Breast Core Biopsy - ER PR HER2 and Reporting

Key Points

• Core biopsy should distinguish invasive carcinoma from in situ disease and provide clinically actionable histology.
• ER, PR, and HER2 testing require validated assays and proper controls.
• ER 1% to 10% is reported as ER-low positive with appropriate interpretive context.
• HER2 IHC 2+ is equivocal and requires guideline-directed ISH resolution.
• The 2023 HER2 update did not redefine HER2-low as a new HER2-positive category.
• Fixation quality and cold ischemia affect biomarker reliability.
• Radiologic-pathologic discordance requires additional action even when the core is benign.
• Preserve pretreatment core tissue when neoadjuvant therapy or molecular testing is anticipated.

Algorithm

1. Confirm the image-guided target and specimen identity.
2. Diagnose histologic lesion and determine invasive versus in situ disease.
3. For invasive carcinoma, report type and grade and perform ER, PR, and HER2 testing.
4. Resolve HER2-equivocal results according to ASCO/CAP criteria.
5. Compare pathology with imaging and procedural findings for concordance.
6. If benign pathology is discordant with suspicious imaging, repeat biopsy or excision.
7. Preserve tissue for genomic or additional predictive testing when clinically relevant.
8. At definitive surgery, integrate residual disease, margins, nodes, treatment response, and stage.

Clinical Synopsis & Reasoning

Core needle biopsy is the standard tissue diagnosis for most suspicious breast lesions because it preserves architecture and supports receptor testing while avoiding the morbidity of diagnostic surgery. The pathology report should identify whether carcinoma is invasive or in situ, histologic type, provisional grade when feasible, lymphovascular invasion if present, associated DCIS, and any special features that affect management. For invasive carcinoma, ER, PR, and HER2 status are key predictive biomarkers. Ki-67 may be reported in selected practices but is not a substitute for validated genomic assays or established receptor testing.

ER and PR are assessed by immunohistochemistry with appropriate controls. Invasive cancers with at least 1% ER staining are considered ER positive under ASCO/CAP guidance, but tumors with 1% to 10% expression are reported as ER-low positive with a comment reflecting limited data and biologic heterogeneity. PR is also reported for invasive disease and is largely prognostic within ER-positive cancers. ER testing is recommended in DCIS to inform endocrine risk-reduction options; PR testing in DCIS is optional.

HER2 testing uses IHC and, when required, in situ hybridization. IHC 3+ is positive, IHC 0 or 1+ is negative for traditional HER2 amplification classification, and IHC 2+ is equivocal and requires ISH or another guideline-directed resolution. The 2023 ASCO/CAP update reaffirmed the existing testing framework while acknowledging therapy concepts such as HER2-low; laboratories should not invent a new HER2-positive category outside validated criteria.

Treatment Strategy & Disposition

Preanalytic quality matters. Cold ischemia should be minimized and formalin fixation performed within validated time parameters because ER, PR, and HER2 results can be affected by poor fixation. Decalcified bone metastases can lose antigenicity or compromise ISH, so testing strategy should be planned carefully. If a core contains very little invasive tumor, repeating biomarkers on the excision may be necessary.

Radiologic-pathologic concordance is mandatory. A benign core that does not explain a spiculated BI-RADS 5 mass is not reassuring and usually requires repeat sampling or excision. Likewise, atypical or high-risk lesions such as atypical ductal hyperplasia, lobular neoplasia, radial scar, or papillary lesion require management based on upgrade risk and concordance rather than the word 'benign' alone.

When neoadjuvant therapy is planned, the pretreatment core may be the only untreated tissue available for biomarker and molecular analysis. Preserve blocks accordingly. At surgery, the resection report documents residual tumor size, treatment response, margins, lymph nodes, and pathologic stage. Receptor retesting may be appropriate when the original result is questionable, tumor biology changes, or metastatic disease is sampled. Clinical treatment decisions should use the integrated pathology, not a single isolated stain.

Epidemiology / Risk Factors

• Breast cancer is biologically heterogeneous, and receptor-defined subgroups guide systemic treatment.
• Most breast cancers are ER positive.
• HER2 amplification or overexpression identifies a subset with major benefit from HER2-targeted therapy.
• Core needle biopsy has largely replaced open diagnostic biopsy for suspicious imaging lesions.

Investigations

Key Report Elements

Pharmacology

Prognosis / Complications

• Prognosis depends on stage, tumor size, nodal status, grade, receptor biology, genomic risk, and treatment response.
• ER-positive tumors often have a more prolonged recurrence risk profile.
• HER2-positive disease prognosis has improved substantially with targeted therapy.
• Triple-negative cancers have different systemic treatment and recurrence patterns.

Patient Education / Counseling

• Explain that receptor testing helps select treatment and does not simply describe the tumor.
• For an ER-low result, discuss that it is technically positive but may behave differently from strongly ER-positive disease.
• Tell patients that an equivocal HER2 test means additional testing is needed, not that the result is halfway positive.
• If pathology and imaging do not match, explain why another biopsy or excision may be recommended.
• Encourage patients to keep copies of pathology reports because biomarker details matter throughout treatment.

Notes

The words 'ER positive' or 'HER2 negative' are not enough for a high-quality report. Percent staining, assay category, controls, and whether additional testing was performed can all change interpretation and treatment.

Core biopsy is also a finite tissue resource. In the era of neoadjuvant therapy and molecular testing, unnecessary stains can exhaust the only pretreatment material available.

References

1. CAP/ASCO HER2 Testing in Breast Cancer 2023 Update — Link
2. CAP/ASCO ER and PgR Testing in Breast Cancer — Link