Hi J.R. -
Unfortunately, I have some experience and knowledge about spleen mets. This past June, a spot showed up on my PET/CT on my spleen. The doctors really didn't think that it was cancer, because it is so rare for it to spread to the spleen. In fact, before me, there was only 8 documented cases in the history of the US of colon cancer that spread to the spleen. I don't know if this was just mets to the spleen alone, or if it included mets to other places, including the spleen. In either case, because of the way that the spleen functions, it is extremely uncommon. However, I was case #9, which they found out after removing my spleen. There is no way to biopsy the spleen because of the type of organ that it is.
So you can remove the spleen without much trouble - it is an important organ, but not a vital one. I was told I could live a very long and healthy life without it (not taking into account the cancer aspect, of course...). I am now undergoing another 6 months of chemo as a precautionary measure.
To answer your question simply, it is possibe for it to spread to the spleen, although rare. If you have any more questions, let me know. I'll be happy to help any way that I can. Especially since you'll be hard pressed to find others with spleen mets...
Good luck and stay strong!
Skier03