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3(a)(b)Upright / supine dose ratio

Source region= Urinary bladder content

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Liver

Heart

Lungs Effectivedose90(d)Numbers of target organs                     126BaCs178
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70
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34

Upright / supine organ dose ratio

In order to estimate the health effects in humans due toradiation exposures, organ dose needs to be calculated using
human models which reproduce body sizes and shapes andelemental compositions of organs. In recent years, voxel
phantoms (phantoms), which consist of small rectangular
block units called ‘voxel’, have been developed on the basisof medical images of actual persons of various races, ages
and body sizes, and are used for dose assessment. The
reference phantoms adopted by the International Commission
on Radiological Protection (ICRP) have been constructed on
the basis of Caucasian anatomical data. The ICRP reference
phantoms will be used to calculate organ doses for thepurposes of radiation protection.
During radiation work, the posture is usually upright.
However, the posture of previously developed phantoms issupine only. This is because the medical images are usuallyobtained from supine subjects. Therefore, it was impossible
to clarify the variation in organ doses caused by changes inposture. We developed a Japanese adult male voxel phantom
with upright posture on the basis of CT images of uprightsubjects for the first time in the world (Fig.7-16). The CT
images in upright were again obtained from a subject for thephantom in supine previously developed by JAEA.

Therefore, it is possible using these two phantoms tocompare directly organ doses and to analyze the effects ofposture on dose assessment. The voxel size of thesephantoms is about 1mm3, which enables us to accurately
represent e.g. the thyroid with its complicated shape.
Fig.7-17 shows examples of organ doses calculated using
upright and supine phantoms. The upright/supine ratio ofdoses in the kidneys and liver were about 270% and 170%,
respectively, larger than those of heart and lungs. This result
was due to the differences in the movement distances oforgans when changing posture; the movement distances ofkidneys and liver due to the changes of posture were longer
than those of heart and lungs. The organ doses were changedby the changes in posture, but differences in the organ doses
are within a factor of 2 in most cases (Fig.7-18). These
results indicate that the impact of posture on the organ doses
is not significant. Therefore, it can be concluded that there is
no problem in using ICRP reference phantoms which havebeen developed from tomographic images obtained in thesupine posture for the calculation of dosimetric quantities inradiation protection. The knowledge obtained from this study
was reflected in the ICRP Publication 110 dealing with adultreference computational phantoms.


Sato, K. et al., Analysis of Effects of Posture on Organ Doses by Internal Photon Emitters Using Voxel Phantoms, Physics in Medicine and Biology, vol.53,
2008, p.4555-4572.

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