When to euthanize pet rat with cancer?

Question
Our 2-year old loving pet rat developed a lump under her rear leg four
months ago and after a biopsy the vet confirmed cancer.  She now has at
least three other lumps that are about the size of my fist, making it difficult for
her to walk. She was still her happy self, though.  But now she has beco e
lethargic, one of the tumors is infected and leaking puss, her appetite is
almost gone and her droppings are tiny and hard.  I don't know whether to let
her pass naturally or save her the discomfort by putting her to sleep.  My
question is how does a vet euthanize a rat?  Could I hold her during an
injection?  Will it take long?  Please let me know if you think it's time and
what I can expect.  Thank you.

Answer
 There are a few different ways to euthanize a rat, and each vet has their own reasons for which way they use.  The first way is a intraperitoneal injection, which is a needle of euthanizing agent that is inserted into the belly.  This injection can be painful but it is very quick.  The euthanasia solution is absorbed into the body in about 1 - 2 minutes and the rat falls into a deep slumber then passes away.  Some people don't like this method because the rat has to be injected and they claim that the euthanasia solution can irritate the belly.  I'm not sure if this is true, but I've euthanized many rats using this method and none of them ever appeared to be in discomfort except while the needle was in their belly, which only lasted a second.  One reason I like this method is that it doesn't cause death instantly and you can hold your rat and cuddle him while he passes away, rather than leaving him on a cold hard table or in a stranger's hands.

 The other method involves gassing the rat down with anesthesia, then injecting them with the euthanasia solution while they are asleep and can't feel it.  Though this seems like the more humane solution, it takes a while to gas down a rat and to do so safely they must be kept in a small chamber into which the gas is pumped.  While they are falling asleep they run around frightened and thump against the sides, meaning their last experience on earth is one of fear, which I don't agree with.  If the rat is really sick and lethargic, however, a mask can sometimes be placed over their face to gas them down, or they will lay comfortably in the gas chamber, and therefore this method may be preferable.

 As for when to choose euthanasia, that is a very difficult decision.  You have to consider her quality of life, and whether or not she is suffering.  Some rats can live a long time with many tumors, but once they start to interfere with daily activities and the rat becomes very obviously unwell it is probably about time to consider euthanasia.

 Well I'm very sorry to hear about your rat, and I hope I have answered your questions!  Let me know if there is anything else I can do.