Overview
This lecture covers how cancer spreads (metastasis) and how that spread is staged clinically. It first works through the metastatic cascade step by step (invasion, intravasation, circulation, extravasation, colonisation), then covers the named routes by which tumours reach distant sites (contiguous, haematogenous, lymphatic, transcoelomic), and finishes with cancer staging: why it is done, the techniques used, and the TNM system (illustrated with AJCC 8th edition colorectal cancer criteria) and its link to prognosis.
The metastatic cascade
- Metastasis is one of the Hallmarks of Cancer (invasion & metastasis), alongside sustaining proliferative signalling, evading growth suppressors, avoiding immune destruction, replicative immortality, tumour-promoting inflammation, inducing vasculature, genome instability, resisting cell death and deregulating metabolism; newer/emerging hallmarks added to this framework include unlocking phenotypic plasticity, nonmutational epigenetic reprogramming, polymorphic microbiomes and senescent cells.
- The metastatic cascade has five sequential steps: invasion → intravasation → circulation → extravasation → colonisation.
- Invasion: tumour cells leave the primary site and enter surrounding tissue/vessels.
- Intravasation: tumour cells enter (cross into) a blood vessel.
- Circulation: tumour cells travel in the bloodstream as circulating tumour cells (CTCs).
- Extravasation: tumour cells exit the vessel at a distant site.
- Colonisation: tumour cells establish and grow at the distant site, via micrometastasis then macrometastasis.
Invasion and EMT
- Invasion is the first step of the metastatic process and requires: loss of cell adhesion, degradation of the extracellular matrix, and gain of motility/migration.
- Tumour cells then penetrate the basement membrane and invade surrounding structures: blood vessels, lymphatic vessels, and nerves.
- Epithelial-to-mesenchymal transition (EMT) is the process underlying invasion. Epithelial cells (tight junctions, adherens junctions, desmosomes, gap junctions, cytokeratin intermediate filaments, apical-basal polarity, attached to basement membrane via integrin α6β4 at hemidesmosomes) transition to a mesenchymal phenotype (minimal cell-cell junctions, actin stress fibres and vimentin intermediate filaments, front-rear polarity, attachment to ECM via β1/β3 integrins at adhesion plaques). The reverse process is mesenchymal-to-epithelial transition (MET).
- Tumour cells can disseminate individually (via full EMT, single cell dissemination) or collectively (as a cohesive group with only partial EMT, led by a “leader cell” at the invasive front).
Intravasation
Intravasation (tumour cells entering the vasculature) is shaped by three categories of factors:
- Cellular microenvironment: surrounding adipocytes, neutrophils, platelets, macrophages, pericytes, fibroblasts.
- Intrinsic tumour cell properties: gene expression, genetic mutations, epigenetic regulation, metabolism.
- Mechanical cues: fluid pressure, solid stress, tissue stiffness.
Circulation and circulating tumour cells (CTCs)
- CTCs survive in circulation despite fluid shear stress (FSS), which differs between macro- and microcirculation:
- Macrocirculation: residence time seconds, flow velocity 100-500 mm/s, shear stress 4-30 Dyn/cm² (up to 100 Dyn/cm² in the heart).
- Microcirculation (capillary bed): residence time seconds to days, flow velocity 0.01-1.5 mm/s, shear stress 10-20 Dyn/cm².
- CTCs can form homotypic (tumour cell-tumour cell) or heterotypic clusters (with other cell types) in circulation.
- Interactions that help CTCs survive and progress: FSS and vessel constrictions promote a stiffness-regulation response; platelet interactions promote long-term arrest of CTCs; activated neutrophil interactions promote extravasation and increased CTC proliferation; monocyte interactions (via cytoplasmic shedding) promote successful metastasis.
Extravasation
- Sequence: initial attachment of the tumour cell to the endothelium, motility/rolling along the vessel wall in the direction of blood flow, then diapedesis (transmigration through the endothelial layer) and angiopellosis (movement out of the vessel into surrounding tissue).
- Rolling and adhesion use specific receptor-ligand pairs: tumour cell sLe^x/sLe^a bind endothelial P-selectin; tumour cell PSGL-1 binds endothelial E-selectin (rolling); firmer arrest and adhesion then involve tumour cell N-cadherin, L1CAM and integrins binding endothelial VCAM-1 and ICAM-1.
- Molecular signalling around the adherent tumour cell (CXCL12, VEGF, ANGPTL4, CCL2, ANGPT2, CXCL5/7, IL-1β, MMP9) disrupts endothelial VE-cadherin junctions and recruits monocytes/macrophages and neutrophils, opening a “microtrack” through the endothelium that the tumour cell uses to extravasate.
