Principles of Neoplasia
Overview
Neoplasia is defined as pathological, autonomous, uncontrolled cellular proliferation independent of normal growth signals. The fundamental distinction between benign and malignant rests on the capacity for local tissue invasion and distant metastasis.
Benign vs Malignant
- Benign: Well-circumscribed, encapsulated, slow-growing, well-differentiated (resembles parent tissue), does NOT metastasise.
- Malignant: Irregular margins, invasive (no true capsule), rapidly growing, poorly differentiated (high nuclear:cytoplasmic ratio, pleomorphism, mitoses), capable of metastasis.
The Dysplasia Spectrum
- Hyperplasia: Increased number of normal cells. Reversible.
- Metaplasia: One differentiated cell type replaced by another (e.g., columnar to squamous in Barrett's oesophagus). Reversible but pre-malignant.
- Dysplasia: Disordered growth with cellular atypia, mitoses, and loss of polarity. Pre-malignant.
- Carcinoma-In-Situ (CIS): Full-thickness severe dysplasia still ABOVE the basement membrane. No invasion.
- Invasive Carcinoma: Neoplastic cells breach the basement membrane — this is the defining moment of malignancy.
WarningThe Basement Membrane: The Critical Threshold
FNAC (cytology) cannot assess architectural relationships — it cannot distinguish CIS from invasive carcinoma. A histological core biopsy is always required to confirm basement membrane breach and thus confirm invasion.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Carcinomas (epithelial) metastasise via lymphatics first, e.g., cervical nodes in head and neck SCC. Sarcomas (mesenchymal) disseminate via blood first: lung is the primary site of haematogenous secondaries. Renal cell carcinoma is a classic exception to the epithelial rule — it spreads haematogenously via the renal vein and IVC.