Radiation doses in computed tomography

BMJ 2000; 320 doi: http://dx.doi.org/10.1136/bmj.320.7235.593 (Published 4 March 2000)
Cite this as: BMJ 2000;320:593

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I found the editorial by Rehani and Berry a thought provoking read as we sometimes rush forward headlong and forget to question what is becoming the status quo. CT is becoming a more accessible investigation and as such our reasons for choosing to order one become less thoughtful. In addition our reason not to order what may be the “gold standard” investigation becomes harder to defend. I have worked in Australia where I saw a tendency to CT the heads of anyone with even brief loss of consciousness following closed head injury. If the investigation is readily available what is the defence if a significant intracranial injury, however unlikely, is missed due to not performing the scan? In the NHS it might be the queue to the scan room. Litigation plays on the doctor’s mind. Will anyone be sued for a cancer developed later in life? The cause and effect is not strong enough.

I am sure that there are a good number of us practicing who either were ignorant of the radiation doses involved or had never heard anyone using them as a reason to alter practice. The Rehani and Berry editorial may at least cause a few of us to stop and think.

Competing interests: None declared

Adam Dangoor, Registrar in Oncology

Christchurch, New Zealand

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I was interested in the editorial by Rehani and Berry regarding the rising contribution from computed tomography (CT) examinations to UK collective dose and possible ways to limit this development [1]. While this trend is concerning, in reality individual patients and clinicians often have little incentive to forego a quick, painless and accurate CT study for the remote possibility of neoplasia [2]. Apart from certain recognisable groups e.g. children and pregnant women, refusing to perform procedures involving radiation is often difficult to justify on grounds of risk alone.

As the authors suggest, a partial solution to this problem may be achieved by shielding radiosensitive tissues, a technique already widely practised for relatively low dose plain radiography. The reduction in patient dose by shielding radiosensitive organs during CT has been the subject of two further publications [3-4]. In our recent study, reductions in gonad dose of 77% and 82% were achieved during abdominal and pelvic scan protocols in a phantom by protecting the testes from scattered radiation. The protection device, which retailed at less than £100, did not impair image quality [3].

While the current enthusiasm for CT examination continues unabated, radiologists must ensure each patient receives maximum benefit from imaging whilst minimising the attendant risks.

Dr R Price MBBS MRCP FRCR

References

1) Rehani M, Berry M. Radiation doses in computed tomography. British Medical Journal 2000;320:593-4.

2) Dixon AK, Dendy PP. Spiral CT: how much does radiation dose matter? Lancet 1998;352:1082-3.

3) Price R, Halson P, Sampson M. Dose reduction during CT scanning in an anthropomorphic phantom by the use of a male gonad shield. British Journal of Radiology 1999;72:489-94.

4) Hidajat N, Schroder RJ, Vogl T, Schedel, Felix R. Effektivitat der Bleiabdeckung zur Dosisreducktion beim Patienten in der Computertomographie. Fortschr. Rontgenstr.1996;165:462-5.

Competing interests: None declared

R Price

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Dear Editor,

We found the paper concerning patient doses in computed tomography written by Dr Rehani and Berry (BMJ 2000;320:593-4, 4 March 2000) very interesting. However, In spite of the fact that we totally agree on the substance of the article, we think that it is quite important to comment the numbers chosen.

The authors used an effective dose value for CT chest examination of 8 mSv. Taking into account the European guidelines concerning CT /1/, this effective dose corresponds to a dose-length product (DLP) of roughly 571 mGy.cm. The reference DLP published in the European guidelines is 650 mGy.cm. This value is quite high and can be drastically reduced by optimising the acquisition protocols. In our hospital most of our chest CT's are performed with a DLP lower than 220 mGy.cm, which leads to an effective dose of 3.1 mSv. Anyway, concerning the CT chest acquisitions the authors choose a value which is in the higher range of doses.

The European guidelines in the field of radiography gives also a reference value for chest radiography /2/. The entrance dose given in this document is 0.3 mGy, which leads to an effective dose of roughly 0.06 mSv. This dose is also in the higher range of dose and can be noticeably reduced. Comparing this effective dose with the value mentioned by the authors (0.02 mSv) it appears that for this examination, the lower range of dose has be chosen. From our point of view the comparison is not fair.

The comparison between chest CT and radiography examinations should done either by choosing published reference values which can often be lowered, or by using optimised protocols. The ratio is then closer to 150 than 400. This number is still quite high, but one has to realise that many countries had for many years tuberculosis screenings using radiophotography units which delivered a dose of 0.2 mSv.

