Radiation dose quantities in PCXMC

All organ doses calculated with PCXMC are given in proportion to the incident air kerma (Ka,i) which is measured free-in-air, without backscatter, at the point where the central axis of the x-ray beam enters the patient. The incident air kerma – or, alternatively, the exposure (X; in mR), the air kerma-area product (PKA, KAP or DAP; in mGy.cm2) or the exposure-area product (in R.cm2) – must be supplied by the user of the program. This datum can be calculated from the technique factors [x-ray tube voltage (kV), tube current-time product (mAs), total filtration and focal spot-to-skin distance (FSD)] and measured data of the radiation output of the x-ray source, or it can be obtained from entrance surface air kerma measurements or DAP measurements of actual patient examinations. If no actual radiation measurements are available, one can use the ability of PCXMC to estimate Ka,i with a reasonable accuracy from the x-ray tube current-time product (mAs); the other necessary parameters [the x-ray tube voltage (kV), the total filtration in the radiation beam and the distance from the x-ray tube focal spot to the patient’s skin (FSD)] must anyway be input by the user.

If the entrance surface air kerma (Ka,e) or the entrance surface dose (ESD) at the centre of the x-ray beam has been measured on the patient's skin and includes radiation back-scattered from the patient, the dose must be divided by the back-scatter factor (BSF) before using it in PCXMC. The BSF depends on the x-ray spectrum and beam size and is typically of the order of 1.3. The practical range of BSF is 1.1–1.6. For data on the BSF, see, for example, Grosswendt (1990), Petoussi-Henss et al. (1998) or ICRU (2005); some typical BSF values (Petoussi-Henss et al. 1998) are shown in Table 1.

Table1.
Backscatter factors (BSF) for three field sizes and some x-ray spectra  typical in diagnostic radiology. (FSD 100 cm, ICRU phantom  tissue.) (Petoussi-Henss et al 1998).

X-ray tube voltage
(kV)
Filtration HVL
(mm Al) 
BSF
      10x10 cm2 20x20 cm2 25x25 cm2
50 2.5 mm Al 1.74 1.25 1.27 1.28
60 2.5 mm Al 2.08 1.28 1.32 1.32
70 2.5 mm Al 2.41 1.31 1.36 1.36
70 3.0 mm A+0.1 mm Cu 3.96 1.39 1.47 1.47
80 2.5 mm Al 2.78 1.33 1.39 1.39
80 3.0 mm A+0.1 mm Cu 4.55 1.40 1.50 1.51
90 2.5 mm Al 3.17 1.34 1.41 1.42
90 3.0 mm A+0.1 mm Cu 5.12 1.41 1.51 1.53
100 2.5 mm Al 3.24 1.34 1.41 1.42
100 3.0 mm A+0.1 mm Cu 5.65 1.42 1.53 1.55
110 2.5 mm Al 3.59 1.35 1.43 1.44
120 3.0 mm A+0.1 mm Cu 6.62 1.42 1.54 1.56
130 2.5 mm Al 4.32 1.36 1.45 1.47
150 2.5 mm Al 4.79 1.36 1.46 1.48
150 3.0 mm A+0.1 mm Cu 8.50 1.41 1.54 1.57
 

If the patient’s entrance dose has been measured in terms of tissue dose instead of air kerma, the measured datum must be converted to air kerma before using it as an input to the program. Strictly, the conversion from tissue dose, Dtissue, to air kerma, Ka, depends on the composition of the tissue and the energy spectrum of radiation. For the energies of diagnostic radiology, the approximate relationship   Ka = 0.94.Dsoft tissue can be used. In the photon energy range considered in PCXMC (photon energies less than 150 keV), kerma in tissue and absorbed dose in tissue can be considered equivalent (except in bone-soft tissue interfaces, see the chapter on the Monte Carlo method below).

PCXMC calculates the mean values of absorbed doses, averaged over the organ volume, for the organs shown in Table 2. In addition to these organ doses, the program calculates the effective dose for both the present tissue weighting factors wT (ICRP 2007) and the old ones (ICRP 1991). PCXMC also calculates the average whole-body dose, and the fraction of the x-ray beam energy that is absorbed in the phantom. In PCXMC, all absorbed doses (and air kerma) are in milligray (mGy). For photons, the numerical values of the equivalent doses of organs in millisieverts (mSv) are equal to the corresponding organ doses in mGy. The unit of the effective dose is mSv.

