1. Core Principle – Cancer develops step-by-step

• A single mutation is rarely sufficient.
• Multiple cooperating genetic alterations accumulate over years/decades.
• Analogy: combo lock – multiple digits needed.

2. Driver vs Passenger Mutations

Driver mutations

• Present early – even in precursor lesions.
• Confer selective growth advantage.
• Initiate clonal expansion.
• Alone are insufficient for malignancy → require cooperating mutations.

Passenger mutations

• Neutral mutations acquired during proliferation.
• Do not contribute to malignant phenotype.

3. Tumors evolve by Darwinian selection

Mechanism:

• Mutations occur randomly.
• Cells with favorable mutations (growth/survival/invasion) outcompete others.
• These successful clones expand.

Consequences:

1. Tumor progression → increasing aggressiveness.
2. Declining responsiveness to therapy over time.

4. Tumor Progression – What it means

• Progressive accumulation of mutations.
• Leads to:
• uncontrolled proliferation
• invasion
• metastasis
• angiogenesis
• resistance to apoptosis
• immune evasion

Molecular events:

• New mutations arise continuously.
• Some are deleterious → kill the cell.
• Some confer selective advantage → retained and expanded.

5. Clonal Expansion and Heterogeneity

• Tumors originate monoclonally from a founding cell.
• Repeated mutation + natural selection → genetically diverse mass.
• Diagnosed tumors = mixture of many subclonal populations.

Memory analogy:

“One seed → many branches.”

6. Therapy Resistance – Why it happens

• Pre-existing resistant subclones survive treatment.
• Chemotherapy eliminates sensitive clones → selects for resistant ones.
• Recurrence often contains predominantly resistant clones.

Thus:

• Tumor aggressiveness increases over time.
• Responsiveness to therapy declines.

💡 Clinical + Exam Pearls

• Presence of a driver mutation in a benign lesion does not mean cancer—cooperating mutations must accumulate.
• Genetic heterogeneity explains variable clinical behavior within same tumor mass.
• Recurrent cancers typically contain cells that survived previous therapy due to resistant mutations.
• Cancer progression mirrors natural selection: mutation → selection → clonal expansion.

🧠 Concept Summary for Long-term Memory

1. Multistep model

cancer develops through sequential mutations

2. Early driver mutation

initiates proliferation but cannot create full malignancy alone

3. Darwinian selection

fittest clones survive and dominate

4. Tumor progression

increasing malignancy, invasion, metastasis

5. Genetic heterogeneity

monoclonal origin → polyclonal tumor at diagnosis

6. Therapy resistance

resistant subclones expand after treatment destroys sensitive clones