Bone cancer

Question
I have a four year old rottie with bone cancer.

She was diagnosed in March and given two months.  She has done well but in the past two weeks - the tumor has grown rapidly.

I have prearranged for the vet to come to the house - but I am finding the decision hard to make for the time to put her to sleep.  I feel like I am being judge, jury and executioner.

I understand from what I've read and been told that since a dog is a pack animal - they do not want to show weakness to the rest of the group - animals or humans.  She seems her old self at times but other times she is very listless.  

I know I am being selfish but I want to know I am doing the right thing.

Thank you for your time.

Answer
You will know when it is time.  Bone cancers spread rapidly to the lungs and liver.  In addition they are very painful as they grow.  If there is severe pain or trouble breathing, it is time. Spend as much quality time as possible with her until that time.