Tumours: definition, classification and tumorigenesis

Tumors – Definition, Classification, and Theories of Tumorigenesis

1. Definition of a Tumor (Neoplasm)

• Tumor / Neoplasm: an abnormal mass of tissue
• Growth is:
  • Excessive
  • Uncoordinated with normal tissues
  • Persistent after removal of the initiating stimulus
• Term applies to both benign and malignant lesions
• Key concept: loss of normal growth control

Distinction

• Tumor ≠ inflammation, hyperplasia, or regeneration
• Hyperplasia: controlled, reversible increase in cell number
• Neoplasia: autonomous, irreversible growth

2. General Characteristics of Neoplasms

Cellular Characteristics

• Clonality: tumor arises from a single transformed cell
• Altered cell cycle regulation
• Genomic instability
• Progressive accumulation of mutations

Biological Behavior

• Benign tumors:
  • Localized growth
  • Well differentiated
  • No metastasis
• Malignant tumors:
  • Invasive growth
  • Destruction of surrounding tissues
  • Ability to metastasize

3. Classification of Tumors

A. Classification According to Biological Behavior

Benign Tumors

• Slow-growing
• Well circumscribed
• Often encapsulated
• Do not infiltrate adjacent tissues
• Do not metastasize

Malignant Tumors

• Rapid or uncontrolled growth
• Poorly circumscribed
• Infiltrative and destructive
• Metastatic potential
• Cause systemic effects (cachexia, anemia)

B. Classification According to Tissue of Origin

1. Epithelial Tumors

• Benign:
  • Papilloma
  • Adenoma
• Malignant (Carcinomas):
  • Squamous cell carcinoma
  • Adenocarcinoma
  • Basal cell carcinoma
  • Transitional cell carcinoma

2. Mesenchymal Tumors

• Benign:
  • Lipoma (adipose tissue)
  • Fibroma (fibrous tissue)
  • Chondroma (cartilage)
  • Osteoma (bone)
  • Leiomyoma (smooth muscle)
• Malignant (Sarcomas):
  • Liposarcoma
  • Fibrosarcoma
  • Chondrosarcoma
  • Osteosarcoma
  • Leiomyosarcoma

3. Hematopoietic and Lymphoid Tumors

• Leukemias
• Lymphomas
• Plasma cell tumors (multiple myeloma)

4. Neuroectodermal Tumors

• Gliomas
• Neuroblastoma
• Melanoma

C. Classification According to Degree of Differentiation

• Well differentiated: resembles tissue of origin
• Moderately differentiated
• Poorly differentiated
• Anaplastic:
  • Severe pleomorphism
  • High mitotic activity
  • Loss of normal tissue architecture

D. Classification According to Growth Pattern

• Expansive (pushing margins)
• Infiltrative (finger-like invasion)
• Exophytic (growing outward)
• Endophytic (growing inward)

4. Nomenclature of Tumors

General Rules

• Benign mesenchymal tumors: tissue + "-oma"
• Malignant epithelial tumors: carcinoma
• Malignant mesenchymal tumors: sarcoma

Important Exceptions

• Lymphoma – malignant
• Melanoma – malignant
• Seminoma – malignant
• Hepatoma (hepatocellular carcinoma)

5. Theories of Tumorigenesis (Carcinogenesis)

A. Genetic Mutation Theory (Modern Central Theory)

• Cancer is a genetic disease at the cellular level
• Caused by accumulation of non-lethal mutations affecting genes controlling:
  • Cell proliferation
  • Cell differentiation
  • DNA repair
  • Apoptosis

Main Gene Groups Involved

• Oncogenes
• Tumor suppressor genes
• DNA repair genes

B. Oncogenes

Definition

• Oncogenes are mutated or overexpressed forms of normal genes (proto-oncogenes)
• They promote uncontrolled cell proliferation
• Mutation leads to gain of function

Mechanisms of Activation

• Point mutation
• Gene amplification
• Chromosomal translocation
• Increased gene expression

