Linear No-Threshold Model May Not Be Appropriate for Estimating Cancer Risk from CT

Letter to the Editor published in Radiology

Linear No-Threshold Model May Not Be Appropriate for Estimating Cancer Risk from CT, 

Mohan Doss

 Radiology Jan 2014, Vol. 270, No. 1:307 –308. 

Available at: http://pubs.rsna.org/doi/pdf/10.1148/radiol.13131661

Editor,

I read with interest the recent article by Zondervan and colleagues in the May 2013 issue of Radiology (1) regarding the increased cancer risk from CT scans.  They cited the references (2-4) which claimed increased risk of cancer from low dose radiation based on comparison to atomic bomb survivor data (5) and/or the linear no-threshold (LNT) model for radiation-induced cancers, justified by the atomic bomb survivor data. 

The latest comprehensive report from the atomic bomb survivor study was published in 2012 by Ozasa et al. (6). With the additional statistics, a significant curvature has been reported in the dose-response relationship for cancers in the dose range of 0-2 Gy, as seen in  Table 7 of (6).   Ozasa et al. said on p.238 of their report that the curvature is apparently due to “relatively lower than expected risks in the dose range 0.3–0.7 Gy, a finding without a current explanation”.  Whereas there is no explanation for the shape of dose-response curve in this dose range using the LNT model, there is an explanation using the radiation hormesis model (7), implying there may be a reduced risk of cancer from low dose radiation.  Since the observed dose-response in the atomic bomb survivor data played a key role in the establishment of the LNT cancer risk model in BEIR VII report, and since the shape of dose-response in the latest update cannot be explained using the LNT model, the cancer risk models in the BEIR VII report should no longer be used. 

Hence, when discussing CT radiation concerns, radiologists should inform patients about the new findings from the atomic bomb survivor study, and dismiss such concerns as being baseless and potentially harmful due to missed diagnoses if the indicated diagnostic studies are not performed due to the concerns.  Also, the present widespread efforts to monitor and reduce CT radiation dose should be discontinued, as they consume considerable resources while providing no health benefit to patients, and may be harming patients’ health because of the potential for misdiagnoses from the use of alternative suboptimal imaging modalities or reduced image quality.

References

1.         Zondervan RL, Hahn PF, Sadow CA, Liu B, Lee SI. Body CT scanning in young adults: examination indications, patient outcomes, and risk of radiation-induced cancer. Radiology. 2013; 267(2):460-9.

2.         Berrington de Gonzalez A, Darby S. Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries. Lancet. 2004; 363(9406):345-51.

3.         Brenner DJ, Hall EJ. Computed tomography--an increasing source of radiation exposure. N Engl J Med. 2007; 357(22):2277-84.

4.         NRC. Health risks from exposure to low levels of ionizing radiation : BEIR VII Phase 2, National Research Council (U.S.). Committee to Assess Health Risks from Exposure to Low Level of Ionizing Radiation. Washington, D.C.: National Academies Press, 2006.

5.         Preston DL, Ron E, Tokuoka S, et al. Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiat Res. 2007; 168(1):1-64.

6.         Ozasa K, Shimizu Y, Suyama A, et al. Studies of the mortality of atomic bomb survivors, report 14, 1950-2003: an overview of cancer and noncancer diseases. Radiat Res. 2012; 177(3):229-43.

7.         Doss M. Linear No-Threshold Model vs. Radiation Hormesis. Dose Response. 2013:(In Press). Available at: http://dose-response.metapress.com/openurl.asp?genre=article&id=doi:10.2203/dose-response.13-005.Doss.

Letter to the Editor published in Radiology

Available at: http://pubs.rsna.org/doi/pdf/10.1148/radiol.13131661