RELATIONSHIP BETWEEN PERSONALDOSEMETERS READINGS AND HUMAN DOSES*

Eri Hiswara**

ABSTRACTRELA TlONSHIP BETWEEN PERSONAL DOSEMETERS READINGS AND

HUMAN DOSES. The relationship between personal dosemeter reading and the effectivemale dose (equivalent), the effect of skin-dosemeter distance and the effect of angularresponse of dosemeters, were studied using a computational model developed in JAERIand a realistic body phantom. The relationship was calculated for five locations of dose­meters worn at the skin surface of the trunk. The calculation showed that the dosemeters

at the left breast pocket gave the best estimation of the effective male dose (equivalent),although from the measurement it was shown that the same location had the worst angularhorizontal response. The study on the effect of skin-dosemeter distance indicated acontradictory result which requires further investigation, whereas the angular verticalresponse showed less than 10% response variation even for 900 rotation of the dosemeter.

ABSTRAK

HUBUNGAN ANTARA BACAAN DOSIMETER PERORANGAN DAN DOSIS

MANUSIA. Hubungan antara bacaan dosimeter perorangan dan dosis (ekivzlen) efektifpria. efek jarak kulit-dosimeter dan cfek tanggapan sudut dosimetcr teiah dipelajari denganmenggunakan model komputasi yang dikembangkan JAERI dan fantom tubuh realistik.Hubungan dihitung untuk lima lokasi dosimeter yang dipasang pada permllkaan dada.Perhitllngan menllnjukkan bahwa dosimeter yang dipasang pada saku kiri memberikanpcrkiraan terbaik dalam dosis (ekivalen) efektif, meskipun pengukuran pada lokasi yangsarna memiliki tanggapan sudut horisontal yang terburuk. Studi ten tang efek jarak kulit­dosimeter menunjukkan hasil yang berlawanan yang memerlukan penyelidikan lebih lanjut,semcntara tanggapan sudut vertikal menunjukkan variasi tanggapan yang kurang dari 10%bahkan untuk rotasi dosimeter sejauh 90°.

INTRODUCTION

For the purpose of a quantitative estimation of risk, the InternationalCommission on Radiological Protection (ICRP) has introduced the effectivedose quantity [J]. This quantity is defined as the sum of weighted equivalentdoses in various organs and/or tissues, and is given by

E (1)

where HT is the equivalent dose in the tissue T and WT is the weightingfactor for each respective tissue. Since the effective dose cannot bemeasured directly, the International Commission on Radiation Units and

*

**

This work was carried out at the Japan Atomic Energy Research Institute(JAERI), Tokai Research Establishement, Japan.Center for Standardization and Radiation Safety Research, SATAN

9

Measurements (ICRU) has introduced the operational quantities for areamonitoring - H*(lO), H'(0.07,a) or H'(3,a), and for individual monitoring- Hp(lO), Hp(0.07) or Hp(3) [2]. Notation a in the case of area monitoringis referred to as the direction of the incident radiation field.

For the individual monitoring quantities, the ICRU has given extensiveconversion coefficients relating the personal dose equivalent to air kerma,exposure and fluence [3]. These coefficients, however, are applied onlyfor personal dosemeters facing a uniform radiation field, whereas the actualexposures are mostly not aligned and expanded as required. This resultsin a problem of how to relate the personal dosemeter reading to individualmonitoring quantity.

AUSTERLITZ et.al [4] have previously studied the relationship betweenthe effective male dose equivalent and readings of personal dosemetersworn at nine locations on a computational model. In their work, conversioncoefficients for a wide range of photon energies were calculated applyingthe modified GSF-Adam computer code [5].

In the case of diagnostic radiology fluoroscopy, FAULKNER andMARSHALL [6] have measured the ratio of effective dose to film badgedosemeter reading. The use of a lead apron in diagnostic radiology causessome difficulties in interpreting the effective dose from a single personaldosemeter, so that their study suggested that in the future the use of twopersonal dosemeters, instead of one, is preferred.