Important Examples of Oncogenes

• RAS
  • Encodes GTP-binding signal transduction protein
  • Constant activation → continuous growth signaling
  • Common in colon, pancreatic, lung cancers
• MYC
  • Transcription factor
  • Increases cell proliferation and metabolism
  • Amplified in lymphomas and solid tumors
• HER2/neu (ERBB2)
  • Growth factor receptor
  • Amplified in breast and gastric cancer
• BCR-ABL
  • Fusion gene from t(9;22) Philadelphia chromosome
  • Constitutive tyrosine kinase activity
  • Chronic myeloid leukemia

C. Tumor Suppressor Genes

Definition

• Genes that inhibit cell proliferation or promote apoptosis
• Loss of function mutations
• Both alleles usually need to be inactivated ("two-hit hypothesis")

Important Tumor Suppressor Genes

• TP53 (p53)
  • "Guardian of the genome"
  • Induces cell cycle arrest or apoptosis after DNA damage
  • Mutated in >50% of human cancers
• RB (Retinoblastoma gene)
  • Controls G1 → S phase transition
  • Loss leads to uncontrolled cell cycle progression
• APC (Adenomatous polyposis coli)
  • Regulates β-catenin and Wnt signaling
  • Mutation leads to colorectal adenomas
• BRCA1 / BRCA2
  • DNA repair genes (homologous recombination)
  • Loss increases risk of breast and ovarian cancer

D. DNA Repair Gene Defects

• Not directly oncogenic
• Increase mutation rate
• Lead to genomic instability

Examples

• Mismatch repair genes (MLH1, MSH2)
• Defective in Lynch syndrome (HNPCC)

E. Clonal Evolution Theory

• Tumor originates from a single mutated cell
• Progressive genetic instability
• Selection of aggressive subclones
• Explains:
  • Tumor heterogeneity
  • Drug resistance

F. Viral Theory of Oncogenesis

• Certain viruses induce malignant transformation
• Mechanisms:
  • Integration into host genome
  • Viral oncogenes
  • Chronic inflammation

Examples

• HPV – cervical carcinoma
• EBV – Burkitt lymphoma, nasopharyngeal carcinoma
• HBV / HCV – hepatocellular carcinoma

G. Chemical Carcinogenesis

• Exposure to carcinogenic substances

Types of Carcinogens

• Direct-acting carcinogens
• Indirect-acting (procarcinogens)

Stages

• Initiation
• Promotion
• Progression

H. Physical Carcinogenesis

• Ionizing radiation
• Ultraviolet radiation
• Causes DNA damage and mutations

I. Immunologic Theory

• Immune system normally eliminates transformed cells
• Tumor development occurs when immune surveillance fails
• Tumor immune evasion mechanisms:
  • Reduced antigen expression
  • Immunosuppressive cytokines

J. Stem Cell Theory

• Tumors arise from tissue stem cells
• Stem cells have:
  • Long lifespan
  • High proliferative potential
• Explains tumor recurrence and resistance

6. Adenoma–Carcinoma Sequence (Polyp to Cancer Pathway)

Definition

• Stepwise progression from normal epithelium → adenoma → carcinoma
• Classic model for colorectal cancer development

Molecular Stages

• Normal epithelium
  • No mutations
• Early adenoma formation
  • APC gene mutation
  • Activation of Wnt signaling
  • Increased β-catenin activity
• Intermediate adenoma
  • KRAS oncogene activation
  • Increased proliferation
• Late adenoma (high-grade dysplasia)
  • Loss of tumor suppressor genes (e.g. SMAD, DCC)
• Carcinoma
  • TP53 mutation
  • Invasion through basement membrane

Clinical Significance

• Explains rationale for colonoscopic polypectomy
• Early detection interrupts cancer development
• High-yield exam topic

7. Clinical Significance for Surgeons

• Basis for surgical oncology
• Determines:
  • Extent of resection
  • Margin requirements
  • Lymph node management
• Guides adjuvant therapy decisions
• Essential for exam and clinical reasoning

8. Exam-Oriented Key Points

• Tumor growth is autonomous and irreversible
• Carcinoma = epithelial malignancy
• Sarcoma = mesenchymal malignancy
• Metastasis defines malignancy (except locally aggressive tumors)
• Carcinogenesis is a multistep genetic process

9. Summary

• Tumors represent uncontrolled cell proliferation
• Classification is essential for diagnosis and treatment
• Tumorigenesis is multifactorial
• Understanding mechanisms underpins modern surgery and oncology