Navigating the 2022 ASCO Update: Using Biomarkers to Personalize Breast Cancer Treatment

1. Introduction: The Shift Toward Precision Oncology

Breast cancer management has transitioned from a standardized, "one-size-fits-all" model to a precision oncology framework. This evolution relies on validated biomarkers to determine which patients can safely de-escalate treatment by omitting chemotherapy and which require intensified systemic therapy.

The 2022 ASCO Guideline Update serves as the definitive clinical framework for integrating biomarker assay results into the management of early-stage breast cancer. By synthesizing high-level evidence from pivotal trials—including TAILORx, RxPONDER, and MINDACT—these recommendations provide clinicians with the clarity needed to personalize adjuvant endocrine and chemotherapy plans.

2. Genomic Testing for ER-Positive, HER2-Negative Breast Cancer

In the most common breast cancer subtype (ER+/HER2-), genomic assays are the standard of care for guiding chemotherapy decisions. However, clinical utility depends heavily on nodal status and menopausal age.

• Oncotype DX (21-gene RS):
  • Node-Negative: RS $\ge$ 26 indicates a clear chemotherapy benefit (Evidence Quality: High; Strength: Strong). For patients $\le$ 50 years with RS 16–25, clinicians may offer chemoendocrine therapy, noting that absolute benefit increases with score (e.g., for RS 21–25, the 5-year IDFS is 92.1% with chemo vs. 86.3% without).
  • Node-Positive (1–3 nodes): Recommended for postmenopausal patients with RS $\ge$ 26 (Evidence Quality: High; Strength: Strong). For premenopausal patients with 1–3 positive nodes, current data suggest chemotherapy benefit regardless of RS; therefore, the assay should not routinely guide the omission of chemotherapy in this group.
• MammaPrint (70-gene signature):
  • Recommended for patients > 50 years with High Clinical Risk (0–3 nodes) to guide chemotherapy omission (Evidence Quality: Intermediate; Strength: Strong).
  • Clinician Warning: MammaPrint is not recommended for patients $\le$ 50 years with High Clinical Risk, nor is it recommended for patients of any age with Low Clinical Risk (Evidence Quality: Intermediate; Strength: Moderate).
• EndoPredict & Prosigna:
  • EndoPredict: Recommended for postmenopausal patients with 0–3 positive nodes (Evidence Quality: Intermediate; Strength: Moderate).
  • Prosigna: Recommended for postmenopausal, node-negative patients (Evidence Quality: Intermediate; Strength: Moderate).

Special Consideration: The "4+ Nodes" Omission

For patients with $\ge$ 4 positive nodes, evidence is currently insufficient to recommend any genomic assay (Oncotype DX, MammaPrint, EndoPredict, Prosigna, or BCI) for routine chemotherapy or extended endocrine therapy decisions. In this high-burden population, these assays are prognostic only and should be limited to shared patient-physician decision-making rather than standard guidance.

Clinical Utility Table: Adjuvant vs. Extended Therapy

Assay Name	Adjuvant Chemotherapy Decision (Nodes 0-3)	Extended Endocrine Decision (Nodes 0-3)	Primary Population (Adjuvant)
Oncotype DX	Recommended	Insufficient Evidence	Premenopausal (N0); Postmenopausal (N0-3)
MammaPrint	Recommended (High Clinical Risk)	Insufficient Evidence	Age > 50 Only
EndoPredict	Recommended	Insufficient Evidence	Postmenopausal
Prosigna	Recommended (N0)	Insufficient Evidence	Postmenopausal (N0)
BCI	Recommended (N0)	Recommended	Postmenopausal or Age > 50

3. Protein Markers: Ki67 and IHC4

While genomic assays are preferred, protein-based markers provide specific utility for targeted therapy and resource-limited settings.

