10.3 Principles of Neoplasia and Carcinogenesis

5. Principles of Neoplasia and Carcinogenesis

Neoplasia represents an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of normal tissues, persisting in an excessive manner after cessation of the inciting stimuli.

Benign versus Malignant Neoplasms

Molecular Basis of Carcinogenesis

Carcinogenesis is a multistep process driven by the sequential accumulation of genetic and epigenetic alterations:

1. Proto-Oncogenes to Oncogenes: Proto-oncogenes normally promote physiological cell growth and survival. Gain-of-function mutations transform them into dominant oncogenes (mutation of a single allele suffices):
   • RAS (H-RAS, K-RAS): Encodes membrane-bound GTP-binding proteins. Missense point mutations lock RAS in a permanently active GTP-bound conformation, continuously transmitting mitogenic signals through the MAPK/ERK pathway independent of external growth factor stimulation.
   • MYC (c-MYC): Nuclear transcription factor that activates cell cycle progression. Overexpressed via amplification in oral squamous cell carcinoma; chromosomal translocation t(8;14) in Burkitt lymphoma moves MYC adjacent to the immunoglobulin heavy chain promoter, driving uncontrolled B-cell proliferation.
2. Tumour Suppressor Genes: Genes that normally inhibit cell proliferation, repair damaged DNA, or promote apoptosis. Follows Knudson's "two-hit" hypothesis, requiring loss-of-function inactivation of both alleles:
   • TP53 ("Guardian of the Genome"): Located on chromosome 17p13.1, encodes the transcription factor p53. In response to DNA damage, hypoxia, or oncogenic stress, p53 is phosphorylated and stabilized. p53 transactivates p21(CIP1/WAF1), a cyclin-dependent kinase inhibitor (CKI) that arrests the cell cycle at the G1/S checkpoint by inhibiting Cyclin D/CDK4 and Cyclin E/CDK2, allowing time for DNA repair. If repair fails, p53 transactivates pro-apoptotic BAX and PUMA, directing the irreversibly damaged cell into apoptosis. Inactivation or missense mutation of TP53 occurs in over 60–70% of oral squamous cell carcinomas.
   • RB1 (Retinoblastoma Gene): Governs the G1 to S phase cell cycle transition. Hypophosphorylated pRb sequesters E2F transcription factors. When Cyclin D/CDK4/6 hyperphosphorylates pRb, E2F is released to drive S-phase entry. Inactivation of pRb (or its functional neutralization by high-risk HPV oncoprotein E7) allows unchecked cell division.
3. Angiogenesis: Tumours cannot expand beyond 1–2 mm in diameter without recruiting a blood supply. Tumour hypoxia stabilizes Hypoxia-Inducible Factor 1-alpha (HIF-1α), which transactivates Vascular Endothelial Growth Factor (VEGF) and basic fibroblast growth factor (bFGF), stimulating the sprouting of leaky, disorganized neovessels.

The Metastatic Cascade

Metastasis is the unequivocal hallmark of malignancy, occurring through a defined sequential cascade:

1. Loss of Intercellular Adhesion: Malignant epithelial cells downregulate and lose membrane E-cadherin (CDH1), severing adherens junctions and undergoing epithelial-mesenchymal transition (EMT).
2. Degradation of Extracellular Matrix: Neoplastic cells secrete matrix metalloproteinases (MMP-2 and MMP-9 gelatinases) and cathepsins that digest Type IV collagen in the epithelial basement membrane and interstitial matrix.
3. Locomotion and Intravasation: Tumour cells migrate through cleaved basement membrane defects via altered integrin signaling, penetrating low-resistance capillary or lymphatic lumens (intravasation).
4. Circulation and Immune Evasion: Within the bloodstream, tumour cells form heteroaggregates with host platelets (tumour emboli), shielding themselves from high shear stress and natural killer (NK) cell surveillance.
5. Extravasation and Colonization: Tumour cells adhere to distant endothelial beds, extravasate into target organ parenchyma, initiate neo-angiogenesis, and establish secondary metastatic colonies.

Grading, Staging and Prognosis

Candidates frequently confuse grade with stage, and examiners exploit it. Grade is a histological judgement about how closely the tumour resembles its tissue of origin: well, moderately or poorly differentiated. Stage is an anatomical judgement about how far the tumour has spread, expressed through the TNM system. Stage is by far the stronger predictor of survival in oral squamous cell carcinoma, which is why early detection — finding a small, node-negative lesion — matters more to prognosis than any refinement of treatment.

Field Change and Second Primaries

Field cancerisation, described by Slaughter, is the concept that the whole mucosa exposed to a carcinogen accumulates genetic damage, so the visible tumour sits within a wider field of genetically altered but clinically normal-looking epithelium. It explains three clinically important observations: multiple synchronous primary tumours, the appearance of metachronous second primaries years after successful treatment, and local recurrence at the margin of an apparently completely excised lesion. For the dental team, it is the reason a treated head and neck cancer patient needs lifelong mucosal surveillance at every recall, and the reason continued tobacco and alcohol use after treatment carries such a high risk of a second primary.