Dont worry I am sure your dog will be fine! I know its an awful thing to go through. Many people are writing in for questions like these and to be honest I will love to give you an answer but dont think its fair as I think the best thing is to get them checked out by the vet properly. Good thing is your lab is not unwell or in pain that is a good sign. It might not be a tumour it could just be a blocked oil gland under its arm. Best to get it professionally checked out. Good luck but dont worry!

 My dog Milo 13 years old Shin Tzu has on the rear leg some swelling about 2" dia.
I recognized 3 months ago.
It's no aggressive grow and no discomfort for Milo.
Vet recommended biopsy before surgery.
A biopsy is a sample of tissue collected from a swelling.
A biopsy can be done by cutting a small piece of the tissue or by using a needle and syringe (NAB) to remove a sample, which then examined about pathology.
The NAB is safer and less traumatic (with out general anesthesia) than an open surgical biopsy.
But NAD don't use for animal. I don't understand why?
I'm afraid of general anesthesia for old Milo and I think at least in this situation Vet could cut a sample of the tissue with local anesthesia and suturing very small wound.
I understand this way no convenient for Vet but it is safer for old dog and I should be next to Milo and hold him for relaxation during shortly procedure. So, first of all we need to know how biopsy can be done without general anesthesia?