Computed Tomography Pulmonary Angiography (CTPA)
- Both CTPA and radionuclide imaging have been advocated as being the first investigation of choice for pregnant patients with suspected PE. 5,8,9
- One recent study has shown equal diagnostic quality and negative predictive value of both CTPA and Perfusion Scanning for interpreting Pulmonary Embolism in pregnant women. 39 The authors suggested that the choice between the two should therefore be based on other considerations like radiation exposure (greater with CTPA), chest radiograph findings, clinical suspicion of an alternate diagnosis and availability of equipment. They preferred Perfusion Scanning in patients with normal chest radiographs and without suspicion of an alternate diagnosis. 39
Information regarding the effects to the fetus
- CTPA has been recommended in the first two trimesters of pregnancy, as it is associated with less fetal radiation than V/Q scanning. However, V/Q examination is a valid alternative and there is likely to be little difference in the diagnostic utility between the two.
- Research into fetal radiation dose in pregnancy has shown a definite trend in favour of CTPA in the first two trimesters of pregnancy. 10-12
- Phantom studies performed have demonstrated that fetal radiation dose varies with gestational age. Early in pregnancy, fetal radiation exposure occurs due to scatter and doses are as little as one-tenth that of V/Q. Later in pregnancy, doses increase towards that of V/Q scanning. 10
- The National Radiological Protection Board (NRPB) estimates the risk of inducing a fatal or non-fatal cancer in a fetus till the age of 15 to be approximately 1/33,000 per mGy. 9 Hence the risk from a single V/Q study would be approximately 1/165,000. The risk from a CTPA would be as little as one-tenth of this, early in pregnancy.
- Careful attention must be made to scanning protocols in pregnant patients. Variation in fetal radiation dose for both CTPA and V/Q scanning (up to 30 times that for CTPA and 3 times that for VQ scanning) have been reported by one author. 14
- No evidence exists that contrast administration during pregnancy is deleterious to the developing fetus. However, neonatal thyroid function should be checked in the first week of life to ensure normal thyroid function. 23
Information regarding the effects to the mother
- Maternal doses of radiation for CTPA are higher than for V/Q. Average whole body doses for CTPA range from 2-10 mSv and 0.6-1.5 mSv for V/Q scanning respectively. 11,15,16
- Of importance is the radiation dose to the breast caused by CTPA. Contention has plagued the literature and mistakes have been made in the interpretation of the evidence. 17-19 The average radiation dose to the breast from a CTPA is typically 10-20 mSv and 0.28-0.5 mSv for V/Q respectively. 11,17,20,21
- The Biological Effects of Ionizing Radiation, seventh report (BEIR VII) estimates that the lifetime attributable risk for breast cancer from a dose of 20mGy is approximately 1/1200 for a woman aged 20, 1/2000 for a woman age 30 and 1/3500 for a woman aged 40. 22 That is, if a woman aged 30 has a CTPA with a breast dose of 20 mGy, there would be an additional 1/2000 chance of her developing breast cancer. The lifetime risk of breast cancer for women is approximately 1/8.
- Studies using bismuth breast shields have shown radiation dose reductions of 34-57% to the breast, without significant decrease in image quality or diagnostic accuracy. 17,27,28
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