The radiation that you get from an HRCT, is so small less than 1mSv, which is way below any sort of danger levels. Also the effect, of radiation on body is so minimal from an HRCT, than the benefit that you get by doing a test like that. So absolutely nothing to worry about, also this is a one off procedure, and you won't be doing it frequently. Only thing is repeat the test only when indicated and suggested my a qualified medical practitioner. If Iam not wrong I had answer similar question previously , so if you are repeating this question because of any unusual symptoms of difficulty that your are facing please let me know you can consult me if you need , because unnecessary anxiety of cancer is not good for your general health .