That is great to hear!
My vet recommended the same dose of doxy (10mg/kg) we used to treat her tick disease but for 14 days on and off (whenever I find her symptoms getting worse). I have also read some use 5mg/kg for cancer treatment but I am just doing what my vet suggested. If you suspect any tick disease, I would use it for a longer period of time.
Doxycycline seems to be a wonder drug. I am just about to pick up another refill to give Timber. I noticed her nose was sounding a little more stuffy and when that happens I am to put her back on the doxy again for 14 days. I think that people with nasal tumors are more prone to infection so maybe the doxy acts on that as well as reduce the inflammation. I know it is also used as an anti-tumor drug. I think I read that it has been shown to induce cell death and prevent cell proliferation in certain types of tumors but I am not sure if it has been studied with nasal tumors specifically. My vet mentioned it is generally helpful for respiratory problems.
It is hard on the liver so good to take a break and use it on and off. Milk thistle and the curcumin may help to support the liver as well.