Can radiation therapy lead to carotid artery blockage?

I do not agree. Radiation to the head and neck area could potentially increase the risk for carotid artery disease years after the treatment. I routinely discuss this risk with my patients whom are being treated for a head and neck cancer. Unfortunately, the treatment with radiation is necessary for their disease so the risk of treatment is outweighed by the benfits.
Not likely. Radiation causes vessel injury and small arteries are damaged while other vessels are created. Arteries can get harder and scarred. In a larger vessel like the carotid these changes can take place but to completely block is not likely. There would have to be very high levels above normal such as in a case perhaps retreated. One more likely worry in that situation would be carotid blowout.
Depends on age. Chang in a 2009 paper suggested that in a study scoring atherosclerotic plaques in the carotids in patients after XRT that while patients less than 41 at the age of tx had a higher correlation those patients older than 50 actually had a negative correlation with plaque scores.