Answer to Question #2372 Submitted to "Ask the Experts"

Q

I am caught in a raging debate on effective dose versus real dose. The National Lung Screening Trials have been underway for some time now. Asymptomatic patients are randomly chosen to receive either a chest x ray or a chest helical CT. Self-referred chest x rays have been acceptable for some time now, but I have a problem with the concept of a self-referred CT. Based upon the material I have read, I question if the patient understands the risks versus the benefits well enough to give informed consent. Are the benefits of the helical CT chest screening sufficient to justify exposing an asymptomatic patient to this procedure? I would appreciate a dialogue.

A

To best answer your question, let us pause first to clarify some terms and definitions.

Radiation dose is simply the amount of energy absorbed by a medium, most often the medium being the human body or, more specifically, a particular tissue or organ of the body. The typical units used to quantify absorbed dose are, in international units, the sievert (Sv), which is equivalent to the absorption of 1 Joule of energy per kg of tissue.

Effective dose is another term used to quantify the biological impact of the absorbed dose by taking into account the varying sensitivity of the different tissues and organs of the body to radiation injury. A weighting factor is applied to the individual organ doses to yield a single effective dose number for a particular procedure, like a chest x ray. The International Commission on Radiological Protection (ICRP) Reports 60 and 72  explain in great detail the concept and use of effective dose. So there is a difference in the terms and use of dose-organ dose versus effective dose.

Let’s consider the procedures in more detail. Spiral, or helical CT, will result in doses up to four times higher than that for a conventional CT scan, and certainly greater than that incurred in standard x rays. In helical CT, the patient is continuously moved through the scanner as the source and receptor rotate around the patient. To achieve such a detailed and expansive feat, it is necessary to expose the entire thickness of the patient to a greater quantity of x-ray photons. The pitch is the ratio of distance the patient has moved through the scanner per rotation per slice thickness. Usually, the number is between one and two. Higher pitch values lower the radiation dose, but at the expense of partial volume effect, i.e., loss of detail. Larger patients receive a relatively higher skin dose in order to penetrate their bodies, but smaller patients have a relatively higher dose to internal organs. Furthermore, since internal organs are more radiosensitive than skin, smaller patients generally incur a higher effective dose for the same CT settings and, hence, higher long-term cancer risk.

With either a chest x ray (CXR) or a CT scan, the part of the body receiving the greatest exposure is the entrance skin in the field of view. Conventional CT using serial, noncontiguous slices gives a much higher dose than a CXR. The typical CXR effective dose is about 0.06 mSv*; a conventional chest CT scan with slice thickness of about 5 mm would give about 2 mSv and a helical CT of the chest results in about 8 mSv (Invest Rad, 2000; Pediatric Radiol, 2002; BJR, 1997). These skin doses are well below the levels required to produce any immediate or short-term effects such as reddening of the skin or blistering. Hence, the concern is the long-term risk from such exposure—the development of cancer later in life. Based upon the calculated effective dose, the patient’s theoretical increased fatal cancer risk from a standard CXR is 0.0002%, about 0.008% for conventional CT and about 0.03% for a helical CT of the chest. Hence, the helical CT of the chest delivers an effective dose, and increased cancer risk, that is over 100 times greater than that for a standard CXR. Note that the natural incidence of fatal cancer in the United States is 25%. Therefore, the theoretical total risk of fatal cancer for the group of adult subjects participating in this study in which they receive a helical chest CT is predicted as 25.03 %. Conventional wisdom is to minimize exposure, and risk, as much as possible, unless there is the prospect of a direct benefit to the patient from the exposure.

What is the benefit of receiving a helical CT scan? If a small slice thickness and low pitch is used, it is possible that a very small lesion might be detected that would otherwise not be detected with a CXR. We qualify this comment to say that the utility of this is very suspect. A lesion small enough to escape detection, less than a centimeter, with a standard CXR is so small that its clinical significance is doubtful. Only serial x rays will prove the lesion to be benign or malignant. Hence, more than one helical CT scan would still have to be performed to give the study any meaning. Perhaps the same benefit could be achieved with serial chest x rays with a fraction of the dose. Another combination that might be even more worthwhile would be a standard CXR and follow-up standard CT scan (only in the area of the lesion) with small slices (1-2 mm) to evaluate a suspicious lesion.

The main advantages of helical CT are not germane to the current situation of screening healthy individuals. Often helical CT scans are very useful in emergency rooms where a fussy child needs a critical x ray, or even for a delirious or uncooperative adult. The advantage is that the entire helical CT can be done in less than a minute that may be long enough to keep a child still, whereas conventional CT scans would take ten minutes or so. Furthermore, the speed of helical CT is an advantage in trauma cases where minutes are vital. With the information stored in the computer files, one can even convert serial horizontal slices to vertical slices without having to repeat the study in some cases of facial trauma where multiple views are important to assess fractures and soft tissue entrapment. For simply detecting a lesion in a routine outpatient setting, a helical CT scan would be considered excessive.

Regarding your concern with the patient’s understanding of the risks versus benefits of performing helical chest CTs in the absence of symptoms, it is extremely important that the patient is provided full disclosure of the associated radiation dose, increase in potential cancer risk, and alternatives for screening in this indication. Most importantly, it appears that the helical CT scan is not necessary for the patient’s medical care, but rather is indicated to fulfill clinical research objectives. Thus, the patient should be informed that the associated radiation exposure is approximately one hundred times greater than that from a standard chest x ray. Typically, a standard chest x ray is all that is really necessary for screening for most pulmonary disease.

Jamey West, MD
Lisa Coronado, Senior Health Physicist
National Institutes of Health
Bethesda, MD

References:

• BJR 1997: British Journal of Radiology, Volume 70, pages 437-439 (1997).
   
• ICRP 60: ICRP Publication 60, Recommendations of the International Commission on Radiological Protection. Annals ICRP 21(1-3): 1-201, 1991. Pergamon Press.
   
• ICRP 73: ICRP Publication 73, Radiological Protection and Safety in Medicine. Annals ICRP, Vol. 26/2, 1996. Pergamon Press.
   
• ICRP 80: ICRP Publication 80, Radiation Dose to Patients from Radiopharmaceuticals. Annals ICRP, Vol. 28/3, 1999. Pergamon Press.
   
• Invest. Radiol. 2000: Investigational Radiology, Volume 35, (9); pages 557-563 (2000).
   
• Pediatric Radiol 2002: Pediatric Journal of Radiology, Volume 32, pages 700-706 (2002).

*Note: To convert to traditional radiation exposure units: 1 mSv = 100 mrem.