In PCXMC the calculation of effective dose is not strictly done according to the specifications in ICRP Publication 103 (2007): in PCXMC, the effective dose is calculated using size-adjustable hermaphrodite phantoms, whereas the present ICRP (2007) definition specifies that the organ doses are calculated in a reference male phantom and in a reference female phantom, the equivalent organ doses in these two phantoms are averaged, and the effective dose is obtained as a weighted sum of these sex-averaged organ doses. This prescription cannot be easily followed in partial body exposures, such as x-ray imaging, where the field size and the quality and amount of radiation are adjusted according to the patient’s size: an x-ray beam of specified size cannot be unambiguously directed similarly to two phantoms of different size and shape. This difficulty is avoided by using hermaphrodite phantoms.  

The effective dose has been introduced to express a radiation detriment-related dose for radiation protection purposes in situations where the dose to the body is not uniform and the absorbed doses are low enough for avoiding deterministic radiation effects. The effective dose is given as a weighted average of the equivalent doses in various organs and tissues. In deriving the tissue weighting factors, an equal number of males and females and a wide range of ages in the exposed population are assumed. Therefore, and because of the health status difference of patients and general population, it may not always be reasonable to use this quantity in reporting doses from medical radiology; for critical views on the use of the effective dose in diagnostic radiology see Drexler et al. (1993) and Martin (2007). If the patient material considered differs greatly from the average population, the use of a different set of weighting factors and risk coefficients should be more appropriate (BEIR 1990, Stokell et al. 1993, Almén and Mattsson 1996). Such age- and sex-dependent weighting factors have not been agreed on; therefore, for example the ICRP uses the same set of tissue weighting factors for all ages and even for the developing foetus, although with caution (ICRP 2007).

The ICRP specifically stresses that effective dose should not be used for, e.g., the assessment of individual risk, assessment of the probability of causation of cancer, or for epidemiological studies. Absorbed doses to irradiated tissues should be used for these purposes. However, the ICRP acknowledges that the effective dose can be of value for comparing doses from different diagnostic procedures and for comparing the use of similar technologies and procedures in different hospitals and countries as well as the use of different technologies for the same medical examination (ICRP 2007). Effective dose has widely been used for such purposes: for example in assessing the population dose from diagnostic x-ray examinations (e.g., UNSCEAR 2000, Hart and Wall 2002 and Scanff et al. 2008).

However, it should be kept in mind that in partial body exposures local absorbed doses may be large even if the mean doses in organs or the effective dose are small. Therefore, low organ doses or a low effective dose do not necessarily imply avoiding deterministic radiation effects (tissue reactions) (ICRP 2007).

PCXMC calculates the effective dose for allowing easy risk comparisons e.g. between different diagnostic procedures. If a more detailed risk assessment is needed, the risk calculation capability of PCXMC should be used. This risk model is based on the report of BEIR VII committee (BEIR 2006), and takes into account, e.g.,  the sex, age at exposure and attained age of the patient (see section 6 for more details). Still, it has to be remembered that individual risk estimates are highly uncertain because of inherent uncertainties in the risk models, the health status of the individual in question and, e.g. individual sensitivity to radiation-induced cancer. 

Table 2:
The organs considered in PCXMC, and their tissue weighting factors for the calculation of the effective dose according to both the present and the old ICRP definitions.

Organ or tissue

Tissue weighting factor
WT (ICRP 103) 7)

Tissue weighting factor
WT (ICRP 60) 8)

Active bone marrow

0.12

0,12

Breasts

0.12

0,05

Colon1)

0.12

0,12

Lungs

0.12

0.12

Stomach

0.12

0.12

Ovaries (female gonads) 2)

0.08/2

0.20/2

Testicles (male gonads) 2)

0.08/2

0.20/2

Liver

0.04

0,05

Oesophagus

0.04

0.05

Thyroid

0.04

0,05

Urinary bladder

0.04

0.05

Brain

0.01

r

Bone surface3)

0.01

0.01

Salivary glands

0.01

-

Skin

0.01

0.01

Adrenals

0.12/13

r

Extrathoracic airways4)

0.12/13

r

Heart

0.12/13

-

Kidneys

0.12/13

r

Lymphatic nodes5)

0.12/13

-

Muscle6)

0.12/13

r

Oral mucosa

0.12/13

-

Pancreas

0.12/13

r

Prostate (male)

0.12/26

-

Small intestine

0.12/13

r

Spleen

0.12/13

r

Thymus

0.12/13

r

Uterus (female)

0.12/26

r

 

1) The dose in the colon is calculated as the mass-weighted average of the upper large intestine and the lower large intestine.

