Fetal dosimetry in diagnostic radiology

Diagnostic radiology examinations are frequently performed in all countries because of the benefit that the patient derives from the resultant diagnosis. Given that so many examinations are performed it is inevitable that there will be occasions when the planned exposure of a woman who is known to be pregnant is contemplated. In these circumstances, there must be rigorous justification of the examination and the procedure itself must be optimised as well. Radiation risks from fetal irradiation are well established. These risks fall into three categories: 1) a cancer induction risk (mainly leukaemia); 2) hereditary effects (as the fetus is a potential parent); 3) a risk of serious mental retardation (if the fetus is exposed in the critical 8-15 weeks period when the forebrain is being developed). Risk factors for these effects have been reviewed by the International Commission on Radiological Protection. Special rules apply to the radiology of women who are or who may be pregnant. These rules have been developed to avoid he unintended irradiation of the fetus. These rules have been variously referred to as the 10-day rule and the 28-day rules, in which radiology of potentially pregnant women is restricted to the first 10 or 28 days following menstruation. It is apparent that the advice provided by national bodies varies, as different rules apply internationally, due presumably to a lack of an international consensus on the subject. The advice from the National Radiological Protection Board, the College of Radiographers and the Royal College of Radiologists applies in the United Kingdom. In summary, the advice is that women of child bearing age are asked before a diagnostic radiology examination in which the pelvis is in, or near, the primary beam are asked if they are, or may be, pregnant. If pregnancy can be excluded then the examination can proceed. If it is likely that the patient is pregnant, then the proposed examination must undergo rigorous justification. If pregnancy cannot be excluded, but the examination results in a low dose to the fetus, then the advice is to proceed with the examination, provided the woman' period is not overdue (i.e. the missed period rule). For high dose procedures (taken to be examinations where the fetus receives a dose of a few mGy) then the 10-day rule applies