The objective of this study is to investigate the relationship betweenpersonal dosemeter reading and the effective male dose (equivalent) fordosemeters worn at five locations on the skin surface of the trunk. The

effect of skin-dosemeter distance and the angular response of dosemeterswere also investigated. Comparisons were made between experiment andcomputer simulation.

MATERIALS AND METHOD

A realistic body phantom and a computational model were used in thiswork. A brief description of the realistic body phantom is presented, whilea complete description of the computational model has been reportedelsewhere [7].

The realistic body phantomThe realistic body phantom used in this study is for an adult Japanese.

The assembly consists of an artificial skeleton embedded in a flexibleplastic-based tissue substitute. Simulated internal organs (heart, lung,stomach, intestine, testis and breast) are included. The overall height ofthe phantom is 168 cm with a mass of 63 kg. In this study, however,

10

the breast was not attached to the trunk, since the object was for a male.

Handling and use of dosemetersPhosphate glass dosemeters of GO-403 type were used throughout this

study. These dosemeters have shown an excellent longterm stability witha measurement accuracy in the order of 2% for ambient temperatures upto 40°C during irradiation [8].

The GO-403 type glass dosemeter consists of an RPL glasscard with four fluorescence detection positions, a capsuleof 25 mm x 60 mm x 9 mm to accommodate the glass card, and acapsule holder (see Figure I). The holder, however, was not used duringthe experiment.

All dosemeters received a pre-experiment calibration with a 137Csgamma source to determine the calibration factor for each dosemeter. Thedosemeters were mounted on the surface of a slab phantom of 40 cm x40 cm x 15 cm and exposed with a dose of 6 mSv in term of H* (10).Before reading was taken, all dosemeters were pre-annealed for 30 minuteat 100°C for buildup and kept in room temperature for 2 hours. The readout were carried out by an FGO 502 reader. Post-annealing for 12 hoursat 400°C was also performed to eliminate the residual dose informationbefore the dosemeters were exposed. The next irradiation was carried outafter the dosemeters were cooled down in room temperature for at least12 hours.

In this study, the effect of dosemeter location, the effect of variationof skin-dosemeter distance and the angular dependence of dosemeter wereinvestigated. Table I lists the coordinates for the positions of the dosemetersat five locations in the realistic body phantom. The origin of the coordinatesystem was taken the same in this body phantom as in the computationalmodel, i.e., at the center of the base of the trunk. The x-axis was directedto the left from the center of the phantom, the z-axis was directed up tothe head and the y-axis was to the back (see Figure 2).

Table 1 also lists the coordinates for the positions of the dose metersfor computational model. The differences of the y-axis for the compu­tational model compared to the body phantom were due to the thicknessof the dosemeter which were taken into account in the model.

Irradiation conditions

A 137Cs gamma source of 740 GBq (20 Ci, August 1979) wasused throughout the experiment. The source-skin distance was set to be243.8 cm to get a dose equivalent of 6 mSv in the skin. To study theeffect of horizontal angular dependence of dosemeter, the phantom wasrotated clockwise in increments of 45°, whereas the effect of vertical

11

angular dependence was investigated by rotating the dosemeter against thephantom in increments of 30°.

An isotropic 137Cs parallel beam source was assumed for simulating

phantom irradiation in the computational model. Calculations were

performed for whole-body irradiation and anterior-posterior (AP) geometry

Personal dosemeter reading and the effective male dosePersonal dosemeter reading is the dose equivalent measured at a certain

position of the dosemeter, or the dose equivalent for soft tissue in the case

of calculation. The quantity used in the dose equivalent in sievert (Sv),

and denoted by R.

The mathematical model available has three types of model : male,

female and unisex. In this study only the male model was used so that

the quantity calculated is called effective male dose equivalent denoted by

HEm [10], or effective male dose (Em) [I]. In this context, the tissue

weighting factors used slightly differ from those recommended by the ICRP

(see Table 2). The relationship between R and HEm or Em is given by:

~ orf = HEm

R

f = Em(2)

where f is the rati,) of personal dosemeter reading to the effective male

dose equivalent or to the effective male dose.

