Estimation of the dose to a pregnant patient - a case study
Marija Surić Mihić, Ivica Prlić
Institute for Medical Research and Occupational Health, Zagreb, Croatia
Introduction
In the Republic of Croatia, a considerablenumber of female patients are exposed to ionising radiation throughmedical radiological procedures. A large number of these exposuresare considered an extraordinary event (accidental or unintentional medical exposure).They usually happen when radiological procedures are performed on a patient unaware of her pregnancy. The legislative [1] states that in every such event, the patient’s dose should be estimated andthe patient and her doctor informed in order to decide whether a termination of pregnancy is indicated. Unfortunately, dose estimation is not calculated for most cases due to the fact that the necessity of such procedureshas not yet been fully recognised. There are also situations when medical practices on pregnant patients are planned or urgent (lifesaving) and should always be justified. In all cases, foetal dose and potential foetal risk should be estimated by a competent expert – medical physicist or radiation protection expert. Such experts are usually not employed in radiological institutions; medical physicists are employed only in clinical hospitals but radiation protection experts are available in dosimetry services contracted by radiological institutions.
This paperdescribes dose estimation in the case of a pregnant patient undergoing KUB X-ray (kidney, ureter and bladder) procedure.
Materials and methods
A pregnant patient (38 years) underwent a radiological procedure at a gestational age of 5 weeks, unaware of the pregnancy. The procedure was justified and the patient was asked about a possible pregnancy (she denied pregnancy). The pregnancy was confirmed at the gestational age of 12 weeks and the patient’s gynaecologist requested a dose estimation from the hospital’s contracting dosimetry service.
The hospital provided all of the radiological procedure data: 80 kV, 50 mAs, SID 120 cm, FOV 35*43 cm, large focus, AEC mode. Since the dosimetry service in question also performed regular QC procedures for the X-ray unit used for the procedure, ESD rates, filtration and other necessary data were available and already familiar. As data on the patient’s weight and trunk thickness at the time of the procedure were not available, a standard female phantom was used as shown on Figure 1. In order to perform calculations and dose estimations,the Monte Carlo-based PC program PCXMC 2.0 [2]was used.
Results
The patient’s effective dose and organ doses were calculated. The results are presented in Table 1. Maximal foetal dose was estimated to be equal to the dose to the uterus(1.08 mGy). The calculated effective dose for the patient was 0.45 mSv.
Discussion
The radiation-related risks for a foetus exposed in medical radiological procedures are related to the stage of pregnancy and absorbed dose. The risk is highest during organogenesis and in the early foetal period, less in the 2nd trimester and least in the 3rdtrimester. ICRP 84 [3] states that malformations have a threshold of 100-200 mGy and are typically associated with central nervous system problems. For a foetal dose of 1 mGy, the probability that the child will have no malformation is 97 % and that the child will not develop cancer (age 0-19) is 99.7%.
The organ dose to the uterus of1.08 mGy was estimated to be lower than the one found in relevant literature [4] (2.5 mGy) which is probably due to specific procedure parameters.
Doses to the foetus in conventional x-ray examinations reach up to 4 mGy, in fluoroscopy and computed tomography up to 80 mGy, all of which is lower than the 100 mGy threshold. Radiation therapy and interventional procedures using fluoroscopy give foetal doses that could be higher than 100 mGy and have to be estimated for each procedure. In cases of doses above 100 mGy, there is a risk for significant foetal damage and the decision regarding a termination of pregnancy should be based upon the individual case [3].
Taking into account that the public, due to inadequate information and lacking knowledge, considers any radiation exposure during pregnancy to be very dangerous and highly risky to the foetus, it is very important to estimatethe foetal dose and risk of possible health effects. These data must be clearly communicated to a patient in order to prevent additional stress and provide a reliable base for future decisions.
References
[1] Ordinance on the conditions for application of ionizing radiation sources in medicine and dentistry, Official Gazette 89/13
[2] Markku Tapiovaara, TeemuSiiskonen. User’s guide PCXMC 2.0, STUK-TR 7, Helsinki, Finland 2008
[3] Annals of the ICRP ICRP Publication 84 Pregnancy and Medical Radiation. International Commission on Radiological Protection, Pergamon, 2000.
[4] Wagner LK, Lester RG, Saldana LR. Exposure of the pregnant patient to diagnostic radiations: a guide to medical management 2nd ed. Madison, Wis: Medical Physics Publishing, 1997.
Figure1. Standard female phantom used in calculations
Figure 2. Entry data used in calculations
Table 1. Dose calculation results