From: assassinpandora	Date: May 19th, 2003 03:06 pm (UTC)		(Link)

Although I took a "Biology of Cancer" class at the University of Oregon, it's been 16 years and my memory is dim. Not to mention there's been a lot of new knowlege integrated since then. Anyway, my immediate recollection is that a cell breaks off from the primary tumor and travels in the body to set up shop somewhere else, and it's called "mestastasis". Here's a basic paragraph from this website:

http://www.azcc.arizona.edu/in_print/N_spring01/n_how-cancer-spreads.htm

"Metastasis is the spread of cancer cells from a tumor to other parts of the body through the blood or lymph system. Before the cancer cells can break away from the tumor, changes must occur in the attachment or adhesion of cells to each other. In order for the cells to enter the bloodstream or lymph system, the molecules that normally anchor the cell in place are disassembled or modified. Once the cells have become unattached from the surrounding tissue, other changes allow their survival and migration."

	From: dichotomous	Date: May 19th, 2003 03:10 pm (UTC)		(Link)

I'm not a scientist, but what I think happens is the DNA in one or more cells gets fubared for one reason or another, and those cells go crazy making copies of themselves. The renegade cells use up oxygen and nutrients meant for healthy cells. Eventually the cancerous cells spread to where they can get transported around the body, I think by the bloodstream, and take root in other places.

I think.

	From: josefinek	Date: May 19th, 2003 05:44 pm (UTC)		(Link)

Keeping in mind that while I am getting a doctorate in the field of cancer genetics, I'm a programmer and not a clinician:

Cancerous cells proliferate abnormally quickly, but do not "infect" or convert other cells. So, cancer does not spread in the same fashion that a bacteria or virus does. Some viruses themselves are oncogenic (cancer-causing; specifically encourage cell proliferation); the oncogenic viral DNA is incorporated into the host cell, the host cell goes berzerker - but the transformed cell itself doesn't infect others. Cancers have a primary location and may metastasize to remote locations (ie., kidney to lung); however, the remote tumor is made up of the *same* type of cells as the initial site - you'd get a kidney-cell tumor on your lung, NOT a lung-cell tumor.

So, how does this happen....

In general, cancer occurs when a normal cell undergoes a series of genetic mutations. A localized tumor/pre-cancerous lesion forms when a single mutant cell proliferates abnormally and spawns a colony of daughter cells (blood cancers, etc. don't have tumors of course). These cells are structurally similar to the other normal cells around them, but appear less differentiated; a cancerous bone cell stays rudimentarily a bone cell, and can't be confused with a cancerous skin cell, etc.. They also *act* pretty similar; a bone cell expects bony nutrition and would not survive in the blood or whatever. So, these mutated cells must gain the additional ability (through more mutation) to survive wherever they want in the body, completely unattached to their normal environment (ie., they pack up their metabolic bags and get ready to go).

These cells must also be able to continually divide. Normal cells are hardwired to divide a certain number of times, and then poof no more divisions - ie., you get old and die. Cells count down divisions due to things like losing a bit of chromosome tip each division (telomeres); once enough tip is snapped off, no more divisions. Cancer cells obviously grow fast, but they must do so continuously as well; thus, they must gain more mutations which cause them to be "immortalized". I think that's the whole point behind that one X-files where the guy eats cancer to stay young, by the way...

Next, cancer cells must actually move to a remote location. They do so by travelling through the blood or lymph. They need to actually get into the blood, and do so by gaining the ability to detach from their local environment ("lose contact inhibition"). They then migrate into the blood by sneaking through little spaces between the cells that make up the vessel walls - this is how white blood cells get out of your blood and into your damaged tissue by the way, so it's generally a useful action. Next, they have to survive the blood (pretty hostile environment for most cells), exit the blood vessel, and end up in a tissue type that isn't hostile. Then they can proliferate merrily again.

So, cancer doesn't infect or change other tissue; metastasis occurs when one cell type mutates, gains the ability to survive solo, moves to another area, and takes up residence. Most things typically move to the lungs, since that's a highly capillated area and next on the circulation path; liver cells go to the intestines or something. My advisor for example has kidney cancer, but the mutated kidney cells metastasized to his lungs - resulting in (basically) untreatable terminal disease.

Last little bit on inherited vs. sporadic cancers:

Cancer occurs when a normal cell undergoes a series of genetic mutations. Only one single cancerous cell is required to start tumor formation, because that cell divides and produces identical daughter cells. These required mutations may occur randomly in normal body cells (on the order of one mutation for every 10^7 cell cycles) OR a person can inherit some mutations at birth; since these mutations are present in all cells, those individuals will potentially get cancerous tumors in numerous places - in that way it might look as though the cancer is metastasizing to other areas, when in fact there are just multiple independent cancer-causing mutations going on simultaneuosly.

Anyway, hope that's a decent explanation. Ask questions or whatever and I'll *try* to keep it short :)