1. Core Concept of Metastasis

• Definition: Spread of tumor cells from primary tumor to physically discontinuous distant sites.
• Diagnostic meaning: Presence of metastasis = tumor is malignant.
• Benign tumors never metastasize.
• Key phrase: Benign stays, malignant strays.

2. Invasion Enables Spread

• Before spreading, cells must first invade → breach tissue planes + vessel/cavity walls.
• Routes become accessible once invasion occurs:
• blood vessels
• lymphatic vessels
• body cavities / CSF spaces

3. Frequency at Diagnosis

• ≈30% of solid tumor patients have clinically evident metastases at presentation.
• ≈20% have occult (hidden) metastases.
• Rough estimate: ~50% show spread at diagnosis.

4. Size, Differentiation & Exceptions

• Increased metastatic risk correlates with larger primary tumor + poorer differentiation (anaplasia).
• But: small tumors may metastasize early; large tumors may remain localized.

Malignant but rarely metastatic:

• Basal cell carcinoma of skin
• Primary CNS tumors (gliomas)

Reason: highly locally invasive but seldom spread beyond site (especially CNS tumors).

5. Blood-Derived Cancers

• Leukemias + lymphomas arise from circulating/immune cells.
• These cells already migrate systemically → widespread involvement at diagnosis.
• → Always considered malignant.

6. Main Routes of Metastasis

1. Seeding of body cavities
2. Lymphatic spread
3. Hematogenous (bloodborne) spread

7. Seeding in Body Cavities / CSF

• Tumor cells exfoliate + implant on cavity surfaces.
• Classic peritoneal seeding: ovarian carcinoma.

CNS seeding via CSF:

• Medulloblastoma
• Ependymoma

8. Lymphatic Spread

Typical associations

• Carcinomas → lymphatics
• Sarcomas → blood (but overlap exists)

Spread follows natural drainage pathways

Lung carcinoma nodes

• Bronchial nodes
• Tracheobronchial nodes
• Hilar nodes

Breast carcinoma drainage

• Upper outer quadrant → axillary nodes
• Medial breast → internal mammary nodes
• Later → supraclavicular/infraclavicular nodes

Skip metastases

• Spread to distant nodes without involvement of expected regional nodes.

Lymphatics → blood link

• Thoracic duct drains lymph → venous system.

Sentinel node biopsy

• First draining node from tumor field.
• Identified via blue dye or radioactive tracer.
• Determines nodal metastasis + guides surgery.

9. Lymph Node Enlargement Caveat

• Enlarged node ≠ metastasis automatically.
• Could be reactive:
• lymph follicular hyperplasia
• sinus histiocytosis
• Diagnosis requires biopsy.

10. Hematogenous Spread

Preferences

• Sarcomas commonly spread via blood.
• Veins invaded more frequently than arteries.

First metastatic organs (commonly)

• Liver (portal drainage)
• Lungs (caval drainage)

Special venous pathways

• Paravertebral venous plexus → vertebral metastases
• Important in prostate and thyroid carcinoma.

Tumors extending through veins

• RCC (renal cell carcinoma) → renal vein → IVC
• HCC → portal/hepatic veins

Organ tropism (“homing” tendencies)

• Prostate → bone
• Bronchogenic carcinoma → adrenal glands + brain
• Neuroblastoma → liver + bone
• Skeletal muscle mets are rare.

Grand Summary Points

• Metastasis proves malignancy.
• Three routes: seeding, lymphatic spread, hematogenous spread.
• Ovarian carcinoma → peritoneal seeding.
• Medulloblastoma/ependymoma → CSF spread.
• Lymphatic spread → carcinomas; sentinel node biopsy key for staging.
• Hematogenous spread → sarcomas; veins > arteries; liver + lungs most common.
• Paravertebral plexus → vertebral metastases.
• RCC/HCC show intravascular venous growth.
• Basal cell carcinoma + CNS tumors rarely metastasize.

If needed next, I can convert this into:

• a summary table
• MCQs
• flowchart/diagram text
• stepwise short notes for quick revision