ZME Science reports on a Nature article (full article is paywalled) (DOI: 10.1038/nature18599) about a disease called disseminated neoplasia. The disease is a group of cancers which are thought to spread via seawater. They affect mussels, cockles, and golden carpet shell clams.
Among mussels and cockles, the cancer cells come from the same species, but the cancer infecting golden carpet shell clams comes from a different species, Venerupis corrugata , the pullet carpet shell.
Well, there is nothing in principle that stops any cancer cells, capable to survive in a transport medium, to go find another host of either their original host specie or some other but compatible specie. Also, if they can survive in vitro, they probably can make living on any suitable nutritious substrate in the world, perhaps as parasites in saprophyte single-cell organism colonies where the nutrients are not readily available. Most infection vectors we are aware of, e.g. parasites and blood-sucking organisms, bodily fluids, even their airborne droplets, under favorable conditions could be vectors for dissemination of cancerous cells, as well.
The fact that it has never been observed tells us that it is highly improbable, but on the other hand none ever considered the possibility that human cells could act as microbes. When a person contracts a cancer, there is no need for explanation, we "know" it was spontaneous.
Cancer cells from one person would be "rejected" in a recipient similar to how an un-matched donor organ would be.
If the dose was high enough (probably more than 10^7 cells) or if the recipient was immunodeficient (remember the story about the AIDS patient who got worm-cancer) then there is a possibility that some cells might survive the immune response and form a tumor.
Contagious cancer, including one that was cross-species, has been observed in humans too:
- a surgeon caught cancer (malignant fibrous histiocytoma) from a patient- Kaposi's sarcoma appeared in recipients of transplants- a man who had a weakened immune system developed a tumour composed of cells from a tapeworm