Error propagationThe errors associated with the measurement and calculation were derived

from the coefficient of variation. The propagation of error was treated by

the method described by KNOLL [9]. For the measurements using the

phosphate glass dosemeter, the uncertainty in personal dosemeter reading

was in the range of 0.2 - 1.3%, the highest coefficient of variation value

was detected for dosemeter positioned at 9 mm skin-dosemeter distance.

The uncertainty in the calculation generally was within ± 1.5%.

RESUL TS AND DISCUSSION

The personal dosemeter response to the effective dose equivalent(R/HEm) and to effective dose (RlEm) is given in Figure 3. It shows the

skin-dosemeter distance dependence of the response at five locations whenthe model was irradiated at 234.8 em of source-to-skin distance with AP

geometry.From Figure 3 it can be seen that R/Em is always higher than RlHEm.

As mentioned by ZANKL et.al [11], the differences were mostly due to

12

the modified weighting factor of the remainder tissue. The weighting factor

for remainder tissue in the latest ICRP recommendation [1] was reduced

by a factor of 6 compared to the previous figure [10] (see Table 2), as

well as its dose contribution is higher to HE than to E since it is averaged

over the maximum five organs doses in case of HE and over ten fixed

organs in case of E.

Figure 3 also shows that the value recorded at the left breast pocket

gives the best estimate of both Hm and Em. This can be compared with

the results obtained by AUSTERLITZ et.al [4] who found those on the

pocket and wrist locations result in the best approximation of the HEm.

In all cases, the ratio of RlEm differs by a factor of 1.03 compared

with RlHEm, whereas the personal dosemeter reading was reduced by afactor of 0.07 from the skin to the SO mm skin-dosemeter distance. The

reduction is thought to be due to the backscatter from the body whichbecomes less as the dosemeter moves away from the skin.

Figure 4 shows the personal dosemeter reading as a function of

skin-dosemeter distance. While the calculated result, as it might be

expected, showed a decrease of the relative dose as the distance increases,

the result of measurement showed a completely different pattern. Theincrease in the relative dose with the skin-dosemeter distance in case of

measurement seems to be caused by the fact that the phantom did not give

much scattering, and the law of inverse square concerning with the dosewas dominantly applied. The examination of the validity of the law reveals

that all the measured results differ only by 0.1 - 2.2 % from the calculation

applying the inverse square law.Figure S shows the angular horizontal response of personal dosemeter.

It is worth to mention that the objective of this study of angular response

was not to evaluate the response of Toshiba GD-403 phosphate glassdosemeters used, but to evaluate the response of personal dosemeters worn

at the realistic body phantom. The back location in Figure S is almostin the same axis as the middle of the chest, except that it was located

at the back of the body (x = 0, z = 37, y = -14).As it is clearly seen from Figure S, the overall pattern of angular

horizontal response for the back location was the same as for left breast

pocket, middle chest and lower chest locations. The response at 180° forleft breast pocket was found to be higher than that for middle chest and

lower chest, and this is thought to be due to smaller linear attenuation

coefficient of the lungs compared with soft tissue and bone.

The phantom used in the experiment was held by a four-leg stand. It

is interesting to note that the responses at 4So and 90° are always higherthan those at 3ISo and 270°, respectively, and the response at 4So itself

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is also higher than that at 90°. This is suspected to be due to the frontleg of the stand on the right side of the phantom when it was irradiatedfrom the front, which contributed more scatters than that on the left side.

The above result was also observed with the dosemeter worn at left

breast pocket, where the response at 90° was much higher than that at270°. The left arm is also thought to give and additional scatter.

For dosemeters worn at the back, similar results were observed.However, in this case, the responses at 315° and 270° were higher thanthose at 45° and 90° respectively, and the response at 270° was higherthan that at 315°.

By assuming that the responses at 315° and 270°, or responses at 45°and 90° in the case of back location, were in a scatter-free condition, itcan be concluded that the responses of personal dosemeters are at bestwhen worn at the middle chest and at worst when worn at left breast pocket.

