Other benign tumors

Microglandular adenosis

Editorial Board Member: Gary Tozbikian, M.D.
Editor-in-Chief: Debra L. Zynger, M.D.
Julie M. Jorns, M.D.

Last author update: 11 February 2021
Last staff update: 25 April 2022

Copyright: 2002-2023,, Inc.

PubMed Search: Microglandular adenosis [title] breast

Julie M. Jorns, M.D.
Page views in 2022: 16,730
Page views in 2023 to date: 15,860
Cite this page: Jorns JM. Microglandular adenosis. website. Accessed September 28th, 2023.
Definition / general
  • Rare benign lesion composed of haphazard, irregularly distributed, small, uniform, rounded, open glands with eosinophilic secretions
  • Mimics invasive carcinoma due to infiltrative stromal growth and single epithelial layer devoid of myoepithelium
Essential features
  • Disordered proliferation of small open glands comprised of bland epithelial cells
  • Retains basement membrane but lacks myoepithelium
  • Infiltrates adipose and fibrous stroma without a stromal response
  • S100+, ER-, PR-
  • Potential precursor to triple negative invasive breast cancer
  • Microglandular adenosis (MGA)
  • Less frequently referred to as:
    • Microglandular hyperplasia
    • Microglandular adenoma
ICD coding
  • ICD-10:
    • N60.8 - other benign mammary dysplasias
    • N60.9 - unspecified benign mammary dysplasia
  • ICD-11:
    • GB20.Y - other specified benign breast disease
    • GB20.Z - benign breast disease, unspecified
  • Rare, represented in case reports and series and small studies
  • No specific known epidemiologic associations (reported in women over a wide age range)
  • No specific location in breast
  • Gain of somatic mutations including TP53, PIK3CA pathway related and tyrosine kinase receptor signaling elated genes have been identified in MGA and MGA associated triple negative breast cancers, supporting a role as a nonobligate cancer precursor (J Pathol 2016;238:677)
  • Unknown
Clinical features
  • Can present as a thickening, palpable mass or no clinical abnormality (seen on imaging only or incidental finding)
  • Histologic examination of tissue often with immunostains performed
Radiology description
  • Mammographic density or calcifications
Radiology images

Contributed by Julie M. Jorns, M.D.
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Mammographic density: MGA / atypical MGA

Prognostic factors
Case reports
Gross description
  • Indiscernible from surrounding tissue
  • Mass / nodule: more frequently identified with coexisting carcinoma
  • Ill defined margins
Microscopic (histologic) description
  • Haphazardly infiltrating collection of small, uniform, rounded, open glands with eosinophilic secretions, irregularly distributed in fibrous or adipose tissue
  • Glands lined by single layer of cuboidal / flat cells with vacuolated / granular cytoplasm and bland nuclei
  • No apocrine snouts, no nucleoli, no myoepithelial layer but thick basement membrane
  • Atypical microglandular adenosis:
Microscopic (histologic) images

Contributed by Julie M. Jorns, M.D.
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Scattered round glands

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Eosinophilic secretions

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With atypia, cribriforming

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With atypia, complex architecture

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With atypia, S100

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With atypia, collagen IV

Cytology description
  • Sparse cellularity, monotonous population of medium sized cells with vacuolated clear cytoplasm, round and uniform nuclei, small nucleoli
  • Also clear cells that are solitary or clustered with spindly fibroblasts (Diagn Cytopathol 1993;9:72)
Positive stains
Electron microscopy description
Molecular / cytogenetics description
  • Clonal in some cases, array CGH and sequencing studies demonstrate molecular progression to matched invasive carcinoma, supports the role of MGA as a nonobligate precursor to triple negative breast cancer (Hum Pathol 2019;85:65, Histopathology 2012;60:E115)
  • Mutations in PI3K pathway (PIK3CA, PTEN, INPP4B and BRCA1) in MGA and atypical MGA (Mod Pathol 2017;30:69)
Sample pathology report
  • Right breast, core biopsy:
    • Microglandular adenosis (MGA) (see comment)
    • Comment: Immunohistochemistry shows the lesion to be S100 positive (diffusely, strongly), estrogen receptor negative and to lack myopithelium via p63 and calponin, supporting the diagnosis. Controls are appropriate.
Differential diagnosis
Board review style question #1

    The pictured lesion shows 3 regions in a breast core biopsy. What is the expected biomarker profile of the invasive carcinoma?

  1. ER-, PR-, HER2-
  2. ER+, PR-, HER2-
  3. ER+, PR+, HER2-
  4. ER+, PR+, HER2+
Board review style answer #1
A. ER-, PR-, HER2-

The pictured lesion is microglandular adenosis (MGA) with associated invasive carcinoma, which is supported by S100 positivity in the MGA. Both MGA and associated carcinomas are most frequently ER-, PR- and HER2- (triple negative).

Comment Here

Reference: Microglandular adenosis of breast
Board review style question #2
    What is a feature that is characteristic of atypical microglandular adenosis (versus typical microglandular adenosis)?

  1. Bland cytology
  2. Fused / cribriform glands
  3. Intraluminal secretions
  4. S100 positivity
Board review style answer #2
B. Fused / cribriform glands

Typical microglandular adenosis (MGA) is characterized by haphazardly infiltrative tubules with characteristic luminal secretions and lined by a single layer of bland epithelial cells. Atypical forms have atypical cytology and architecture, including multilayered epithelium, gland fusion and cribriform architecture. Atypical MGA may not have as prominent secretions as in more typical forms. Both typical and atypical MGA are S100 (diffusely, strongly) positive.

Comment Here

Reference: Microglandular adenosis of breast
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