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Q: CT Scan radiation dose ( Answered 5 out of 5 stars,   0 Comments )
Subject: CT Scan radiation dose
Category: Health > Medicine
Asked by: crash1977mtl-ga
List Price: $20.00
Posted: 29 Jun 2006 11:26 PDT
Expires: 29 Jul 2006 11:26 PDT
Question ID: 742075
I just had a CT Scan of my sinus and neck. I am concerned because I
thought the procedure was uncessesary, and I am trying to determine
how much radation I was exposed to. I have the following data which
the technician provided to me. Can someone please translate this into
MSV or MREMS, or tell me if , overall, the dose was low or high, and
within normal values for this type of procedure, and what the
associated risks are for this type of exposure?

   CTDI vol mgy 23.30
   DLP 766.93
   Effeciency 97.4 %

   CTDI vol mgy 12.88
   DLP 179.15
   Effeciency 97.4 %


CTDI vol mgy 19.51
DLP 312.89
Effeceincy 85.7 %

Thank you
Subject: Re: CT Scan radiation dose
Answered By: welte-ga on 30 Jun 2006 10:03 PDT
Rated:5 out of 5 stars
Hi crash, and thanks for your question.

The better measure of total exposure for a study is the DLP (Dose
Length Product), rather than CTDI (CT Dose Index), which is related
more to the dose per single CT slice.  Going from DLP to exposure
requires some assumptions about the types of scan performed.  Here is
a good paper that looks at exposure relative to DLP for different
types of exams:

Van Unnik JG, Broerse JJ, Geleijns J, Jansen JT, Zoetelief J, Zweers
D. 	Survey of CT techniques and absorbed dose in various Dutch
Br J Radiol. 1997 Apr;70(832):367-71.

You can find the full free text here:

See particularly Tables 3 and 4 and Figure 3 on page 559.  This gives
a conversion factor of 1.90 x 10^-2 mSv / (mGy-cm) for neck exams and
0.62 x 10^-2 mSv / (mGy-cm) for head CTs (which are similar to sinus
CTs).  These factors are dependent on the specific scanner used, but
give reasonable estimates on exposure.

So... You had 2 scans of the neck which gives an exposure of

766.93 mGy-cm x  1.90x10^-2 mSv/(mGy-cm) = 14.57 mSv


179.15 mGy-cm x  1.90x10^-2 mSv/(mGy-cm) = 3.40 mSv

For the sinus CT, we get

312.89 mGy-cm x 0.62 x 10^-2 mSv/(mGy-cm) = 1.94 mSv  (the potential
error here is on the order of 10%)


The FDA gives a good, brief overview of the risks associated with
various exposures from CT examinations here:

For an CT exam with an exposure of 10 mSv, the increased risk of a
fatal cancer is 1/2000.  Your total exposure was (14.57 + 3.40 + 1.94)
 mSv = 19.91 mSv, conveying an increased risk of
(19.91 mSv / 10 mSv) * 1/2000 = 1/1005 or about (0.1%).  The natural
risk of fatal cancer in the US population is about 1/5 (about 20%),
meaning that your risk was increased from 20% to about 20.1% of
getting a fatal cancer.  The Health Physics Society states that
adverse effects have not been reliably demonstrated below 100mSv:

You can find more detailed information on the effects of ionizing
radiation from the UN Scientific Committee on the Effects of Atomic

The types of cancers typically associated with low level radiation
exposures are various types of sarcomas (soft tissue tumors),
leukemia, and lung cancers among others.  See the reference above for
more information.


Based on Table I in the above source, your exposure for the sinus CT
was similar (they give 2mSv) to the typical head CT dose and is the
equivalent of 243 days of exposure to background radiation or 100
chest x-rays.

