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Letters to the Editor |
and
Fergus Gleeson, FRCR
* Department of Radiology, St James' University Hospital, Leeds Teaching Hospitals NHS Trust, Beckett St, Leeds, LS9 7TF, England
e-mail: andrew.scarsbrook{at}leedsth.nhs.uk
Department of Radiology, Churchill Hospital, Oxford Radcliffe NHS Trust, Headington, Oxford, England
We read with interest the article by Dr Groves and colleagues (1), in the September 2006 issue of Radiology, on physicians' knowledge of radiation exposure during imaging for suspicion of pulmonary embolism (PE) in pregnancy. The authors highlight some important aspects; however, some of the conclusions regarding lung scintigraphy are misleading.
Most research on imaging for PE has been performed in nonpregnant patients, and the largest studies specifically excluded pregnant women (2,3). As a consequence, recommendations for imaging of PE in pregnancy are often based on extrapolation from trials performed in nonpregnant patients.
In nonpregnant patients, up to 50%–70% of scans may be classified into the nondiagnostic category (2,4). Findings of a prospective study of 120 pregnant women with suspicion of PE undergoing lung scintigraphy showed a high percentage of normal scans (74%), a smaller percentage of nondiagnostic scans (24%), and a low proportion of high-probability scans (2%) (5). To our knowledge, the diagnostic utility of computed tomographic (CT) pulmonary angiography in pregnant patients has not been validated.
CT pulmonary angiography has a nondiagnostic rate of 5%–19% (6–8) and imparts a substantially higher maternal radiation exposure than scintigraphy, with breast dose ranges from 10 to 35 mGy (9–11). The latent carcinogenic effects of breast irradiation are uncertain, but it seems likely that radiosensitive, proliferating maternal breast tissue is at increased risk. The lifetime risk of breast carcinoma has been estimated to increase by 14% above the background rate following a single 10-mGy dose to the breast in a 35-year-old woman (11). Data from our institution showed the average breast exposure from half-dose (40 MBq) perfusion scintigraphy to be 150 times lower than that from CT pulmonary angiography (0.11 mGy versus 16 mGy).
In pregnancy, concern over fetal radiation exposure is paramount, and it is common practice to perform half-dose perfusion scintigraphy, without a ventilation study (12). This imparts a lower fetal dose than standard lung scintigraphy. We are not aware of any evidence to support the statement by Dr Groves and colleagues that this may result in a less diagnostic study (1).
A major limitation of their discussion is a failure to consider fetal radiation exposures in context. A threshold dose of 50 mGy is required before deterministic effects, including fetal malformation, mental retardation, or death, become specific risks (13). The only possible risk following low-level radiation is induction of malignancy (14). No increased incidence of congenital or developmental abnormalities was reported in the offspring of 120 women undergoing perfusion scintigraphy during pregnancy, and no pediatric malignancies occurred during follow-up (5). The estimated incidence of childhood malignancy following in utero radiation exposure is 1 in 16 000 per milligray (14), which equates to a trivial additional risk of malignancy of 1 in 560 000 following half-dose perfusion scintigraphy and 1 in 1 000 000 after CT pulmonary angiography (9).
In our experience and that of others, perfusion scintigraphy performed in pregnant patients is diagnostically helpful in most cases (5,15). While the technique is associated with a higher (albeit insignificant) fetal dose, it imparts a lower maternal dose and a significantly lower breast dose. The conclusion of Dr Groves and colleagues that it is difficult to justify the use of lung scintigraphy in pregnant patients suspected of having PE is not supported by the available evidence. Conversely, the judicious use of half-dose perfusion scintigraphy in pregnant patients suspected of having PE seems entirely warranted.
