fetal radiation doses in pregnancy Xrays and CT scans

Introduction

the risk-benefits of radiologic investigations in pregnancy and lactation need to be considered
exposure of less than 50mGy at any gestation is associated with negligible additional risk to the fetus although they may be an increased life time risk of cancer, for example, a 10-20mGy dose to a fetus may give a 1.5-2.0x risk of childhood leukaemia but the background rate is 1 in 3000 so absolute risk is small.
there is no evidence of teratogenic or mutagenic risk with iodinated contrast and no cases of fetal or neonatal hypothyroidism have been reported, these are also safe in lactation as only 0.01% of maternal dose is absorbed by the infant in breast milk
gadolinium contrast is also safe in lactation as 0.0004% of maternal dose is absorbed by the infant in breast milk
MRI in any trimester does not seem to confer harm to the fetus or infant

lead shielding for patients is no longer recommended
the benefits even for pregnant woman are neglible - radiation from scatter from the body far exceeds any benefit from the shielding ¹⁾

2023 recommended a maximum radiation exposure limit of 1 mSv over the course of a 40-week pregnancy the longest international flight exposure is no more than 1/6th of this limit. ²⁾

gestation	threshold radiation dose (mGy)
congenital anomalies or growth restriction during organogenesis (4-10wks gestation)	200
intellectual disability 10-17wks gestation	60-310
intellectual disability 18-27wks gestation	250-280

NB. external lead shielding of maternal pelvis reduces radiation dose by ~3% at all stages of pregnancy ³⁾

breast exposure during pregnancy or lactation:
- CTPA gives 20-70mGy breast exposure compared to ~0.25mGy for V/Q scan (ie. over 100x dose)
  - CTPA dose can be reduced by up to a third by use of a bismuth breast shield &/or organ-based tube current modulation
  - those with a normal CXR should be offered V/Q scan rather than CTPA

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²⁾

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Circulation 2001; 104:893-7, Damilakus et al