Francis R Verdun and Reto Meuli
Institute of Radiophysic and Department of Radiology University Hospital Lausanne, Switzerland

/1/ Quality criteria for computed tomography, CEC Working document, EUR 16262, (May 1998).

/2/ European guidelines on quality criteria for diagnostic radiographic images, CEC document, EUR 16260 (1996).

Competing interests: None declared

Francis R Verdun, Medical Physicist

Reto Meuli

Lausanne

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I found Rehani and Berry's editorial (BMJ 2000;320:593-4, 4 March 2000) wearisomely predictable. I wonder how many practising clinicians are familiar with the radiologist who appears to regard any patient exposure as a privilege to be eked out grudgingly. Only recently I was involved in the case where two A&E Consultants and a surgical registrar were all of the opinion that a patient in a high-energy road crash with soft-tissue abdominal imprinting required abdominal CT only for it to be refused until a surgical Consultant had examined the patient.

We are all very well aware that radiation exposure needs careful thought, but as radiation doses from conventional exposure drop (for two reasons; the dose per exposure has reduced dramatically recently for several reasons and fewer unnecessary exposures are being performed as clinicians follow rules such as the Ottawa Ankle Rules (1)) so, pari passu the percentage of total exposure from CT must rise. Doubtless the same may be said of other specialist investigations such as coronary angiography.

There is little doubt that the development of CT has revolutionised many areas, as Rehani and Berry admit. Furthermore, a good case can be made that it has both saved lives and improved morbidity - the scales seem firmly on the side of the benefit compared to the risk. Can I plead for radiology colleagues to consider the clinician's viewpoint - the clinician is the doctor with responsibility for that patient.

(1) Stiell IG, Greenberg GH, McKnight RD, Nair RC, McDowell I, Reardon M et al. Decision rules for the use of radiography in acute ankle injuries: refinement and prospective validation. JAMA 1993;269:1127-32.

Rowland Cottingham BSc FRCS (Ed) FFAEM
Consultant in Emergency Medicine
Eastbourne District General Hospital, Eastbourne, East Sussex

Competing interests: None declared

Rowland Cottingham

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Editor - Rehani and Berry give a timely reminder that computed tomography (CT) of the abdomen involves so high a dose of radiation that its use must be "really necessary..and appropriate" (1). This is highly relevant in the light of recent papers enthusing over the ability of to improve diagnosis in patients with suspected acute appendicitis (2-4).

The investigators applied CT to the 50 - 60% of their patients who, at the first examination, did not require urgent operation, but retained equivocal signs of appendicitis. The main claim is that this use of CT enabled them to reduce their negative appendicectomy rate from about 20% to 7-10%. This is useful, but it is a serious matter to apply CT so widely when it is known that about one-third of patients who present with suspected appendicitis will settle spontaneously over 24-36 hours (acute non-specific abdominal pain or NSAP). There is now substantial evidence that close bedside supervision (Active Observation) can identify these patients; among those others who go on to develop signs of the need for operation the negative appendicectomy rate is 5-7%, and there is no significant rise in the perforation rate (5). In the occasional obscure case CT will be helpful, but before CT is used with any freedom in this field it must be remembered that most acute appendicitis occurs in the young, and some of the women scanned will carry an early unsuspected pregnancy.

These remarks do not detract from the value of CT in the management of other aspects of the acute abdomen, especially in closed abdominal trauma, and in selected patients with severe acute pancreatitis, acute colonic diverticulitis, and possible small bowel strangulation. Here the balance is quite different, and concern over radiation dosage takes second place to the production of vital information which may, for example, allow preservation of the spleen in an injured child.

Peter F Jones
Emeritus Clinical Professor of Surgery
University of Aberdeen

1. Rehani MM, Berry M. Radiation doses in computed tomography. BMJ 2000; 320: 593-4.(4 March).

2. Cho SS, Buckingham JM, Pierce M, Hardman DT. Computed tomography in the diagnosis of equivocal appendicitis. Austr NZ J Surg 1999; 69: 664- 7.

3. Stroman DL, Bayouth CU, Kuhn JL, Westmoreland M, Jones RC, Fisher TL, et al. The role of computed tomography in the diagnosis of acute appendicitis. Am J Surg 1999; 178: 485-9.

4. Rao PM, Rhea JT, Rattner DW, Venus LG, Novelline RA. Introduction of appendiceal CT. Impact on negative appendicectomy and appendiceal perforation rates. Ann Surg 1999; 229:344-9.

5. Jones PF, Bagley FH. Acute appendicitis. In: Jones PF, Krukowski ZH, Youngson GG,eds. Emergency Abdominal Surgery. London: Chapman and Hall, 1998: 48-52.

Competing interests: None declared

Peter F Jones

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