Colonisation
Colonisation has two phases:
- Homing: (1) mechanical trapping of the tumour cell in a vessel; (2) site-specific adhesion or chemoattraction; (3) formation of a pre-metastatic niche (stromal cells, cancer stem cells, inflammatory cells within ECM); (4) quiescence of the disseminated tumour cell.
- Colonization (growth): (5) micrometastasis, a small cluster of differentiated cancer cells and cancer stem cells; (6) macrometastasis, a larger vascularised tumour mass.
Routes of spread
Four named routes of cancer spread: contiguous spread, haematogenous spread, lymphatic spread, and transcoelomic spread. Related terms: LVI (lymphovascular invasion) and PNI (perineural invasion).
Warning
- Nodal (lymphatic) route: shown on PET/CT with focal high tracer uptake in pelvic and axillary/chest lymph nodes, indicating nodal metastatic disease.
- Haematogenous route: shown on imaging as multiple liver lesions (CT) and multiple round pulmonary nodules (chest CT), i.e. blood-borne spread to liver and lungs.
- Transcoelomic (body cavity) spread: peritoneal fluid flow within the abdominal cavity carries tumour cells around the peritoneum; imaging shows ascites outlining bowel loops. Multi-modality example: CT shows peritoneal/pelvic deposits and lymph nodes, gross pathology shows tumour deposits on resected bowel/omentum, and histology confirms tumour cells/mitotic figures.
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Cancer staging
- Stage refers to the extent of cancer spread, both locally and more distantly.
- Staging procedures aim to determine whether disease is localised only, or distant/metastatic disease is present.
- Staging determines treatment intent: curative vs non-curative.
- Staging is also used for prognostic purposes.
Techniques of staging
- Examination: lymph node basins, skin, pleural effusion, ascites, organomegaly (e.g. hepatomegaly).
- Imaging: chest X-ray, nuclear medicine bone scan, staging CT (chest/abdomen/pelvis), MRI, FDG-PET.
- Surgical techniques: endoscopic ultrasound, laparoscopy, colonoscopy, bronchoscopy, pan-nasal endoscopy.
- Blood tests (tumour markers): LDH (melanoma), hCG (gestational trophoblastic tumours), LDH/hCG/AFP (testicular cancer), CEA.
TNM staging system
- TNM criteria differ by anatomical site.
- T (Tumour): depth of invasion through the organ, or size of the primary tumour, depending on cancer type (e.g. breast cancer T-stage is by size of the primary; mucosal-based cancers such as colorectal cancer are T-staged by depth of invasion).
- N (Nodes): number of regional lymph nodes involved.
- M (Metastases): presence of distant metastases.
- Combined TNM generates an overall stage: Stage I (small local invasion only, best survival), Stage II (locally advanced), Stage III (involving loco-regional lymph nodes), Stage IV (distant metastases, worst survival). Equivalently: Stage 0 = carcinoma in situ, Stage I = localised, Stage II = early locally advanced, Stage III = late locally advanced, Stage IV = metastasised.
AJCC 8th edition colorectal cancer staging (worked example)
T category: TX (cannot be assessed), T0 (no evidence of primary tumour), Tis (carcinoma in situ/intramucosal, confined to lamina propria), T1 (invades submucosa), T2 (invades muscularis propria), T3 (invades through muscularis propria into pericolorectal tissues), T4a (invades through the visceral peritoneum), T4b (directly invades or adheres to adjacent organs/structures).
N category: NX (cannot be assessed), N0 (no regional node metastasis), N1 (1-3 positive regional nodes, or tumour deposits present with all identifiable nodes negative) subdivided into N1a (1 positive node), N1b (2-3 positive nodes), N1c (no positive nodes but tumour deposits in subserosa/mesentery/pericolic or perirectal/mesorectal tissue), N2 (4+ positive regional nodes) subdivided into N2a (4-6 positive) and N2b (7+ positive).
M category: M0 (no distant metastasis identified; not assigned by pathologists), M1 (metastasis to one or more distant sites, or peritoneal metastasis) subdivided into M1a (one distant site/organ, no peritoneal metastasis), M1b (two or more distant sites/organs, no peritoneal metastasis), M1c (peritoneal surface metastasis, alone or with other site/organ metastases).
Stage grouping (T/N/M → stage): Tis N0 M0 → 0; T1-T2 N0 M0 → I; T3 N0 M0 → IIA; T4a N0 M0 → IIB; T4b N0 M0 → IIC; T1-T2 N1/N1c M0 or T1 N2a M0 → IIIA; T3-T4a N1/N1c M0 or T2-T3 N2a M0 or T1-T2 N2b M0 → IIIB; T4a N2a M0 or T3-T4a N2b M0 or T4b N1-N2 M0 → IIIC; any T any N M1a → IVA; any T any N M1b → IVB; any T any N M1c → IVC.
