Intraductal atypia

Flat epithelial atypia

Editor-in-Chief: Debra L. Zynger, M.D.
Julie M. Jorns, M.D.

Last author update: 30 April 2020
Last staff update: 17 February 2021

Copyright: 2002-2023,, Inc.

PubMed Search Flat epithelial atypia [title] breast

Julie M. Jorns, M.D.
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Cite this page: Jorns JM. Flat epithelial atypia. website. Accessed October 2nd, 2023.
Definition / general
  • Introduced by World Health Organization (WHO) Working Group on Pathology and Genetics of Tumours of the Breast to encompass columnar cell lesions with low grade / monomorphic cytologic atypia that lack architectural features of atypical ductal hyperplasia (ADH) or low grade ductal carcinoma in situ (LG DCIS)
Essential features
  • Atypical epithelial proliferation with one to several layers of disorganized, monotonous, low columnar to cuboidal cells without architectural atypia
  • WHO acceptable:
    • Flat epithelial atypia
    • Also columnar alteration with atypia (columnar cell change with atypia or columnar cell hyperplasia with atypia)
  • Not recommended by WHO:
    • Low grade clinging carcinoma (monomorphous type)
    • Atypical cystic lobules
    • Atypical lobules type A
  • Other (historical):
    • Flat ductal intraepithelial neoplasia grade 1A (DIN1A)
    • Atypical columnar alternation with prominent apical spouts and secretions
ICD coding
  • ICD-10: N60.9 - unspecified benign mammary dysplasia
  • ICD-10: N60.89 - other benign mammary dysplasias
  • Breast terminal duct lobular units (TDLUs)
Clinical features
  • Imaging guided (frequently stereotactic due to mammographically detected calcifications) needle core or surgical excisional biopsy with tissue histologic examination
Radiology description
Radiology images

Contributed by Julie M. Jorns, M.D. and Shadie Majidi, M.D.

Calcifications corresponding to FEA

Calcifications, craniocaudal view

Calcifications, mediolateral view

Prognostic factors
Case reports
  • 47 year old woman with sonographic features of flat epithelial atypia manifesting as a nonmass-like lesion (Am J Case Rep 2019;20:340)
Gross description
  • No discernible lesion (typically a microscopic finding only)
Microscopic (histologic) description
  • Atypia that does not fulfill criteria of atypical ductal hyperplasia or low grade ductal carcinoma in situ due to flat growth pattern (i.e. cytologic atypia and lack of architectural atypia)
  • Distention of terminal duct lobular unit with dilated glands
  • Glands usually more blue than normal terminal duct lobular units at low power
  • Frequent secretions and calcifications within cystically dilated glands
  • Atypical epithelial proliferation with lack of regular orientation perpendicular to basement membrane
  • One to several layers of low columnar or cuboidal cells with low grade cytologic atypia resembling low grade ductal carcinoma in situ
  • Increased nuclear:cytoplasmic ratio (imparts blue low power appearance)
  • Prominent apical tufting (snouts)
  • Nuclei are usually round with variably prominent nucleoli
  • Apocrine features such as eosinophilic cytoplasm may be present
Microscopic (histologic) images

Contributed by Julie M. Jorns, M.D.

Bluer appearance

Luminal secretions

Lack of polarity


Apocrine features

features and
luminal secretions

Prominent nucleoli

Cytology description
  • Flat sheets of atypical cells with enlarged nuclei, distinct cell borders; indistinguishable from well differentiated in situ and invasive carcinoma (Diagn Cytopathol 2007;35:73)
Positive stains
Negative stains
Molecular / cytogenetics description
  • Phosphatidylinositol 3 kinase catalytic subunit (P13KCA) mutations frequently observed similar to other breast epithelial proliferations (Mod Pathol 2014;27:740)
Sample pathology report
  • Right breast, core biopsy:
    • Flat epithelial atypia (FEA) with microcalcifications
Differential diagnosis
Board review style question #1

Acceptable WHO (5th edition) terminology for the pictured lesion (H&E, 10x) includes

  1. Atypical cystic lobules
  2. Atypical lobules type A
  3. Flat epithelial atypia
  4. Low grade clinging carcinoma (monomorphous type)
Board review style answer #1
C. Flat epithelial atypia

The pictured lesion has classic features of flat epithelial atypia (FEA), at low power showing a terminal duct lobular unit (TDLU) with dilated, blue glands owing to increased nuclear:cytoplasmic ratios, which at high power show loss of orientation toward lumina and prominent apical tufting or snouts. Other current WHO accepted names include columnar cell alteration (change / hyperplasia) with atypia. Atypical cystic lobules, atypical lobules type A and low grade clinging carcinoma (monomorphous type) are terms that are not recommended for use by the WHO.

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Reference: Flat epithelial atypia of breast
Board review style question #2
Flat epithelial atypia most commonly has the corresponding imaging feature of

  1. Calcifications on mammography
  2. Enhancing mass on magnetic resonance imaging (MRI)
  3. Filling defect on ductogram (galactogram)
  4. Mass on ultrasound
Board review style answer #2
A. Calcifications on mammography

Flat epithelial atypia (FEA) is typically diagnosed by stereotactic biopsy of mammographically detected intraluminal calcifications. Less frequently, it constitutes an incidental finding on biopsy or excision. It does not typically present as mass on imaging nor a filling defect on ductogram or galactogram.

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Reference: Flat epithelial atypia of breast
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