Summary:
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) proce
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
- Valentin J. Biological effects after prenatal irradiation
(embryo and fetus). Publication 90.
Ann ICRP.
2003;33(1-2):1-206.
- 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