oh, the pictures look much better than what i envisioned when i wrote my comment!

a cyst or abscess or tumor would look much more localized. i see nothing like that in the picture. i see a buildup to a massive fart / defecation. the way you describe the feel of it, probarbly a fart of epic proportions. its an extreme cse of sausage butt.

wait for others to confirm it, im not the expert, but if others see the same, i would maybe suggest a bath in lukewarm water (not too warm). and then a gentle massage in the water. that often causes badly constipated snakes to go.

i have to reverse my suggestion: i dont think a vet can help (unless its a real snake and reptile expert). in the case of a sausage butt, vets cannot do much, because any orally administered laxative wont reach that region in the current condition. and if there is a trick to get them to go, (other than bath + massage) you are more likely to find out about it on this forum, compared to visiting a vet who may or may not be a snake expert.

based on the pictures, its also hard to believe that it has not eaten in 8 months. just feed him/her, or at least try, even in cases of extreme sausage butt and constipation, you should continue feeding. but i dont see severe malnourishment.

one thing thats weird is that in cases of extreme sausage butt and/or gas buildup, it usually goes a bit further down, almost to the vent, but in your case, there seems to be 1 or 2 inches missing.

is it a male or female? if you dont know, get it sexed using the popping method. if its female, maybe its an issue of being egg-bound or something.

but dont take my word for it, wait for other forum members to either confirm or shred apart what i said. the only thing im quite sure about is that a vet can only help if it is a real snake and reptile expert. if it really would be an abscess / cyst, or cancer, any vet may help, but thats not the case, its a digestive issue, or if its a female, maybe an egg-related issue.

a biopsy or puncture is out of the question because the issue is too deep, it could puncture the intestine, and that is deadly without emergency surgery to close the puncture wound. i was right about one thing: pictures really do help A LOT.