There are two types of cancer: benign and malignant. Benign cancers are the kind that don't spread and don't threaten one's life. Malignant cancers are the kind that spread, and if no intervention is taken, a malignant cancer will generally not stop until the host is dead. That's the loose definition of a malignancy: its end game is death.
Virtually every subtype of lymphoma is malignant. Without some sort of treatment intervention, and if given enough time, lymphoma cells will eventually crowd out healthy lymphocytes to the point where a person's body or particular organ system can no longer function, leading to a situation that is incompatible with life. This might take the form of an opportunistic infection that the body cannot fight because it lacks the immune system power. It could be that a lymphatic tumor in the upper chest grows so large that it directly obstructs the superior vena cava, a vein that carries deoxygenated blood from the upper body to the heart.
The point is that when a malignant cancer begins to spread, many avenues open up by which to die. The spread of a cancer from its primary site, meaning from the site where the cancer began, to other parts of the body is called metastasis.
Metastasis in lymphoma occurs spontaneously when cells from the primary lymph node or tumor site break off and are carried by either the bloodstream or by through the lymphatic vessels to distant sites. There, those cells begin forming new tumors.
Eventually, cells will break off from these secondary tumor sites and migrate to other parts of the body, forming new tumors. What had been a localized disease is now a metastatic one. Now, different organs and organ systems become infested with cancerous lymphocytes, and in time they will prevent the organ from doing its job. Depending on the organ, this could mean the death of the host. At the very least, it could mean organ failure, which itself is a serious health problem.
Although this has been written to describe lymphoma, it can loosely be applied to any cancer. The difference with lymphomas is that the primary method of metastasis in lymphoma is different than other cancers. Other cancers rely on the bloodstream to spread, and since blood travels to all parts of the body, metastasis presents a very serious threat. However, cancerous lymphocytes will more often spread to distant sites by way of the lymphatic system. Like the bloodstream, lymphatic vessels go to every part of the body, making a person even more susceptible to the threats posed by metastasis.
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