Can rectal cancer return after 20 years?

Hi

Hi. I am very worried about my dad. He has been cancer free after rectal cancer in 1998, after which he had a permanent colostomy. He has become very sick in the past 5-7 years with loss of feeling in his legs and back pain. He also has a permanent incurable infection from a fistula. He is very weak, and exhausted all the time, loss of appetite, withdrawal etc. 

2 months ago when being invstigated for the infection, a 5cm lung mass was found. The biopsy was not conclusive (whatever that means) (although it showed it was partly necrotised.  We have not seen the pathology report), and so a PET scan was ordered.  This PET scan showed an SUVMax uptake of 10.3 in the lung mass AND a 5cm mass was found in his lower pelvis, sacral cavity, next to the rectal stump, with an SUVMax uptake of 10.9. I figured this would explain all his back pain and leg symptons, if it has been pressing on his spinal nerves.

His doctors are saying that in all their career (in a very small town) they have never seen rectal cancer come back after so long and so therefore this is NOT cancer. Without a biopsy being done. They are propsing to resect his lung, without knowing what type of lung cancer it is, if it's secondary or primary, and are saying they are unable to stage the cancer.  I feel this is utterly imcompetent.  The oncologist is saying he is annoyed at the PET scan operators for pushing to daignose the pelvic mass.  ??? I am astonished, angry, and many other things! Dad is too weak to undergo chemo or another treatment.

Can rectal cancer come back after 20 years? If i isnt rectal cancer, what other type could it be?

And, is 10.4 usually a pretty good indicator of cancer??

I want to help them, but I feel like I can't argue with doctors!

Thanks so much.