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Comparative Study
. 2012 Jun;39(6):3404-23.
doi: 10.1118/1.4718710.

Organ doses, effective doses, and risk indices in adult CT: comparison of four types of reference phantoms across different examination protocols

Yakun Zhang  1 , Xiang Li, W Paul Segars, Ehsan Samei
Affiliations
Comparative Study

Organ doses, effective doses, and risk indices in adult CT: comparison of four types of reference phantoms across different examination protocols

Yakun Zhang et al. Med Phys. 2012 Jun.

Abstract

Purpose: Radiation exposure from computed tomography (CT) to the public has increased the concern among radiation protection professionals. Being able to accurately assess the radiation dose patients receive during CT procedures is a crucial step in the management of CT dose. Currently, various computational anthropomorphic phantoms are used to assess radiation dose by different research groups. It is desirable to better understand how the dose results are affected by different choices of phantoms. In this study, the authors assessed the uncertainties in CT dose and risk estimation associated with different types of computational phantoms for a selected group of representative CT protocols.

Methods: Routinely used CT examinations were categorized into ten body and three neurological examination categories. Organ doses, effective doses, risk indices, and conversion coefficients to effective dose and risk index (k and q factors, respectively) were estimated for these examinations for a clinical CT system (LightSpeed VCT, GE Healthcare). Four methods were used, each employing a different type of reference phantoms. The first and second methods employed a Monte Carlo program previously developed and validated in our laboratory. In the first method, the reference male and female extended cardiac-torso (XCAT) phantoms were used, which were initially created from the Visible Human data and later adjusted to match organ masses defined in ICRP publication 89. In the second method, the reference male and female phantoms described in ICRP publication 110 were used, which were initially developed from tomographic data of two patients and later modified to match ICRP 89 organ masses. The third method employed a commercial dosimetry spreadsheet (ImPACT group, London, England) with its own hermaphrodite stylized phantom. In the fourth method, another widely used dosimetry spreadsheet (CT-Expo, Medizinische Hochschule, Hannover, Germany) was employed together with its associated male and female stylized phantoms.

Results: For fully irradiated organs, average coefficients of variation (COV) ranged from 0.07 to 0.22 across the four male phantoms and from 0.06 to 0.18 across the four female phantoms; for partially irradiated organs, average COV ranged from 0.13 to 0.30 across the four male phantoms and from 0.15 to 0.30 across the four female phantoms. Doses to the testes, breasts, and esophagus showed large variations between phantoms. COV for gender-averaged effective dose and k factor ranged from 0.03 to 0.23 and from 0.06 to 0.30, respectively. COV for male risk index and q factor ranged from 0.06 to 0.30 and from 0.05 to 0.36, respectively; COV for female risk index and q factor ranged from 0.06 to 0.49 and from 0.07 to 0.54, respectively.

Conclusions: Despite closely matched organ mass, total body weight, and height, large differences in organ dose exist due to variation in organ location, spatial distribution, and dose approximation method. Dose differences for fully irradiated radiosensitive organs were much smaller than those for partially irradiated organs. Weighted dosimetry quantities including effective dose, male risk indices, k factors, and male q factors agreed well across phantoms. The female risk indices and q factors varied considerably across phantoms.

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Figures

Figure 1
Figure 1
Flow chart of the methodology used in this study. RI: risk index; ED: effective dose.
Figure 2
Figure 2
Lateral and frontal views of the four types of phantoms used in this study. (a) and (b) XCAT reference male and female hybrid phantoms after voxelization. (c) and (d) ICRP 110 reference male and female voxelized phantoms. The organs were relabeled to be consistent with the XCAT phantoms. (e) ImPACT stylized/mathematical hermaphrodite phantom. (f) and (g) CT-Expo male and female stylized/mathematical phantoms.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 3
Figure 3
Comparison between different types of phantoms in terms of organ doses and effective doses (ED) in 13 examination categories: (a) chest–abdomen–pelvis; (b) chest; (c) abdomen–pelvis; (d) abdomen; (e) pelvis; (f) adrenals; (g) liver; (h) kidneys; (i) liver-to-kidneys; (j) kidneys-to-bladder; (k) head; (l) neck; (m) head-and-neck. Note that doses for ImPACT are the same for male and female except for testes and ovaries. Doses are normalized to 100 mAs. The quantitative differences were tabulated in Tables 4, TABLE V.. The coefficient of variation for ED is displayed above the bars for each examination.
Figure 4
Figure 4
Comparison of risk index (RI) over four types of phantoms for the 13 examination categories for male (a) and female (b) phantoms. The coefficient of variation for each examination is displayed above the bars.
Figure 5
Figure 5
Comparison of k factor among four types of phantoms for 13 examination categories. The k factor for XCAT, ICRP 110, and CT-Expo phantoms used gender-averaged effective dose and DLP. The tabulated k values for six protocol classes from the AAPM were also included for comparison in this graph. The coefficient of variation for each examination is displayed above the bars.
Figure 6
Figure 6
Comparison of q factor over four types of phantoms for the 13 examination categories for male (a) and female (b) phantoms. The coefficient of variation for each examination is displayed above the bars.
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