Could radiation therapy cause osteonecrosis of the hips?

Yes. If the hips get an amount of radiation above what this area can handle this complication can happen. This is why there is treatment planning and doctors limit radiation doses to the area of concern. However if treated no more than the maximum tolerance their is still 5 percent chance of complications at that maximum level.
Yes. Femoral head dose is always one of the issues we monitor for this very reason.