I am sorry about your dog. Anal Sac Adenocarcinoma in dogs is uncommon cancerous tumor but is aggressive. It has tendency to metastasize to other parts of the body like lymph nodes, kidney and lungs. This cancer is diagnosed when lot of damage is already done. Initially tumors are small and are not diagnosed. Large size tumors (more than 10cm) are difficult to remove. After surgery recurrence is also common.
Surgery followed by chemotherapy and radiotherapy can improve quality of life to some extent but with poor prognosis.
My dog has the same cancer and we attempted surgery but it could not be removed so we went to plan B. We did radiation and chemo. It has been quite a journey, but we have the best team of doctors and wonderful support. Guinness is quite a fighter and although I am exhausted and poor....I would not change my decision to treat him. He has been with me for five months now since being diagnosed and we have managed to keep the cancer exactly where it was Labor Day weekend. I get to hold him and play with him every day. He is not in pain, his tail wags, he eats...we have some tough days...but don't' lose hope! It is a very aggressive cancer, but there are some very aggressive vet oncologists up to the challenge! If I can be of more help leave your email address.
I would ask for a referral to a veterinary surgical specialist--these board certified veterinarians tackle complicated surgeries that are beyond general practitioners. Many things that are nonresectable for me the surgeons feel comfortable removing.
I would have your dog staged before surgery so you know the prognosis; this will see if the cancer has spread.
Metronomic therapy may be a possibility. I have not used it for this type of tumor. Your veterinarian could consult with a veterinary oncologist. Metronomic therapy is designed to promote the immune system and increase quality and perhaps quantity of life.