I don't know what the usual protocol is. 

My partner has Secondary Breast Cancer and at the time of diagnosis we were told that there were different types. Some spread quickly (like hers), others stay in the breast. I suspect that they saw that cancer as unlikely to spread otherwise they would have offered a blood test. (There's a marker in the blood; ca15-3) and if that is raised it's likely that the cancer is spreading. (I say "likely" as our oncologist has stated that this is not a clear indicator). 

You could write a letter to the hospital seeking answers and if they won't share information because you aren't the patient then ask for their usual protocols. However, my sense is that you are looking for bigger answers  

It seems that as long as the patient is having treatment to keep the cancer under control )which she was), then they are doing all they can. We could question whether staying on the same regime for six years is the best option , but perhaps that was what stopped new cancers appearing in her breasts, even if it didn't stop the original cancer from spreading.  

My partner was receiving a treatment for the first two years which was then changed as her cancer had spread some more and we were told that treatment usually worked for only two years  at one time she was taking Tamoxifen  was this the one that usually only worked for two years, I can't remember;  and if it was does it help? 

You are obviously in pain, but perhaps you are asking the question could "they" have done more because you are wondering... could you? I love that you had celebrations when the all clear came, you clearly showed her love. You could pursue complaints etc but it doesn't change the outcome and personally I think that instead of spending an hour writing a letter you could spend an hour doing something lovely (a walk in the park for example) with someone you love. Losing an important person teaches us to value special moments.