Very difficult decision with pet.
	
	
		The only thing I can offer Diana if you haven't already is I don't think that opting not to have surgery is a bad thing especially considering her age. 
I would how ever if your vet has not already done it have her oxygen saturation levels checked. 
Thats something they probably wont tell you from a blood test is how much the pressure on her diaphragm is impacting the intake of oxygen.
Just because she is not showing signs of respiratory distress doesn't mean that she is getting all the oxygen she should be. 
If it is impacting then that would be a consideration for another discussion about surgery
But I would be doing so after having a specialist ultrasound if you haven't had one already. 
Eddie had an 8cm x 4cm tumour on his spleen unknown until he had a small bleed, which was only found by accident as I had a bloat scare and I rushed him to emergency vet and had them do a just in case xray after he acted normal when we got there  
Dogs are very stoic when it comes to hiding pain. 
A specialist can often tell you far more and give you far better info to fully inform you when taking surgery in to consideration. 
Just my thoughts of course.
I lost eddie at 8yrs old he got through surgery successfully for the removal of the tumour they told me that it was a 50/50 chance as to whether it was benign or malignant. Unfortunately for him it was Malignant and I lost him 4 weeks later. (Although I was given 3-6months)
Do I regret putting him through the surgery which cost me nearly 5grand?!
Not for a second because for those 4 weeks he was like a normal happy dog without a care in the world in fact happier than he was before, it made me wonder how long he had been silently suffering. 
Although his ending was fairly sudden (internal bleeding) 
I was comforted by the fact that for those 4 weeks i gave him the opportunity to live a life pain and discomfort free even if it was a short time
Sometimes its easy to fall in to a trap of whats normal with your pet when deterioration has occurred minimally over a period of time. Because those little behaviours they now do that we put down to them "getting old" can often be in a response to discomfort for something underlying.
I too have fallen in to this trap. With Ormolou my other boxer. 
For ages she had a funny gate behind when walking (only walking) i had her checked by a vet more than once for it as it bothered me as it would come and go it was put down to the spondalosis she has in her back. 
Recently she blew her cruciate she turns 11 in Feb i did the TTA surgery which she is currently recovering from and low and behold that dodgy gate has now disappeared... 
How bad do I feel that she has had a dodgy cruciate which has gone undiagnosed for nearly a year. 
As I said not having surgery is probably the right thing to do as long as you have all the information you could possibly have 
AND
You prepare yourself and make sure that you make the call before she begins to suffer. 
And please look for the subtle signs of things not going well
Please don't judge it by the "she is bright in the eyes and still eating" comment you often here.
Because when your beloved pet is not bright in the eyes or not eating you have already left it too long because at that point they are already suffering. 
You know your family member best so just make sure you always put them before your oen emotions of how you might feel with them not being around [emoji4]
Good luck Diana hope you have more wonderful times together. 
She is a good age
But like many they just don't live long enough!!! [emoji4]