Staging and prognosis
Cancer staging is the most important predictor of prognosis and determines treatment intent. In a cohort of 28,491 colon adenocarcinoma cases (SEER data), 5-year survival fell progressively with stage: Stage I had the highest survival (~74% at 5 years), and survival decreased through stages IIA-IIIC, with Stage IV having the lowest survival (dropping to ~5.7% at 5 years).
Self-test
- List the five sequential steps of the metastatic cascade, in order.
- Describe the three changes a tumour cell must undergo to invade (before penetrating the basement membrane).
- Distinguish the epithelial and mesenchymal cell phenotypes in EMT, in terms of cell-cell junctions, cytoskeleton, and polarity.
- Distinguish individual (single-cell) tumour dissemination from collective tumour dissemination.
- List the three categories of factors that influence intravasation, with one example from each.
- Explain why circulating tumour cells (CTCs) experience different levels of fluid shear stress in macrocirculation versus microcirculation, and give the approximate shear stress range for each.
- Describe the sequence of receptor-ligand interactions that mediate extravasation, from initial rolling to firm adhesion.
- Describe the two phases of colonisation and the steps within each.
- Name the four routes of cancer spread and give one imaging or pathology finding characteristic of each.
- What do LVI and PNI stand for?
- Explain why cancer staging is performed (what three purposes it serves).
- List the four categories of staging techniques, with one example investigation from each.
- In the TNM system, what does each of T, N and M represent?
- Using the AJCC 8th edition colorectal criteria, what T category would a tumour that has invaded through the muscularis propria into pericolorectal tissue but not through the visceral peritoneum receive, and what stage group would result if N and M are both 0?
- Explain the relationship between cancer stage and prognosis, citing the approximate difference in 5-year survival between Stage I and Stage IV colon adenocarcinoma.
Answers
Reveal answers
- Invasion, intravasation, circulation, extravasation, colonisation.
- Loss of cell adhesion, degradation of the extracellular matrix, and gain of motility/migration.
- Epithelial cells have tight junctions, adherens junctions, desmosomes and gap junctions, an actin/cytokeratin cytoskeleton, and apical-basal polarity; mesenchymal cells have minimal cell-cell junctions, an actin stress fibre/vimentin cytoskeleton, and front-rear polarity.
- Individual dissemination involves full EMT of a single cell; collective dissemination is a cohesive group of cells with only partial EMT, moving together behind a leader cell at the invasive front.
- Cellular microenvironment (e.g. macrophages, fibroblasts, platelets), intrinsic tumour cell properties (e.g. gene expression, mutations), and mechanical cues (e.g. fluid pressure, tissue stiffness).
- Macrocirculation has faster flow and much higher shear stress (4-30 Dyn/cm², up to 100 in the heart) but very brief residence time; microcirculation has slower flow and lower shear stress (10-20 Dyn/cm²) but residence time can extend from seconds to days, allowing more interaction with the vessel wall.
- Rolling via sLe^x/sLe^a-P-selectin and PSGL-1-E-selectin interactions, followed by firmer arrest/adhesion via N-cadherin, L1CAM and integrins binding endothelial VCAM-1/ICAM-1, then transmigration between endothelial cells.
- Homing (mechanical trapping, site-specific adhesion/chemoattraction, pre-metastatic niche formation, quiescence) followed by colonization/growth (micrometastasis, then macrometastasis).
- Contiguous spread (direct local extension), haematogenous spread (blood-borne, e.g. liver/lung lesions on CT), lymphatic spread (nodal disease, e.g. increased tracer uptake in nodes on PET/CT), transcoelomic spread (peritoneal seeding, e.g. ascites outlining bowel loops on CT).
- LVI = lymphovascular invasion; PNI = perineural invasion.
- To determine whether disease is localised or distant/metastatic, to determine treatment intent (curative vs non-curative), and for prognostic purposes.
- Examination (e.g. checking for organomegaly), imaging (e.g. staging CT chest/abdomen/pelvis), surgical techniques (e.g. laparoscopy), blood tests/tumour markers (e.g. CEA).
- T = depth of invasion or size of the primary tumour; N = number of regional lymph nodes involved; M = presence of distant metastases.
- T3 (invades through the muscularis propria into pericolorectal tissues); with N0 M0 this is stage group IIA.
- Survival falls progressively as stage increases; in the SEER colon adenocarcinoma cohort, 5-year survival was roughly 74% for Stage I compared with roughly 5.7% for Stage IV.