Guest blog by Kalpana M. Kanal, PhD, Direc­tor of Diag­nos­tic Physics Sec­tion and Pro­fes­sor in the Depart­ment of Radi­ol­ogy at Uni­ver­sity of Washington

In a recent article published online¹, the authors state in their introduction that radiation dose risk is cumulative and an increasing number of patients are undergoing multiple follow-up procedures at regular intervals. Is cumulative dose of concern in patients who have repeated scans? The jury is still out on this question. There is support for tracking cumulative dose² as well as thought that cumulative dose should not be given any importance when making decisions about individual patients^{3, 4}.

Radiation risk is based on the linear no-threshold model which states that all radiation exposure carries some risk but these need to be weighed against the benefits of the radiation exposure. This linear relationship implies that irrespective of which CT scan a patient is receiving, the absolute risk is the same. There is no increase in sensitivity from the increasing dose received from repeated CT scans, only an accumulation of probability. The linear no-threshold model would break down and not make any sense if there was an increase in sensitivity from repeated scans.

Consider the analogy of driving to work every day which has a risk of a fatal automobile accident associated with it. We do not keep track of the number of times we have driven in the past and its influence on whether we drive tomorrow or not. Similarly, as far as medical decisions are concerned, cumulative dose should not play a factor in deciding if a CT scan should be ordered or not. The benefit of getting the CT may far outweigh the risks. Also, individual risks are hard to quantify as all our risk models are based on large population data.

It is very important that we do not misuse the patient history information about previous scans to influence our medical decision today. Educating the physicians and the public on this is paramount to avoid such misuse.

References:

1. Roobottom CA and Loader R. Virtual Special Issue Radiation dose reduction in CT: dose optimisation gains both increasing importance and complexity! Clinical Radiology, 2016; 71(5): 438–441.
2. Sodickson A, Baeyens PF, Andriole KP, et al. Recurrent CT, cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults. Radiology 2009; 251: 175-84.
3. Durrand DJ, Dixon RL, Morin RL. Utilization Strategies for Cumulative Dose Estimates: A Review and Rational Assessment. Journal of the American College or Radiology 2012; 9: 480-485.
4. Eisenberg JD, Benjamin Harvey HD, Moore DA et al. Falling Prey to the Sunk Cost Bias: A Potential Harm of Patient Radiation Dose Histories. Radiology: 2012; 263(3): 626-628.