2) The dose in the gonads is defined as the average of the doses in the ovaries and testicles. The tissue weighting factor for gonads is presently 0.08 (ICRP Publication 103) and was earlier 0.20 (ICRP Publication 60).

3) The tissue weighting factor refers to the dose to bone surface. PCXMC approximates this dose using the dose to the whole skeleton (excluding active bone marrow).

4) In PCXMC only the trachea, pharynx and nasal sinuses are used to represent the extrathoracic airways.

5) In PCXMC the lymph nodes have not been modelled in the phantom. The dose in lymph nodes is calculated as a weighted average of several surrogate organs (see the chapter for Mathematical phantoms below).

6) In PCXMC, the dose in muscle tissue is calculated as the average dose to the whole phantom, but excluding the other organs and tissues given in this table.

7) The weighting factors that are shown as the fraction 0.12/13 or 0.12/26 represent the remainder organs of ICRP 103. The new weighting factor for the arithmetic average of the remainder organs is given the tissue weighting factor 0.12. Sex-specific organs have effectively a lower weighting factor than the other remainder organs. 

8) Weighting factors labelled as “-” denote organs that are not included in the calculation of the effective dose according to the old ICRP 60 definition. Weighting factors labelled as “r” belong to the 'remainder tissues' of ICRP Publication 60. The tissue weighting factor of the ICRP 60 remainder is 0.05, and is applied to the mass averaged dose in the remainder organs and tissues. However, if any of these organs receives a dose that is higher than the dose to any of the twelve organs for which a weighting factor is specified, a weighting factor of 0.025 is applied to that tissue or organ and the rest of the weighting factor, 0.025, is applied to the mass averaged dose in the other remainder organs and tissues (ICRP 1991 and 1995).

Incident air kerma calculation based on the tube current-time product (mAs)

PCXMC can evaluate the incident air kerma from the specified examination factors, when also the tube current-time product (mAs) is known. All other data needed for the evaluation: x-ray tube voltage (kV), total filtration and FSD, must be specified in the examination's input data anyway. This option cannot be used, if the user has specified the FSD to be infinite (actually 100 m).

In practice, the x-ray tube output varies from one unit to the next, however, and one cannot expect an exact agreement between the calculated and measured incident air kerma. Variability between x-ray tubes is caused at least by differences in the following factors:

  • x-ray tube voltage waveform (PCXMC assumes a constant potential or low-ripple generator),

  • x-ray tube anode angle (not used in the incident air kerma calculation in PCXMC),

  • smoothness of the x-ray tube anode surface,

  • actual filter materials in the beam path; [in spite of matching filtration equivalence, the attenuation of the actual filter (including glass and oil) may differ from the attenuation of an aluminium filter],

  • differences in off-focal radiation and its removal by collimation,

  • the error between actual and displayed values of the tube voltage (kV), the tube current-time product (mAs) and the filtration.

The x-ray tube output calculation of PCXMC is based on x-ray tube output measurements from diagnostic x-ray tubes. The basic data have been obtained from 46 different x-ray tubes and/or filter choices. The average of these measurements agrees with the value calculated with PCXMC, and the standard deviation between the individual measured results and the calculated results is 16 %. Therefore, one can expect the calculated output dose to be within about 30 % (2 standard errors) of the correct value.

The ratio of calculated and measured results is typically constant for a given x-ray tube; the ratio stays the same although x-ray tube voltage or filtration is varied. This can be used to improve the accuracy of mAs-based dose calculations. By making x-ray tube output measurements for a given x-ray system and comparing these with the output values calculated by PCXMC one will get an effective 'mAs calibration coefficient' for that x-ray tube, and it can be used at all x-ray tube voltages and filter choices. By doing such a normalisation for each of the 46 x-ray tubes above, the standard deviation between individual measured and calculated results dropped to 5 %, and the accuracy of the calculation can then be taken as 10 %.

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