The thyroid gland is of particular interest in radiation protection practice because cancer risks are highly dependent on the age and sex of exposed individuals. Data presented in the Biological Effects of Ionizing Radiation (BEIR) Committee report (BEIR VII) show that thyroid cancer risks to a newborn are two orders of magnitude higher than to 50 y old males and females. Even more striking is the fact that in uniformly exposed newborns, the estimated thyroid cancer induction risk accounts for 4.5% of the total cancer risk in males and 13.3% of the total cancer risk in females. For these reasons, understanding thyroid doses to patients undergoing diagnostic MBSS radiological examinations is essential. Knowledge of thyroid doses will be helpful in justifying patient exposures to ensure that there is a net patient benefit (Huda et al. 2013 ; Tipnis et al. 2015). In addition, knowledge of thyroid doses will be important in optimizing these types of studies to ensure that patient doses are kept as low as reasonably achievable (ALARA) (NRC 1998). The purpose of this study was to provide a practical tool to MBSS practitioners that will enable thyroid organ doses to be estimated in routine clinical practice.
Doses to salivary glands in MBSS examinations are generally comparable to those to thyroid glands. At 80 kV and with an x-ray beam filtration of 3 mm, the ratio of the dose to the salivary glands to the entrance air kerma (i.e., f salivary) was determined to be 0.45 for lateral projections and 0.23 for the upper GI projection. Organ doses in MBSS examinations are well below threshold doses for induction of deterministic effects that have been described in the literature (Grundmann et al. 2009 ; Konings et al. 2005). Carcinogenic sensitivity of salivary glands is generally considered to be relatively low, with this organ not receiving a tissue-weighting factor in ICRP Publication 26 (1977) or ICRP Publication 60 (1991). It was only in ICRP Publication 103 (2007) that this organ was given a weighting factor of 0.01. In addition, salivary glands are not explicitly mentioned in risk factors provided in BEIR VII, whereas thyroid doses are believed to be responsible for an average of 9% of all cancers induced in newborns from uniform whole-body irradiation. In young patients, thyroid cancer risk will therefore be much higher than any carcinogenic risk from irradiation of salivary glands, whereas in older patients carcinogenic risks from these two organs both will be relatively low.
The dosimetry performed in this study did not consider the presence of barium in the patient, which may affect the resultant patient doses. A recent study (He et al. 2014) investigated the effect of adding iodine to patient doses, which is likely to be applicable to barium given the close similarity between iodine (Z = 53; k-edge at 33 keV) and barium (Z = 56; k-edge at 37 keV). In this study, it was found that the addition of iodine had negligible effects on absorbed tissue doses at locations that are in front of the added iodine, but that substantial dose reductions can occur at x-ray beam locations that are behind the added contrast material. For PA and lateral projections used in MBSS examinations, the x-ray beam will be incident on the thyroid before it reaches the barium, and there will be no shielding of the radiation to this organ from the incident x-ray beam. Overall, it is therefore likely that thyroid dose conversion factors computed in this study will be valid even in the presence of added barium contrast material to patients undergoing MBSS examinations.
We performed a PCXMC simulation to assess how inclusion of additional views (middle and lower GI) would affect thyroid doses. The calculation was performed for an average adult patient using a beam quality of 80 kV and 3 mm aluminum total filtration with 30% of the KAP allocated in the lateral projection. The remaining KAP was distributed between three PA projections that cover the upper GI tract (upper, middle, and lower esophagus) with allocations of 15%, 15%, and 40%, respectively. Adding the middle and lower GI PA projections would have increased the thyroid dose by approximately 2%. This finding is expected given that the thyroid gland is not directly irradiated in these additional views (i.e., middle and lower GI), so that this organ would be exposed only to scatter radiation.
