Letter to the Editor

To the Editor:

The article "Minimizing radiation risks with MDCT in neuroradiology" (2009;1-2:19-24) in the January/February issue, by Cari Buckingham, MD, and Megan K. Strother, MD, listed incorrect radiation dose values in Table 1 on page 20. The effective dose for the computed tomography (CT) procedures should have been listed in mSv, for example, the effective dose for a head CT is 2 mSv or 200 mrem or 0.2 rem. Also, the discussion on the third column of page 20, regarding radiation dose in pregnancy and its effects, is incorrect. Normally, we use an effective dose of 10 rem to the fetus during the first 6 weeks postconception as a threshold dose to consider recommendation for therapeutic abortion. The overall discussion about pregnancy and radiation dose is not consistent, and in fact, the authors seem to switch between effective dose and organ dose with significant errors.

Mahadevappa Mahesh, PhD, FAAPM, FACR
Department of Radiology and Radiological Science,
Johns Hopkins Hospital,
Baltimore, MD

To the Editor:

The authors wish to thank Dr. Mahesh for his careful reading of our article and for pointing out these errors, allowing us to offer these clarifications. Table 1 lists typical effective doses for various radiographic studies.

By way of background, Sieverts is the SI unit for equivalent dose (H = D× weighting factor of radiation). The equivalent dose attempts to quantify the biological damage arising from the deposition of ionizing radiation in tissues by different types of radiation. Radiation equivalent man (rem) is the nonSI unit for equivalent dose. Gray (or mrad) is used for organ doses, and does not include a weighting factor for radiation sensitivity. Thus, in our article, we should have either listed doses in terms of effective dose (using Sv or mrem) orin terms of organ dose (using Gy or mrad).

To Dr. Mahesh's second point, unfortunately, our knowledge of radiation effects in humans for low-dose radiation during the preimplantation phase is limited. Most of our assumptions come from data on atomic bomb survivors and from experimental animal data. The limitations of this data undermine attempts to truly quantify risks to the developing embryo during the preimplantation phase. We appreciate the additional recommendations regarding a threshold dose for consideration of therapeutic abortion. We did not address this in our article.

Our recommendations focused on the varying risks of radiation with respect to fetal age, which is well documented. ¹ It is not clear whether there is a minimal threshold dose at conception, although it is clear that the developing embryo is extremely radiosensitive during the preimplantation phase (days 0 to 8). The Joint Guidance from the National Radiological Protection Board (NRPB) states that there is no threshold dose for death at day 1, and there is a threshold of 100 mGy on days 2 to 7. The NRPB recommends that "any procedure that delivers doses to the fetus of some tens of milliGray … may carry significant risks." ² This is the 10 mGy figure we used in our article. Other studies havefound an LD ₅₀ of 300 mGy during the initial day of the preimplantation phase. ¹ We apologize that we muddled this discussion. It would have been clearer to either present a nonthreshold model or to leave out the >1 rem threshold dose for embryonic death during the preimplantation phase.

REFERENCES

1. Valentin J. Biological effects after prenatal irradiation (embryo and fetus). Publication 90. Ann ICRP. 2003;33(1-2):1-206.
2. Sharp C, Shrimpton JA, Bury RF. Diagnostic medical exposures: Exposure to ionizing radiation of pregnant women. Doc NRPB. 1998;4(4)5-14.

Megan K. Strother, MD
Department of Radiology,
Vanderbilt University
Medical Center,
Nashville, TN