I opted out on a recent trip. The TSO was very professional, and asked me if I had any wounds, implants, injuries, etc. that she should be aware of. I explained that I had a bandage on my abdomen and pointed at the spot. She said, "Thank you, I will be careful when patting down that area." And she was considerate and careful. So there are good, professional TSOs at some checkpoints.

I wouldn't mention the tattoo - I would say, "I have a colostomy bag and a bandaged wound at the surgery site." Completely true, and more likely to get respect and/or sympathy than a voluntary tattoo. If they press the issue, say it was a cosmetic procedure to deal with scarring. (also true)

If your patdown officer acts like a jerk from the beginning, I would ask for a supervisor immediately and explain your concerns - that it seemed like the TSO didn't understand your medical issues, and you wanted to make sure that you had communicated everything clearly before the patdown begins, because there could be medical consequences for you.

If the supervisor acts like a jerk, then call in the people that Dianne47 mentioned.