut server:

Mammogram, contributed by Dr. Mark R. Wick

Case reports
Gross description
  • Usually unilateral, well circumscribed, external surface may resemble a capsule; 2 - 15 cm
  • Cut surface is firm, gray white, resembles fibroadenoma
  • No hemorrhage or necrosis
Gross images

Images hosted on other servers:

Nodular lesion

Multiple nodules (fig 2)

Microscopic (histologic) description
  • Proliferation of stromal elements (fibroblastic / myofibroblastic) mixed with breast ducts
  • Dense keloid-like stroma has anastomosing pattern of slit-like clefts (empty spaces) lined by single layer of flat spindle cells simulating vascular spaces
  • Single cells may form delicate bridges between the walls of a space
  • Cellular areas or plump spindle cells may obscure pseudoangiomatous structure
  • Often gynecomastia-like changes (Mod Pathol 2008;21:201)
  • Associated with columnar cell lesion (Int J Clin Exp Pathol 2009;3:87)
  • Rarely multinucleated giant cells (Breast J 2007;13:568)
  • No mitotic figures, no necrosis, no atypia
Microscopic (histologic) images

Images hosted on PathOut server:

Contributed by Dr. Mark R. Wick

Spindle cells at margins of slit-like spaces

Clefts and anastomosing
spaces in dense
collagenous stroma (small
blood vessel at arrow)

Images hosted on other servers:

Anastomosing pattern of slit-like spaces

Spindle cells at margins of slit-like spaces

With columnar cell lesion

Various images

Clefts (but not endothelium) are CD31 negative

Spindle cells (not ductal cells) are cytokeratin negative

Vimentin+ spindle cells

ER-beta positive


With gynecomastia (A: vimentin, B: CD34)

Cytology description
  • Moderately cellular with cohesive clusters of bland ductal cells (occasionally with staghorn pattern), single naked nuclei, some spindle cells with moderate cytoplasm and fine chromatin
  • Occasional loose hypocellular stromal tissue fragments containing spindle cells and paired elongated nuclei in fibrillary matrix (Acta Cytol 2003;47:373)
  • Findings can confirm benign nature of disease but are nonspecific, resembling fibroadenoma or phyllodes tumor (Indian J Pathol Microbiol 2005;48:260)
  • Finding plump spindled mesenchymal cells is suggestive (Diagn Cytopathol 2005;32:345)
Negative stains
Electron microscopy description
  • Spaces are not true vascular channels but due to disruption and separation of stromal collagen fibers
Differential diagnosis