• Ki67 (Proliferation Marker): Clinicians should identify node-positive patients at high risk of recurrence with a Ki67 score $\ge$ 20% (via FDA-approved test) as candidates for two years of abemaciclib in addition to endocrine therapy (Evidence Quality: Intermediate; Strength: Strong).
  • Clinical Caveat: Ki67 is most technically reliable at the extremes (<5% or >30%). Values between 5% and 30% have limited technical reliability for distinguishing prognosis and should be interpreted with caution.
• IHC4 Score: This may be utilized to guide chemotherapy decisions only if multigene assays are unavailable.
  • Warning: Clinicians must ensure the test is locally validated in an experienced laboratory due to significant interobserver and interlaboratory variability.

4. Extended Horizons: Management Beyond 5 Years

Deciding whether to extend endocrine therapy to 10 years remains a significant challenge for ER-positive disease.

• Breast Cancer Index (BCI): Currently the only genomic tool recommended to predict the benefit of extended endocrine therapy in patients with 0–3 positive nodes (Evidence Quality: Intermediate; Strength: Moderate).
• CTS5 Web Tool: This prognostic resource estimates the risk of late recurrence (years 5–10) in postmenopausal women using clinical variables (age, size, grade, nodes).
  • Technical Restriction: CTS5 should not be used to estimate residual risk for patients already receiving extended endocrine therapy, as it significantly overestimates risk in that context.
• Prohibited Tests for Extended Therapy: Evidence is insufficient to use Oncotype DX, MammaPrint, Prosigna, EndoPredict, Ki67, or IHC4 to guide the decision to extend endocrine therapy beyond 5 years.

5. What Not to Do: HER2-Positive and Triple-Negative (TNBC)

Current multiparameter genomic assays (Oncotype DX, MammaPrint, BCI, etc.) were validated almost exclusively for ER-positive, HER2-negative disease.

Clinicians must not use these assays to guide systemic therapy decisions for HER2-positive or TNBC subtypes. Treatment for these aggressive subtypes must follow established clinical and pathological standards, as these genomic signatures lack mature evidence in these populations.

6. Emerging Biomarkers: The Research Frontier

The guideline identifies several biomarkers that, while promising, currently lack the "clinical utility" required to change standard adjuvant therapy:

• Tumor-Infiltrating Lymphocytes (TILs): Prognostic in TNBC and HER2+, but not yet validated for therapy de-escalation.
• PD-L1 Testing: Critical for metastatic TNBC, but evidence is insufficient to guide early-stage (neo)adjuvant decisions.
• CTC & ctDNA: While these can detect molecular residual disease, no evidence yet confirms that changing treatment based on these results improves patient outcomes.

7. Communication, Disparities, and Comorbidities

Effective clinical application of these guidelines requires a holistic view of the patient.

• Health Literacy: Oncology communication should use simple language and the "repeat back" method. Clinicians must ensure patients understand that biomarker results are tools for tailoring, not just "calculating," their future.
• Health Disparities: A critical finding in the TAILORx trial revealed that Black patients had higher recurrence rates and inferior overall survival despite having similar genomic recurrence scores to White patients. This highlights that biological assays do not capture the full spectrum of risk, necessitating inclusive care and attention to social determinants of health.
• Multiple Chronic Conditions (MCC): Biomarker results should only influence care if the patient is a candidate for the resulting treatment. Comorbidities and patient preferences must remain central to the final treatment plan.

8. Key Takeaways for the Clinic

1. Omitting Chemotherapy: Use Oncotype DX or MammaPrint for node-negative or postmenopausal N1 (1-3 nodes) disease to identify candidates for de-escalation.
2. Premenopausal N1 Exception: Do not use genomic assays to omit chemotherapy in premenopausal patients with 1–3 positive nodes.
3. The 4+ Node Rule: Genomic assays are prognostic only for patients with $\ge$ 4 positive nodes; clinical utility for therapy guidance is insufficient.
4. Targeted Therapy: Use Ki67 $\ge$ 20% to identify high-risk, node-positive candidates for adjuvant abemaciclib.
5. Extended Therapy: BCI is the primary predictive tool for the 5-to-10-year endocrine therapy decision; CTS5 is a supportive prognostic tool for postmenopausal women.
6. Subtype Restriction: Avoid all listed genomic assays in HER2+ and TNBC cases.
7. Evidence over Instinct: Always integrate ASCO ratings (Evidence Quality and Strength) into the risk-benefit discussion with the patient.