samclembeau,
I chose proton in your situation though my cancer was rectal not anal. You'll have radiation exposure to the bone and maybe some bone marrow edema. The primary concern is the nerves of the spine and in this regard it is fortunate that your cancer is low.
Your bones will take an entry level hit from proton but no exit exposure. I had some bone marrow edema which causes a weakening of the bone. I'm not at all worried about fracture of the hip. I had two ports of entry through the hip on each side. And two through the back. I could tell you the radiation exposures in GYs if it has meaning.
You'll have less readiation if you take chemotherapy which has it's own hazards for bone.
Luckily there are strong osteoporosis medications. Bisphosphonates won't be sufficient after in my unschooled opinion. Tymlos is probably the best, but consult with your doctors to know if you should be on them before radiation therapy. I chose to wait.
I should be in a good position to respond if there is anything else you'd like to know.