I have two dogs that have had mast cell tumors. One had a grade 2 on the prepuce (male dog sheath) that was removed with dirty margins. He was only 1 year old at the time. The other had a grade 1 removed from her left shoulder area. She was also young when this happened -- only 3 years old. Neither of these dogs are boxers -- they are a Boston Terrier and Pug respectively. However, these breeds, like boxers, also often exhibit a "less malignant" variety of the mast cell than other breeds.
The grade 2 on the prepuce had 21 daily radiation treatments to clean up his margins. This anatomic location is considered to be more malignant, or associated with more aggressive tumors, than other parts of the body, so that's why we opted for radiation -- that, and the fact that he was only a year old and it offered the best statistics in terms of long-term control. He finished his radiation treatments just about a year ago and continues to do really well (knock on wood). He has had some more suspicicious growths pop up, but all have been aspirated and none have been mast cell tumors. He goes to the veterinary oncologist every 6 weeks for a physical exam.
The grade 1 on the shoulder was removed surgically with very wide margins. That was also a year ago -- they happened at the same time to unrelated dogs -- different breeds even!! Can you believe that? Anyway, she is doing well too. She has also had a couple of odd lumps and bumps here and there and all have been aspirated and have come back normal. She also goes to the veterinary oncologist every 6 weeks for a physical exam.
I agree that if the nodes are clean and the x-rays are clean, then just careful monitoring for tumor recurrence at the same site or new tumors elsewhere is the best thing to do. I have heard that dogs that have had mast cell tumors in the past are more likely to get more. By the way, if they do spread, mast cell tumors usually spread to the lymph nodes, liver and spleen,not the lungs, so chest radiographs are not always the most helpful in these cases.
What I have found is that most mast cell tumors, if caught early, are really quite treatable unless they are grade 3's. Those are usually quite aggressive. Fortunately, most of them are grade 1's and grade 2's. I wish you the best of luck because I absolutely know how scary the diagnosis can be and making decisions for another living creature's life without their input can be very difficult to do.
You are definitely in my thoughts and prayers.