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ImagingCT Imaging > How many CT scans are too many?

Last update: 01/14/2013

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Intro | Questions Needing Answers | Radiation Dose in Computed Tomography
Resources & Research News

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 

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?
Also see Comparing CT and MRI - Lymphomation.org
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Radiation Dose In Computed Tomography – Euclid Seeram, B.SC,. M.Sc., R.T.(R) PMID: 10432536

What’s here:

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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Table 2 Effective Doses from various Examinations
Examination
Effective Dose (mSv)
Skull series
.05
Chest
.05
Lumbar spine
1
Abdomen
1
Pelvis
1.6
  CT head/neck
2
  CT chest
7
  CT abdomen
9
  CT pelvis
9
Total CT (my note)
27
 
 
 
 
 
 
 
 
 
 
Conclusion of the author
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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Resources & Research News
  • Second malignancy after treatment of childhood non-Hodgkin lymphoma.
    Cancer. 2001 Oct 1;92(7):1959-66. PMID: 11745271 - PubMed 
  • Radiation Dose Comparisons  - FDA
  • test

 
Disclaimer:  The information on Lymphomation.org is not intended to be a substitute for 
professional medical advice or to replace your relationship with a physician.
For all medical concerns,  you should always consult your doctor. 
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