invasion and metastasis of cancer

1. INVASION & METASTASIS

2. LOCAL INVASION (DIRECT SPREAD)
BENIGN TUMOURS.
Most benign tumours form encapsulated or circumscribed masses that expand and push aside
the surrounding normal tissues without actually invading, infiltrating or metastasising.
MALIGNANT TUMOURS.
Malignant tumours also enlarge by expansion and some well-differentiated tumours may be
partially encapsulated as well e.g. follicular carcinoma thyroid. They are distinguished from
benign tumours by invasion, infiltration and destruction of the surrounding tissue, besides
distant metastasis .

3. Tumours invade via the route of least resistance, though eventually most cancers recognise
no anatomic boundaries.
Often, cancers extend through tissue spaces, permeate lymphatics, blood vessels, perineural
spaces and may penetrate a bone by growing through nutrient foramina.
More commonly, the tumours invade thin walled capillaries and veins than thick-walled
arteries.
Dense compact collagen, elastic tissue and cartilage are some of the tissues which are
sufficiently resistant to invasion by tumours.

4. METASTASIS (DISTANT SPREAD)
Metastasis (meta = transformation, stasis = residence)
It is defined as spread of tumour by invasion in such a way that discontinuous secondary
tumour mass/masses are formed at the site of lodgement.
Metastasis and invasiveness are the two most important features to distinguish malignant from
benign tumours:
benign tumours do not metastasise
all the malignant tumours with a few exceptions like gliomas of the CNS and basal cell
carcinoma of the skin, can metastasise.
Generally, larger, more aggressive and rapidly-growing tumours are more likely to metastasise
but there are numerous exceptions.

5. Routes of Metastasis :
Cancers may spread to distant sites by following pathways:
1. Lymphatic spread
2. Haematogenous spread
3. Spread along body cavities and natural passages (Transcoelomic spread, along
epithelium-lined surfaces, spread via cerebrospinal fluid, implantation).

6. 1. LYMPHATIC SPREAD.
2. In general, carcinomas metastasise by lymphatic route while sarcomas favour
haematogenous route.
The involvement of lymph nodes by malignant cells may be of two forms:
i) Lymphatic permeation :
The walls of lymphatics are readily invaded by cancer cells and may form a continuous
growth in the lymphatic channels called lymphatic permeation.
ii) Lymphatic emboli.
Alternatively, the malignant cells may detach to form tumour emboli so as to be carried along
the lymph to the next draining lymph node.
The tumour emboli enter the lymph node at its convex surface and are lodged in the
subcapsular sinus where they start growing .
Later, of course, the whole lymph node may be replaced and enlarged by the metastatic
tumour

7. regional nodal metastasis e.g. from carcinoma breast to axillary lymph nodes, from carcinoma
thyroid to lateral cervical lymph nodes,
retrograde metastases at unusual sites e.g. metastasis of carcinoma prostate to the
supraclavicular lymph nodes, metastatic deposits from bronchogenic carcinoma to the axillary
lymph nodes.
lymph nodes in the vicinity of tumour perform multiple roles—as initial barrier filter, and in
destruction of tumour cells, while later provide fertile soil for growth of tumour cells.

8. HAEMATOGENOUS SPREAD.
Blood-borne metastasis is the common route for sarcomas but certain carcinomas also
frequently metastasise by this mode. organs such as spleen, heart, and skeletal muscle generally
do not allow tumour metastasis to grow.
Spleen is unfavorable site due to open sinusoidal pattern which does not permit tumour cells to
stay there long enough to produce metastasis.
• Systemic veins drain blood into vena cava from limbs, head and neck and pelvis. Therefore,
cancers of these sites more often metastasise to the lungs.
• Portal veins drain blood from the bowel, spleen and pancreas into the liver. Thus, tumours of
these organs frequently have secondaries in the liver.
• Arterial spread of tumours is less likely because they are thick-walled and contain elastic
tissue which is resistant to invasion. Nevertheless, arterial spread may occur when tumour
cells pass through pulmonary capillary bed or through pulmonary arterial branches which
have thin walls. Cancer of the lung may, however, metastasise by pulmonary arterial route to
kidneys, adrenals, bones, brain etc.
• Retrograde spread by blood route may occur at unusual sites due to retrograde spread after
venous obstruction, just as with lymphatic metastases.

9. SPREAD ALONG BODY CAVITIES AND NATURAL PASSAGES.
Uncommonly, some cancers may spread by seeding across body cavities and natural passages.
These routes of distant spread are as under:
i) Transcoelomic spread. Certain cancers invade through the serosal wall of the coelomic cavity
so that tumour fragments or clusters of tumour cells break off to be carried in the coelomic fluid
and are implanted elsewhere in the body cavity. Peritoneal cavity is involved most often, but
occasionally pleural and pericardial cavities are also affected.
ex: a) Carcinoma of the stomach seeding to both ovaries (Krukenberg tumour).
b) Carcinoma of the ovary spreading to the entire peritoneal cavity without infiltrating the
underlying organs.
c) Pseudomyxoma peritonei is the gelatinous coating of the peritoneum from mucin-secreting
carcinoma of the ovary or apppendix.
d) Carcinoma of the bronchus and breast seeding to the pleura and pericardium.

10. ii) Spread along epithelium-lined surfaces. It is unusual for a malignant tumour to spread along
the epithelium-lined surfaces because intact epithelium and mucus coat are quite resistant to
penetration by tumour cells. However, exceptionally a malignant tumour may spread through:
a) the fallopian tube from the endometrium to the ovaries or vice-versa; b) through the
bronchus into alveoli; and c) through the ureters from the kidneys into lower urinary tract.
iii) Spread via cerebrospinal fluid. Malignant tumour of the ependyma and leptomeninges may
spread by release of tumour fragments and tumour cells into the CSF and produce metastases
at other sites in the central nervous system.
iv) Implantation. Rarely, a tumour may spread by implantation by surgeon’s scalpel, needles,
sutures, or may be implanted by direct contact such as transfer of cancer of the lower lip to the
apposing upper lip.