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,
Stuart J. Yates, MSc,
Thida Win, MB, BS
,
Irfan Kayani, MB, BS*,
Jamshed Bomanji, MB, BS, PhD*, and
Peter J. Ell, MD*
* Institute of Nuclear Medicine, University College London, Middlesex Hospital, London, England
e-mail: drashleygroves{at}hotmail.com
East-Anglian Radiation Protection Service, Addenbrooke's Hospital, University of Cambridge Teaching Hospital Trust, Hills Road, Cambridge CB2 4EU, England
Department of Chest Medicine, Lister Hospital, Cambridge, England
Our research (1) suggests that there were deficiencies in the knowledge of the fetal dosimetry of imaging PE among health care professionals. Given these deficiencies, we aimed to improve awareness of these exposures and to highlight the lack of clinical guidelines in this setting. We presented the maternal and fetal dosimetry of imaging PE by means of ventilation-perfusion (V/Q) scintigraphy and CT pulmonary angiography, together with the evidence base. Our article emphasized that the European Medical Exposure Directive (2) required that patient exposures be kept to a minimum, especially in pregnancy. However, current American and British guidelines on PE (3,4) do not address imaging recommendations in pregnancy, nor do they make reference to the relevant dosimetry.
We thank Dr Scarsbrook and colleagues for their comments and recognize that there are many facets to the issues discussed, but many of their statements require clarification.
The points made in their letter were covered in the discussion (paragraphs 3, 4, and 6) of our original article: lack of published evidence for the diagnostic performance of CT and V/Q scanning for imaging PE in pregnancy; maternal dosimetry issues including the breast exposure; the relevance of small fetal exposures; and the relative small differences in fetal exposure between CT pulmonary angiography and V/Q scanning.
The value of 14% quoted in the letter by Dr Scarsbrook and colleagues for the increase in the lifetime risk of developing breast cancer, for a 10-mGy exposure, originates with Land et al (5). However, this figure was derived from a group of women who were probably abnormally susceptible to radiation-induced breast cancer (6). For a 30-year-old woman in the general population, a breast dose of 10 mGy is actually estimated to correspond to an increase in the lifetime risk of breast cancer of approximately 0.2% (6).
By using data published by the International Commission on Radiological Protection (ICRP) (7), the dose to the breast from a half-dose perfusion study by using 50-MBq technetium 99m–labeled macroaggregates is estimated as 0.25 mGy. This seems inconsistent with the 0.11 mGy quoted by Dr Scarsbrook and colleagues for a 40-MBq study.
The data presented by Cook and Kyriou (8) describe fatal cancer risks and not total cancer risks. Moreover, the letter by Cook and Kyriou already quotes values for half-dose scintigraphy. Therefore, the total childhood cancer risk from half-dose perfusion scintigraphy is estimated to be 1 in 140 ,000 and not the 1 in 560 000 quoted by Dr Scarsbrook and colleagues.
With respect to scintigraphy, the lower maternal dose and lower breast dose are not independent benefits. The maternal dose is in part lower because the breast dose is lower. In fact, while there might be a factor of 40 difference in the breast dose between the two techniques (8), the difference in the mother's overall cancer risk is much less substantial. This can be estimated from the ratio of the effective dose for CT pulmonary angiography to that for scintigraphy, which is on the order of 3:1 for full-dose perfusion scintigraphy.
Finally, in our article, we clearly state possible counter arguments to the use of CT pulmonary angiography in pregnancy, such as patient choice, maternal dosimetry, lack of CT availability, and the safety of intravenous CT contrast medium in pregnancy. Nonetheless, the ICRP emphasizes that the fetus should be specifically considered in pregnancy and that options for fetal dose reduction should be considered (9). Recent meta-analyses (10,11) and consensus among experts by the British Thoracic Society guidelines (4) provide at least a recognized degree of evidence base for the argument to perform CT pulmonary angiography. Previous personal experience as argued by Dr Scarsbrook and colleagues is not usually accepted as evidence per se.
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This article has been cited by other articles:
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  J. K. Pahade, D. Litmanovich, I. Pedrosa, J. Romero, A. A. Bankier, and P. M. Boiselle Quality Initiatives: Imaging Pregnant Patients with Suspected Pulmonary Embolism: What the Radiologist Needs to Know RadioGraphics, May 1, 2009; 29(3): 639 - 654. [Abstract] [Full Text] [PDF]
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