How should an endometrial biopsy be processed and examined microscopically?

How to Analyze Endometrial Biopsy Under Microscope

Endometrial biopsy specimens should be fixed in 10% neutral buffered formalin, embedded in paraffin, sectioned serially, and examined with hematoxylin & eosin (H&E) staining as the primary method, supplemented by Masson trichrome or elastic trichrome staining when evaluating for specific pathology. 1

Specimen Processing and Fixation

The tissue must be handled with extreme care to minimize artifacts:

• Transfer the specimen from the bioptome using a sterile needle, never forceps, to prevent crush artifacts 2
• Fix immediately in 10% neutral buffered formalin at room temperature (not cold, to prevent contraction band artifacts) 1, 2
• Most of the specimen should go into formalin for routine histology, as this allows the full range of histochemical and immunohistochemical stains 1

Critical Pitfall

If less than 2 cm of tissue is obtained, taking portions for other uses (frozen sections, molecular studies) may compromise standard microscopic interpretation and lead to misdiagnosis or incorrect grading/staging 1.

Tissue Embedding and Sectioning

• Embed tissue in paraffin blocks after adequate fixation 1, 2
• Obtain serial sections that are sequentially numbered 2
• Standard section thickness for routine examination
• For special studies like Congo red (amyloid detection), use 10-15 μm thick sections 2

Staining Protocol

Primary Stains (Routine Examination)

For standard endometrial evaluation:

• Hematoxylin & eosin (H&E) - primary stain for all specimens 1, 2, 3
• Masson trichrome or elastic trichrome - to visualize collagen and elastic tissue, particularly useful when assessing for fibrosis or architectural changes 2

Supplementary Stains (Based on Clinical Indication)

• Iron stain - routinely performed on all men and postmenopausal women regardless of indication 2
• Congo red - when amyloidosis is suspected 2
• Immunohistochemical stains - preserved slides should be available for this purpose 2

Microscopic Examination Framework

Architectural Assessment

Evaluate the glandular-to-stromal ratio:

• Grade 1 endometrioid adenocarcinoma: <5% solid component 4, 5, 4
• Grade 3 endometrioid adenocarcinoma: >50% solid component 4, 5, 4
• Grade 2: intermediate between these values

Glandular Morphology

Examine whether glands:

• Resemble normal endometrium (endometrioid pattern) 4, 5, 4
• Show complex architecture with back-to-back glands (hyperplasia vs. carcinoma)
• Display papillary architecture (serous carcinoma) 5, 4
• Have clear cytoplasm (clear cell carcinoma) 5

Nuclear Features

• Nuclear atypia severity
• Mitotic activity
• Chromatin pattern
• Nucleolar prominence

Stromal Assessment

• Stromal invasion depth
• Inflammatory infiltrate (chronic endometritis requires biopsy for diagnosis) 6
• Vascular invasion
• Decidualization or other changes

Specimen Adequacy

A specimen is considered adequate when:

• Sufficient tissue volume is present for complete evaluation 1
• Endometrial glands and stroma are identifiable 3, 7
• Minimal crush artifact or processing artifact 2

Common inadequacy issues:

• 6.6% of specimens may be insufficient for classification 3
• Blind sampling techniques have limitations - may miss focal lesions like polyps 6, 8
• False-negative rates up to 10% with office biopsies 4, 5, 4

Pathologist Expertise Requirement

Critical consideration: A pathologist experienced in gynecologic pathology should interpret the biopsy, ideally in partnership with the clinician. Non-specialist pathologists have reported diagnostic errors (including clinically meaningful ones) in >25% of cases 1. Liberal use of second opinions from specialist pathologists is recommended 1.

Complementary Cytologic Examination

When available, cytologic smear examination can complement histologic evaluation, particularly for detecting adenocarcinoma and atypical hyperplasia with 100% sensitivity and specificity 7.

Special Circumstances

When Additional Studies Are Needed

• Electron microscopy: Reserve for metabolic diseases or research; requires glutaraldehyde fixation of 1-2 mm piece 1
• Frozen sections: For demonstrating lipids, porphyrins, or labile antigens; requires snap-freezing in embedding medium 1
• Molecular studies: Flash-freeze tissue, but only if sufficient tissue remains for standard microscopy 1

Immunohistochemistry Applications

Useful for:

• Estrogen and progesterone receptor status (absent in clear cell carcinomas) 5
• p53 expression (characteristic of serous carcinomas) 5, 4
• DNA mismatch repair proteins (MLH1, MSH2, MSH6) for Lynch syndrome screening 9