What are the long term effects of radiation therapy on bone?

Radiation therapy tends to kill the stem cells in the bone marrow, which can result in a weakned immune system and aplastic anemia (basically, your body doesn't produce enough red blood cells) since both red blood cells and immune cells are produced from stem cells in the bone marrow. In some cases, this requires a bone marrow transplant, which, when sucessful, typically has minimal long-term consequences.

Radiation can also weaken the bone itself.

Depends on dose and realy should talk to your doctor for the best info. But I had a max dose on my hip and it has weakend the bone to the point I have to watch how much stress I put on it as not to crack it. As well radiation can cause other types of cancer later in life(I've been told about 50 years later, so not a huge risk to most people).

It really depends on the amount and where. I had max dosage to my neck and left facial area for stage 4 squamous in 1992-93. I was told it would cause ostradinarcosis of the jaw bone. A bone scan was done in Feb 2005 and showed the bone was strong and clean. My oral surgeon has more concerns about the blood flow. I needed teeth pulled.
When I had some pulled several years ago I did hyperbaric chamber to force oxygen into the bone and tissue so it would heal. This may be part of the reason my jaw is ok now.
I actually have more damage to other areas, like 10% normal saliva- try living with no spit! Lost my thyroid gland, most of my soft palate, raspy voice, tissue damage in my neck.
Again, like so many treatments for cancer there is no set list of damage or side effects, it's different for every one.

I had radiation therapy in 1997 for a brain tumor. I had my fourth brain surgery last September, during which the neurosurgeon had to replace the dura (outer covering of the brain) and the skull flap at the site, because they had both died. Now I have a titanium plate in my skull.