The issue raised by Brenner et al concerns the general "committed dose" to the population by CT scanners. The NEJM article contains significant estimates/extrapolations on population radiation doses from increasing uses of CT scanners, and in particular the "screening" use of whole body scans without apparent medical indications which are becoming increasingly popular in free standing centers.
The AAPM (American Assoc. Physicists in Medicine), published a response to the NEJM article here:
http://www.aapm.org/announcements/CTScans.asp
Concerning the radiobiology of the concerns, as we know radiation induced carcinogenesis does exist, but the risk v. exposure is uncertain and variable depending on many factors.
The Linear-No Threshold dose concept is a radiation protection and safety concept which assumes that there is no safe level of radiation exposure which would not cause an increased risk, which is the premise of Brenner's article. This is the regulatory environment in which we live and work.
We do know that there is an apparent threshold from both radiobiology and empircally. That this threshold does exist, can be demonstrated by the lack of differences in cancer in persons living at high altitude and those living at sea level. This is also a more or less continuous chronic dose, rather than a "burst" dose such as from a CT scanner.
A typical CT scan on modern equipment delivers a dose of around 15-20 mSv. So, repeated scans can add up over time, and unnecessary scans increase the population committed dose and that has a statistical probability of increasing the population rate of cancer.
This must be offset by the underlying medical question to be answered by the ordering of the CT scan.
For radiotherapy purposes, I go nuts everytime a tech or physicist whines when I order an extra set of scouts or a respiratory gating sequence when doing a planning CT. We are maybe giving an extra 30 mSv of radiation (or 0.03 Sv) to a patient we are planning to give 66-80 Gy which will increase their dose to 66.03 Sv at completion of treatment.
Of course I could go down and spend 30 mSv on the fluoro and give about the same dose while assessing tumor motion...and not hear a single breath of worry about that dose.
The risk, though small does exist. Every medical imaging procedure involving x-rays/ionizing radiation) should have a solid medical indication and question to be answered in order to keep the population and individual doses to as low as reasonably achievable (ALARA).
