Introduction
Metastasis is the development of secondary malignant growths at a distance from the primary site of the cancer. It distinguishes benign from malignant tumours: benign tumours do not metastasize, while all malignant tumours (except gliomas of the CNS and basal cell carcinoma of the skin) can. Malignant tumours spread because they have no capsule.
General points
- Invasion of tumour cells into veins is difficult, therefore the vessels in tumours are newly formed structures; they are connected to the systemic circulation, which is what allows metastasis.
- The more direct the contact with blood vessels, the more intensive the growth of tumour cells and the more rapid the metastasis, e.g. seminoma, clear cell renal carcinoma, chorionepithelioma.
Routes of metastasis
- blood (hematogenous spread);
- lymph (lymphatic spread);
- canalicular spread;
- invasion of neighbouring structures;
- through the serosa (carcinomatosis);
- relapse (recurrence).
Grouped: 1) lymphatic spread, 2) hematogenous spread, 3) other routes (spread along epithelium-lined surfaces, e.g. the GI tract, via CSF, implantation).