I had a very low polyp, before we know it was cancer. I had a transanal resection. When the biopsy with the positive margins came out, the surgeon said she would not operate again, not to put the sphincter at risk. The oncologist said the same thing. I had RT + Xeloda on the days of the RT sessions.

When I developed a recto-vaginal fistula, my original surgeon offered a solution I didn't like and I went for a second and third opinion. The latter two asked for a colonoscopy first, which found a new polyp facing the fistula, both close to the sphincter. The surgeon of the second opinion said he would have operated again after the transanal resection and taken a wait-and-see attitude on the RT. I ended up having a resection of the rectum, mesorectum and sigmoid to get rid of the fistula and polyp, to reduce my risk of having new fistulas and polyps and so reduce the risk of a renewed cancer. When they reached the fistula during the surgery, they found it to have expanded so much it touched the sphincter. They had zero margin for the resection which made it very complicated. Now living without rectum and sigmoid is an enormous change in one's life.

If you can afford it, get a second opinion.

To prepare for treatment, ask all the questions you can think of about how it will be, the side-effects everybody gets and potential side effects not everyone gets.
If you get radiotherapy, be assured that you will suffer from burnt skin. Prepare by moisterizing the perianal and buttcheek area with aloevera for example and get ointments or creams for burns, like Cicalfate+. Be prepared to feel ever more tired as the sessions progress. Make sure you will have the opportunity to rest well at night and take naps during the day. Exercising can help with that.