For the neck scans, we can get an estimate on the typical exposure by
converting from the typical exposure for head CTs and using Table 2
from this source:

The relative normalized effective dose to convert from head to neck CT
is (0.0054 / 0.0023), so using the data from above source for typical
head CT exposures, we expect about

(0.0054 / 0.0023) * 2 mSv = 4.69 mSv

Your exposure for the second scan was in this range.  The exposure for
the second scan was a few times higher, but might be due to some need
to repeat the scan (motion, poor positioning, artifacts, etc.) or
different scanner settings compared to the second scan.  This isn't
that uncommon, since the second scan is usually with IV contrast and
the timing of the scan needs to be different to visualize the vascular
structures in the neck.  Another possibility is that the scans were
done with different slice thickness for finer detail, resulting in
more slices.


I hope this information is helpful.  Please feel free to request
clarification prior to rating.


Request for Answer Clarification by crash1977mtl-ga on 30 Jun 2006 21:27 PDT
Thank you for the information. As I am only 29 years old, I am worried
that due to the dose of close to 20 msv this could have a negative
impact moving forward.

Your assumption on the 2nd scan is correct, I had contrast (iodine) injected.

Could you tell me if 14.57 msv for this type of neck scan with
contrast is within expected ranges, and how much additional msv the
radioactive iodine contains? Does the dose of 20 msv carry any
increased risk for reproduction?

Clarification of Answer by welte-ga on 01 Jul 2006 12:43 PDT
Hi again,

The 14.57 mSv is somewhat higher than the typical range for neck CTs,
and may indicate that multiple scans needed to be taken (and are
counted as one).  Another possibility is that the current was
increased to get a better quality scan, etc.

Your total exposure was about 20mSv, causing an increased lifetime
risk of cancer of 0.1% (total risk 20.1%).  This estimate on increased
risk refers to cancers that typically appear on the order of 20-30
years after exposure, meaning that people who have this level of
radiation exposure, but who are already 90 years old, are unlikely to
live long enough to realize an actual fatal cancer from the radiation.
 Their risk is the same, provided they live long enough.  In your
case, the 0.1% increased risk is more meaningful, since you are quite
likely to live another 20-30 years.

In any case, as I stated, the Health Physics Societies states that
adverse effects have not been reliably documented below exposures of
about 100 mSv, so the 0.1% increased risk is likely more theoretical
than real.  In other words, below about 100mSv, the increased risks of
getting cancer are so small that they are dwarfed by other risk
factors, making it almost impossible to do studies that are large
enough to establish that a cancer was likely caused by prior radiation

In terms of reproductive risks, the Health Physics Society has an
excellent summary of both the risks with regard to gametes (sperm,
eggs) and to a fetus:

"According to published information, the reported dose of radiation to
result in an increase incidence of birth defects or miscarriage is
above 20 rad or 200 mSv."

They also state that during the first 2 weeks of pregnancy, doses of
much greater than 50mSv are required to induce a miscarriage.

In terms of damage to sperm or testes, the same source states

"For those patients who remain fertile after [radiation] therapy,
their reproductive risks are not increased significantly. In other
words, the risk of birth defects in the next generation for those men
who remain fertile and conceive is quite low. Studies of the atomic
bomb survivors indicate even in the high-exposure group that there is
not an increased incidence of chromosome abnormalities or genetic
disease in the next generation. That is also the case for studies from
the National Cancer Institute, which indicate that patients who had
cancer and received chemotherapy and radiation did not have an
increased incidence in genetic disease or birth defects in the next
generation although they did have problems with infertility."

Regarding exposure to ovaries, a question relating to a higher dose,
in this case to the abdomen and pelvis rather than the neck (closer to
the ovaries, so higher dose) was answered by Dr. Brent of HPS,
indicating that the risk of mutations passed to a child approach zero:

Finally, the iodine used for CT contrast studies is not radioactive. 
It is more radio-opaque than blood, and so shows up within the
vessels.  This helps to distinguish, for example, lymph nodes from
blood vessels, etc., which is particularly important in the neck,
where the anatomy is quite complicated.

I hope this information is helpful.

crash1977mtl-ga rated this answer:5 out of 5 stars and gave an additional tip of: $7.50
Very good - excellent - thank you.

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