Although the values of f thyroid were higher in infants and very small children compared to adults, this is unlikely to result in higher thyroid doses in clinical MBSSs. Fluoroscopy is generally performed using AECs, which maintain a constant radiation intensity (K air) at the image receptor. Because children attenuate much less radiation than adults, the entrance air kerma will be lower in children than in adults. At typical beam qualities used in diagnostic radiological imaging, 3 cm of soft tissue will attenuate approximately half the incident radiation. If we assume that a child is 6 cm thinner than an adult, only 25% of a beam of radiation at the same quality would be required to achieve the same radiation intensity at the image receptor. Accordingly, it is very likely that the reductions in entrance air kerma in small children undergoing MBSSs would be much greater than the increases in f thyroid obtained in this study. Assuming comparable imaging times and examinations performed at the same x-ray beam quality, infant and pediatric thyroid doses should be lower than those in adults.
The observed trends in adult f thyroid depicted in Table 4 are expected given that the thyroid is totally visible in all the BMI categories studied. Larger patients will inevitably further attenuate the x-ray beam thereby reducing the values of f thyroid. The highest value of f thyroid was 0.65 in adults with the lowest BMI category (15 kg m−2), and the lowest value of f thyroid was 0.38 in severely obese adults (BMI 42 kg m−2). The ratio of these high-to-low values (i.e., 1.7) is relatively large in the lateral projection and even larger in the upper GI PA projection (i.e., 3.3) and suggest that BMI should be taken into account when estimating thyroid doses in adults.
MBSS examinations are invariably performed on fluoroscopy systems that employ AEC systems where the radiation intensities at the image receptor are kept constant. For this reason, using higher x-ray beam qualities will most likely reduce thyroid doses, even though f thyroid values increase as depicted by the data in Table 3. Increasing beam quality results in a more penetrating x-ray beam, thereby requiring less incident radiation to achieve a given x-ray air kerma at the image receptor (Sheeting 2004 ; Hamer et al. 2005). Reductions in radiation incident on the patient at higher beam qualities are greater than the corresponding increases in f thyroid values.
For any given patient who undergoes a specified examination, the thyroid dose may be readily obtained by multiplication of f thyroid values provided in this paper with the corresponding entrance air kerma incident on the patient in the lateral projection. One way of obtaining the entrance air kerma is by dividing the KAP (Gy cm2) by the estimated area of the x-ray beam incident on the patient. KAP is generally explicitly provided on most modern imaging systems and is also available in the digital imaging and communications in medicine (DICOM) header information that is now standard in radiological imaging. Tube voltage information is also readily available in any radiographic image DICOM header. X-ray tube filtration is generally available in reports issued by medical physicists who are required by regulations to test each radiographic unit that is used clinically.
One important reason for computing thyroid doses from MBSS is to investigate whether increased beam qualities could be used to perform MBSS examinations without affecting diagnostic performance. Optimization of radiation protection (i.e., ALARA) would study how changes in x-ray beam quality, as well as x-ray beam quantity, impact diagnostic performance. Currently, clinicians are reducing fluoroscopy beam pulse rate to decrease radiation exposure from MBSSs. However, this also reduces the temporal resolution of the MBSS and greatly diminishes diagnostic accuracy (Bonilha et al. 2013). Modifications to beam quality, which would preserve the temporal resolution, may be a better approach to optimizing radiation protection. Whenever radiographic techniques are adjusted, it is essential that diagnostic performance is maintained so that patient outcomes are not adversely affected. When diagnostic performance is kept constant, the set of radiographic techniques that provide the lowest patient doses would be deemed to be optimal. The thyroid dose conversion factors generated in this study may be taken to be the first step in any such optimization program.
The thyroid gland is the only radiosensitive organ susceptible to carcinogenesis that is fully exposed to radiation in the lateral projection and the upper GI PA projection of the MBSS. We found that patient size has an impact on f thyroid, with the largest patients having a value of f thyroid that is 40% less when compared to that of a normal-sized patient. Infants and small children have a higher f thyroid. Clinicians can use the conversion factors provided in this manuscript, along with readily available information from fluoroscopy units, to determine patient thyroid dose.
This work was supported by the National Institute of Diabetes and Digestive and Kidney Diseases at the National Institute of Health (Grant R01 DK098222). We thank Katlyn McGrattan, CCC-SLP, for her advice on procedures for pediatric modified barium swallow studies.
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Keywords:© 2018 by the Health Physics Society
dose, organ; fluoroscopy; gastrointestinal tract; thyroid