whipple surgery

My brother had whipple surgery the first of July and initially the surgeon said he had gotten all the cancer and that following repeated evaluations from pathologists, that the type of cancer they found is unknown and the surgeon explained that chemo is not good idea since they don't know the type of cancer they are dealing with. Cat scan last week shows reappearance of tumors. Doctor described it as aggressive and recommends chemo.

Interesting to note is that he had a cancer of unknown primary origin 16 years ago (surprisingly found lump in neck - lymph node area). The doctor who treated him at that time, pretty much hit him with all kinds of chemo to attack all types of cancer and it was obviously successful!

I just don't understand how cancer can not be cell specific. Any thoughts?

Thanks, Kneesox