Dog with colonic tumor.

Hi -
Our nearly 11 year old male yellow lab has had quite a week. He has been very healthy until now.
We brought him to our regular vet a week ago for lameness in rear left leg. We did xrays and our vet concluded that he has a torn cruciate ligament. Additionally, they noted spinal arthritis. They used a mild sedative during the xray procedures and sent us home with vetprofin. 3 days later Vedder began having stomach issues which escalated into diarreah, then vomiting almost 24 hours later. About 12 hours later there was blood in the diarreah and then blood in the vomit soon thereafter. We brought him to a local 24 hr hospital. They ran bloodwork...which all came back normal (aside from the expected signs of blood loss and dehydration). They administered fluids and anti-nausea shot. His condition improved and he was eating small dose of bland food by that evening...and almost back to 100% after another 24 hours. At the hospital that day, we also had an ultrasound taken to look for tear or other GI issues that could have caused the bleeding. Nothing was found. However a "large" mass in his ascending colon was found during the ultrasound (near the ascending/transverse corner of the colon). Everything else on the ultrasound looked good...no perceived cancer/metastasis in the liver, spleen or anywhere else. Chest xrays showed no signs of cancer/metastasis in his lungs. The opinion seems to be that the diarreah and vomiting episode (as well as blood in both) was not related to the mass...that the mass is a completely incidental finding. A needle biopsy was taken, and the results are inconclusive...but "there is no overabundance of malignancy within the observed population". They seemed to think they could rule out lymphosarcoma, however that the tumor could be adenoma, adenocarcinoma, leiomyosarcoma or leiomyoma. We are faced now with the decision of whether to proceed with surgery to have the mass removed. What is your opinion? I understand that adenocarcinoma is much more common than leiomyosarcoma...and that the former typically has much worse prognosis. Do you think it is likely or unlikely that the diarreah/vomiting/blood are related to the mass or potential cancer? What is the likelihood that the tumor could be benign? If the tumor is benign, what is the likelihood it would become cancerous? What is the likelihood it would continue to grow and cause other major problems for Vedder?
Thank you so much in advance for your time and opinion.