3.1 Diagnostic Workup, Tissue Biopsy & Biomarker Evaluation

1.2 Diagnostic Workup, Tissue Biopsy & Biomarker Evaluation

Clinical Blueprint Focus: AOCNP exam candidates must understand the complete diagnostic workflow for solid and hematologic malignancies, including imaging selection (PET-CT, MRI, CT), tissue biopsy acquisition standards, biomarker classification (predictive vs. prognostic), next-generation sequencing (NGS) panels, microsatellite instability testing, and liquid biopsy indications.

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Diagnostic Workup & Staging Imaging

Upon clinical suspicion or detection of a mass during screening, a structured diagnostic workup is initiated to establish histology, determine primary organ site, and establish anatomical disease extent (staging).

Imaging Modalities in Oncology Workups

• Computed Tomography (CT) with IV & Oral Contrast: The primary workup tool for thoracic, abdominal, and pelvic malignancies. Contrast enhancement evaluates vascular invasion, parenchymal visceral metastases, and lymphadenopathy (defined as lymph nodes ≥1.0 cm in short axis).
• Positron Emission Tomography / Computed Tomography (18F-FDG PET-CT): Evaluates metabolic glucose avidity using Fluorodeoxyglucose (FDG). Indicated for initial staging, occult primary search, radiation therapy planning, and treatment response assessment (Deauville criteria in lymphoma; PERCIST criteria in solid tumors). Caution: Granulomatous disease (TB, sarcoidosis) and infection can produce false-positive FDG uptake.
• Magnetic Resonance Imaging (MRI): Gold standard modality for evaluating central nervous system (CNS) tumors, brain metastases, spinal cord compression, soft tissue sarcomas, pelvic malignancies (rectal, cervical, prostate), and hepatic lesion characterization.
• Radionuclide Bone Scan (Technetium-99m Methylene Diphosphonate [99mTc-MDP]): Detects osteoblastic bone turnover in prostate cancer, breast cancer, and blastic skeletal metastases.

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Tissue Biopsy Principles & Acquisition Techniques

Definitive cancer diagnosis requires tissue acquisition prior to initiating systemic chemotherapy, targeted therapy, or radiation. Selecting the correct biopsy modality is vital to obtain adequate tissue volume and preserve cellular architecture.

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Biomarker Evaluation: Predictive vs. Prognostic

Precision oncology relies on molecular biomarker characterization to guide prognosis and predict treatment response.

• Predictive Biomarkers: Provide information regarding the likelihood of response or benefit from a specific therapeutic intervention. Examples:
  • HER2/neu (ERBB2) overexpression/amplification predicts response to anti-HER2 targeted agents (trastuzumab, pertuzumab, fam-trastuzumab deruxtecan-nxki).
  • EGFR Exon 19 deletion / L858R mutation predicts sensitivity to EGFR tyrosine kinase inhibitors (osimertinib).
  • PD-L1 tumor proportion score (TPS) predicts efficacy of anti-PD-1/PD-L1 checkpoint inhibitors (pembrolizumab).
• Prognostic Biomarkers: Provide information regarding overall patient outcome, disease aggressiveness, or recurrence risk regardless of the specific treatment administered. Examples:
  • High Ki-67 proliferation index indicates aggressive cellular proliferation and worse overall survival.
  • TP53 mutation or deletion 17p in chronic lymphocytic leukemia (CLL) confers poor clinical prognosis.

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Key Targetable Biomarkers in Major Solid Tumors

Solid Tumor Type         Key Genomic & Protein Biomarkers            Targeted Therapy Class
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Non-Small Cell Lung      EGFR, ALK, ROS1, BRAF V600E, RET, MET,      TKIs (Osimertinib, Alectinib,
(NSCLC)                  KRAS G12C, NTRK, PD-L1                      Sotorasib, Dabrafenib/Trametinib)

Colorectal Cancer        KRAS/NRAS/BRAF (Wildtype vs Mutant),        Anti-EGFR mAbs (Cetuximab),
(CRC)                    HER2, MSI-H/dMMR, NTRK                      BRAF inhibitor (Encorafenib)

Breast Cancer            ER/PR, HER2, BRCA1/2, PIK3CA, ESR1,         Endocrine tx, CDK4/6i, PARPi,
                         TMB, Trop-2                                 Alpelisib, ADCs (T-DXd, SG)

Melanoma                 BRAF V600E/K, KIT, NRAS, PD-L1              BRAF/MEK inhibitors, Anti-PD-1

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Microsatellite Instability (MSI) & Mismatch Repair (MMR)

• Mechanism: DNA mismatch repair (MMR) proteins (MLH1, MSH2, MSH6, PMS2) correct single-base mismatches and insertion-deletion loops during DNA replication. Loss of function in any MMR protein leads to mismatch repair deficiency (dMMR) and accumulation of hypermutable microsatellite repeats (MSI-High).
• Testing Methods: Immunohistochemistry (IHC) tests for nuclear expression of the 4 MMR proteins (loss of protein signal = dMMR). Polymerase chain reaction (PCR) or NGS measures microsatellite instability directly (MSI-H).
• Clinical Significance:
  1. Therapeutic Selection: MSI-H/dMMR solid tumors express high neoantigen burden, rendering them highly sensitive to anti-PD-1 checkpoint inhibitors (pembrolizumab, dostarlimab) under tumor-agnostic FDA approvals.
  2. Hereditary Screening: Loss of MLH1/PMS2 or MSH2/MSH6 signal serves as the initial screening step for Lynch Syndrome (Hereditary Non-Polyposis Colorectal Cancer).

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Next-Generation Sequencing (NGS) & Liquid Biopsy

• Comprehensive Genomic Profiling (CGP): Multi-gene NGS panels sequence hundreds of cancer-related genes simultaneously, detecting single nucleotide variants (SNVs), copy number alterations (CNAs), insertion-deletions (indels), genomic fusions (e.g., NTRK, RET, ALK), Tumor Mutational Burden (TMB ≥10 mutations/megabase), and MSI status.
• Liquid Biopsy (Circulating Tumor DNA [ctDNA]): Analyzes cell-free tumor DNA shed into peripheral blood by dying malignant cells.
  • Clinical Indications: Used when tissue biopsy is unfeasible or carries high surgical risk; when tissue volume is exhausted; for rapid biomarker identification in advanced NSCLC; and for monitoring minimal residual disease (MRD) or acquired resistance mechanisms (e.g., EGFR T790M or C797S, ESR1 mutations).
  • Key Diagnostic Rule: Liquid biopsy has high positive predictive value (a positive result is actionable), but a high false-negative rate (up to 20–30% due to low tumor shedding). Therefore, a negative liquid biopsy result MUST be followed by a tissue biopsy.