How many CT scans are too many?
In order to make truly informed
decisions, it's important to put the risks of procedures into context
and not to magnify them. The text within is not a promotion of the
idea that CT is inappropriate for staging or monitoring NHL. We are
seeking answers and guidance from professionals in the field to help
us identify and understand the risks, and to put the risks into
perspective. Please provide comment or sources of reputable
information by clicking here.
What raises the concern:
The
paper Radiation Dose in Computed Tomography --
"Directed
Reading" for medical students -- clearly states the dangers of CT
scanning in it's
conclusion:
"CT is one of the highest radiation-exposure examinations
in radiology. Therefore it is mandatory and critical to pay close attention not only to
the dose factors but also to the methods used to reduce the dose to the patient because of
the risks associated with exposure to radiation."
What mitigates the
concern: There is no conclusive data on the incidence of secondary malignancies
resulting from CT exposure in NHL patients. We have only risk calculations
based on experiences with radiation exposures. Children who have received treatment
for cancer with radiotherapy provide important clues. Note: Children are
at increased risk because their organs are still forming during exposures and
they have a longer life expectancy. The risks reported in the following report
are for exposures resulting from treatment, which is much greater than from
exposures from CT. Increased incidence of secondary malignancies (2.1% and 4.8%)
was reported at 10 and 20 years after diagnosis, respectively.
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Second malignancy after treatment of childhood
non-Hodgkin lymphoma.
Cancer. 2001 Oct 1;92(7):1959-66. PMID: 11745271 - PubMed
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What are the Radiation Risks from CT? - FDA.gov
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Questions Needing Answers:
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Is the radiation exposure that results from repeated CT scans
relevant to
patients with indolent cancers who have good, and improving, long-term survival
expectations--especially
patients who when participating in clinical trials may have neck to pelvis scans
ordered every two months?
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Can any of the high incidence of secondary cancers in patients
with NHL be attributed, in part,
to radiation exposure from CTs?
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Can MRIs be used as an alternative imaging technique?
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Can accurate measurements be relied upon that will satisfy the
FDA in the conduct of a clinical trial?
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What is the minimum standard for MRI equipment for monitoring
or staging NHL?
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Is special training required of imaging technicians to ensure
quality results for monitoring or staging NHL?
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| How can the patient ensure that the MRI scans he
or she receives will meet the standards required of their physicians and/or the FDA in
measuring lesions?
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Radiation Dose In Computed Tomography – Euclid
Seeram, B.SC,. M.Sc., R.T.(R) PMID: 10432536
What’s here:
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Table 2 Effective
Doses from various Examinations
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Conclusion (from the last page of the paper)
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Radiation Protection Principles:
Justification, Optimization and Dose Limitation
To be effective, a radiation protection program for CT (or any other radiologic
modality) always should ensure justification, optimization and dose limitation -
principles that are vital to radiation protection regulation.
Justification involves the concept of net benefit, that is, there must be a benefit
associated with every exposure. This requirement is intended for referring physicians and
is one effort to reduce dose to patients undergoing x-ray examinations. As Wolbarst
indicates, 'justification provides an essential moral stance for the intelligent use of
radiation.’
Optimization is a principle intended to ensure that doses delivered to patients are
kept as low as is reasonably achievable (ALARA), taking into account economic and social
factors. In implementing ALARA, radiologic technologists always should apply all relevant
technical radiation protection practices to ensure that the patient receives an exposure
as low as is reasonably achievable without sacrificing image quality.
The concept of dose limitation is a major, integral component of regulatory guidance on
radiation protection. This concept addresses the maximum permissible dose that an
individual may receive annually or accumulate over a working lifetime. These doses should
be within the limits established by international organizations such as the ICRP and
national bodies such as the NCRP. These recommended limits are intended to reduce the
probability of stochastic effects and prevent detrimental deterministic (nonstochastic)
effects.
The 1993 NCRP annual effective dose limit for radiologic technologists is 50
mSv. For
cumulative occupational exposure, the dose accumulated in N years is equal to 10 x N
mSv.
For members of the public (recall that shielding is intended to protect people outside the
CT room), the annual effective dose limit for continuous or frequent exposure is 1
mSv.
This implies that the shielding of the CT room walls would have to ensure that no member
of the public in close proximity to the CT room would be exposed to 1 mSv/year.
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Examination
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Effective Dose (mSv)
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Skull series
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.05
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Chest
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.05
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Lumbar spine
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1
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Abdomen
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1
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Pelvis
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1.6
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CT head/neck
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2
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CT chest
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7
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CT abdomen
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9
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CT pelvis
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9
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Total CT (my note)
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27
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CT is one of the highest radiation-exposure examinations in radiology. Therefore it is
mandatory and critical to pay close attention not only to the dose factors but also to the
methods used to reduce the dose to the patient because of the risks associated with
exposure to radiation. In this regard. Gray emphasizes that:
"The risk of developing a cancer as a result of CT of the liver is 12.5 per
10,000, but only 1.06 per 10,000 AP lumber spine films and 0.009 per 10,000 for a PA
chest. In other words, the CT carries a risk of 208 times higher than that of an AP lumbar
spine and 1388 times higher than for a P.A, chest.1 This article offers one small step
toward understanding the factors affecting the dose in CT and how to reduce the dose to
both patients and personnel.
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