The angular vertical response of personal dosemeter given in Figure6 simulates a condition where the bottom part of a dosemeter is movingaway from the body. This condition occurs frequently when the dosemetersare worn at a loose pocket. At the worst condition of 90°, the responsewas only decreased by less than 10% relative to 0°. Therefore, the angularvertical movement of the dosemeter has little effect to the response.

CONCLUSIONS

The relationship between personal dosemeter reading, R, and theeffective male dose (equivalent), Em (HEm), was studied by means of amathematical model developed in JAERI. In addition, two characteristicsof personal dosemeter worn at a realistic body phantom, i.e, the effect ofskin-dosemeter distance and the effect angular response to the dosemeter,were also investigated.

The calculation on the relationship suggests that the left breast pocketlocation give the best estimation of the effective male dose (equivalent).However, at the same time, actual measurement revealed that the same

location had the worst angular horizontal response. Further study is thenrequired to find the optimum condition where the estimation is closest andthe angular movement of the dosemeter does not affect the response.

The effect of skin-dosemeter distance was studied experimentally andcomputationally by moving the dosemeters away from the skin of thephantom and the model. Since the results indicated a clear contradiction,i.e., experiment showed an incerase of relative dose with distance, whereascalculation showed a decrease, further study is then needed to clarify theresult.

The angular horizontal response of dosemeters studied using a realistic

14

body phantom showed that the response was at the best when a dosemeterwas worn at middle chest, and at the worst when worn at left breast pocket.The angular vertical response, however, showed no substantial variationeven for 900 rotation of the dosemeter.

The present study was only applying a 137Cs gamma source. It mightbe interesting to investigate how the results will vary with the energy ofthe source. Further study applying a wide range of gamma energies isstrongly suggested.

ACKNOWLEDGMENTS

The author wishes his thanks to Mr. Y. Yamaguchi and Mr. F. Takahashiof JAERI for their valuable advice and help. The work was carried outunder Science and Technology Agency (Japan) fellowship.

REFERENCES

1. ICRP, "1990 Recommendations of the International Commission onRadiological Protection". ICRP Publication 60. Pergamon Press, Oxford(1991) 6-9

2. ICRU, "Determination of Dose Equivalents from External RadiationSources - Part 2". ICRU Report 43. ICRU, Bethesda (1988) 4-5

3. ICRU, "Measurement of Dose Equivalents for External Photon andElectron Radiations". ICRU Report 47. ICRU, Bethesda (1992) 22-30

4. AUSTERLITZ, C., KAHN, B., EICHHOLZ, G.G., ZANKL, M. andDREXLER, G., "Calculation of the Effective Male Dose EquivalentRelative to the Personal Dose at Nine Locations with a Free-Arm

Model". Radiat.Prot.Dosim. 36 (1) (1991) 13-21

5. KRAMER, R., ZANKL, M. WILLIAMS, G. and DREXLER, G., "TheCalculation of Dose from External Photon Exposures Using ReferenceHuman Phantoms and Monte Carlo Methods, Part I : The Male (Adam)and Female (Eva) Adult Mathematical Phantoms", GSF- Bericht S-885.Neuherberg, GSF (1984)

6. FAULKNER, K. and MARSHALL, N.W., "The Relationship ofEffective Dose to Personnel and Monitor Reading for SimulatedFluoroscopic Irradiation Conditions", Health Phys. 64 (5) (1993)502-508

7. YAMAGUCHI, Y. DEEP Code to Calculate Dose Equivalents inHuman Phantom for External Photon Exposure by Monte Carlo Method.JAERI-M 90-235. JAERl, Japan (1991)

15

8. PIESCH, E., BURGKHARDT, B. and VILGIS, M., "Progress inPhosphate Glass Dosimetry: Experiences and Routine Monitoring withA Modern Dosimetry System", Radiat. Prot.Dosim. 47 (1/4) (1993) 409­414

9. KNOLL, G., "Radiation Detection and Measurement", 2nd Ed. JohnWiley: New York (1989) 88-92

10. ICRP, "Recommendations of the International Commission onRadiological Protection", lCRP Publication 26. Pergamon Press, Oxford( 1977)

11. ZANKL, M., PETOUSSI, N. and DREXLER, G., "Effective Dose andEffective Dose Equivalent : The Impact of the New ICRP Definitionfor External Photon Irradiation", Health Phys. 62 (5) (1992) 395- 399.

