Atypical intraductal epithelial proliferation (AEIP)

The term ‘atypical intraductal epithelial proliferation‘ is basically just another term for flat epithelial atypia (FEA) and columnar cell hyperplasia. Essentially it describes neoplastic cell growth in the breast ducts, primarily of epithelial cells and not mixed cells types, and with some odd features but without the characteristic cellular mutations and odd formations associated with malignancy. There is considerable difference in opinion as to whether these kinds of proliferations are fully benign or not, as in some instances they do develop alongside ductal carcinoma in situ. It is also not uncommon for flat epithelial proliferations to develop at the site of breast cancer surgical treatments. So,when this term, ‘atypical intraductal epithelial proliferation‘ (AIEP) is used (and it is not in common use) it is likely an attempt to describe a kind of ‘middle ground‘ diagnosis between flat epithelial hyperplasia (which is benign), and atypical ductal hyperplasia, which is considered suspicious for malignancy.

This page is still gives good information if you are researching Atypical Intraductal Epithelial Proliferations. However, we have created a whole new, more up-to-date page on such topic.


Background on proliferative breast neoplasia

Breast cancer screening often reveals potential or developing lesions composed of unexpected new cell growth. In general, new cell growth is often called a ‘neoplasm‘, and where it seems to be very rapid and extensive, it might be called ‘hyperplasia.’ Any time new cells are growing, the finding can be called a ‘proliferative‘ lesion, as opposed to a non-proliferative lesion caused by a ‘mechanical‘ blockage of some kind (usually resulting in a cyst). An ‘epithelial proliferation‘ means that epithelial cells (the kind of cell that usually comprise the lining of an organ) appear to be growing and accumulating more than normal. An ‘intraductal‘ epithelial proliferation indicates that the new growth is occurring around the breast ducts, and may even compromise their function to a degree.

Most proliferative neoplasms, including a generic (usual) epithelial proliferation are considered benign and of zero-to-very low risk of breast cancer development. But, when certain ‘cytological‘ features (cellular characteristic revealed by a microscopic analysis) are highly ‘atypical‘, this raises additional concern that the lesion might possibly indicate a very, very early presentation of breast carcinoma.

Atypical epithelial proliferation will likely emerge as a distinct category of breast lesion

Intraductal epithelial proliferations tend to grow in a ‘flat‘ pattern, or form into columns. But, it is a very consistent and regular formation that is not considered ‘atypical‘ enough to constitute atypical ductal hyperplasia. Cancer researchers and pathologists who use this term ‘atypical intraductal epithelial proliferation‘ are trying to make a point that even very subtle non-typical features may be significant enough to diagnose a specific, potentially malignant or pre-DCIS situation.

Some of these mildly atypical features might include larger nuclei, cellular polarizations, well-developed micropapillations, and complex or odd ‘architectural‘ formations involving bars, bridges, and ‘punched-out‘ fenestrations. However, there is a danger that creating a new ‘category‘ for these subtle differences, which have not yet been convincingly proven to have any direct link to DCIS or definitive increased risk of breast cancer, may cause needless anxiety and over-treatment. However, a body of new data is emerging which seems to suggest that usual ductal hyperplasia and atypical ductal hyperplasia are not related, but should be considered as distinct breast lesions, at least from a genetic point of view. Once this is established, then risk factors can be objectively evaluated.

Everything you pretty much need to know about atypical intraductal epithelial proliferation are listed above, but here are a couple extras below…

  • If you’ve been diagnosed with atypical intraductal epithelial proliferation, you have an increased risk of developing breast cancer in the future. For this reason, doctors often recommend intensive breast cancer screening and medications to reduce the breast cancer risk.
  • What are the symptoms? Atypical intraductal epithelial proliferation usually doesn’t cause any specific symptoms, but it may cause changes to appear on a mammogram.
  • What causes atypical intraductal epithelial proliferation? It’s not entirely clear what the cause is, however, it forms when breast cells become abnormal in size, shape, growth pattern and appearance.
  • What are the known tests and diagnosis? It is usually discovered after a biopsy to evaluate a suspicious area found on a mammogram or during a clinical breast exam.
  • What happens during the biopsy? During the biopsy, tissue samples are removed and sent for analysis by a pathologist. The tissue samples are then examined under a microscope, and the pathologist identifies atypical intraductal epithelial proliferation, if it’s present. For further evaluation, your doctor may recommend surgery to remove a larger sample of tissue to look for breast cancer.
  • What are some ways to reduce your risk? Take preventive medications, avoid menopausal hormone therapy, participate in a clinical trial, consider risk-reducing mastectomy.
  • Be sure to go for follow-up tests to monitor for breast cancer. Your options may include self exams, clinical exams, screening mammograms annually, and MRI (depending on other risk factors such as dense breasts, a strong family history or genetic predisposition to breast cancer).


  1. Ven de Vijver, Marc J., Pterse, Hans. The Diagnosis and management of pre-invasive breast disease: Pathological dignosis-problems with existing classifications. Breast Cancer Research (July 2003). 5:5; p. 269.
  2. Schnitt, SJ. Flat epithelial atypia — classification, pathologic features and clinical significance. Breast Cancer Res 2003, 5:263-268
  3. Lerwill MF. Flat epithelial atypia of the breast. Arch Pathol Lab Med. 2008 Apr;132(4):615-21
  4. Tavossoli, FA. Norris, HJ. A comparison of the result of long term follow-up of atypical intraductal hyperplasia and intraductal hyperplasia of the breast. Cancer, 1990, 65:518-529.
  5. Boecker, W. Buerger, H. Usual and atypical ductal hyperplasia—members of the same family?
    Current Diagnostic Pathology, Volume 10, Issue 3, Pages 175-182

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