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Table I. Locations of the personal dosemeter

Coordinates

Location

Body phantomaComputational modelb

x

yz xyz

10

490 10490.25

Left breast pocket :

10499 10491.2510

4926 10493.2510

4949 10495.25

0

490 0490.25

Upper chest :

0499 0491.250

4926 0493.250

4949 0495.25

0

370 0370.25Middle chest :

()379 ()37 1.250

3726 ()'73.25.),()

'749 ()375.25.) I

-8

25() -8250.25

Right waist :

-8259 -8251.25-8

2526 -8253.25-8

2549 -8255.25

0

250 0250.25Lower chest :

()259 0251.250

2526 ()253.250

2549 ()255.25

a : y coordinate 0 represents surface, whereas or 9, 26 and 49 representskin-dosemeter distances in mm.

b : y coordinate 0.25 represents surface, whereas of 1.25, 3.25 and 5.25represent skin-dosemeter distances of 10, 30 and 50 111m,respectively.

17

Table 2. Tissue weighting factors used for computational model

Weightingfactors

Tissue/organ (CRP 1977This studyICRP 1990This study

Gonads

0.250.290.200.21

(Testes)

(testes)

Bone marow (red)

0.120.140.120.13

Colon

10.120.13- -

Lungs

0.120.140.120.13

Stomach

-- 0.120.13

Bladder

-- 0.050.05

Breast

0.15- 0.05-Esophagus

-- 0.050.05

Liver

-- 0.050.05

Thyroid

0.030.040.050.05

Skin

-- 0.010.01

Bone surfaces2

0.030.040.010.01

Remainder

0.300.350.050.05

In the model it is called GI LLI.2 In the model it is called skeleton.

18

Binary code ~

Capsule holder

f)-window

'"[-window

Capsule

Figure I. Sturcture of GD-403 phosphate glass dosemeter

_______ \ 1]'~'" -. \

~X

Figure 2. Coordinate system in the computational model

Glass card

19

1.134

1.09-

1.07~iI

•

---- '.~',

(a) R/HEm

(a) R/Em

(b) R/HEm

• (b) R/Em

(c) R/HEm

(c) RlEm

" (d) R/HEm

(d) R/Em

(e) R/HEm

~ (e) R/Em

II

1.05 I ._~, --~-------_.I

o 10 20 30 40 50

Skin-dosemeter distance (mm)

Figure 3. Personal dosemeter reading per effective male doseequivalent, RJHEm, and per effective male dose, R/Emcalculated at (a) left breast pocket, (b) upper chest,(c) middle chest, (d) right waist and (e) lower chest.

20

1.1 ~------_._--~

1.05J

Q)

./

I(a) Calc.

e..>

cu.•...

•

:s

-

(a) Meas

(/)

cI

..

(b) Calc

~

,

(/)

·

(b) Meas.

0

•.... 1Q)

(c) Calc.

(/)

0"0

•

(c) Meas

Q)

.2:

-

(d) Calc

ro

Qj

E

(d) Meas

([

c

(e) Calc

0.95~

•

(e) Meas

I

0.9+---r--~-------, ,

o 10 20 30 40 50

Skin-dosemeter distance (mm).

Figure 4. Personal dosemeter reading relative to reading on skin

surface measured and calculated at (a) len breast pocket,

(b) upper chest, (c) middle chest, (d) right waist and(e) lower chest,

21

Figure 5.

Figure 6.

22

Angular horizontal response relative to 0° measured at

(a) left breast pocket. (b) middle chest. (c) lower chestand (d) back.

Angular vertical response relative to 0° measured at

(a) left breast pocket, and (b) middle chest.