Radiation Protection Dosimetry Advance Access published July 12, 2015 Radiation Protection Dosimetry (2015), pp. 1–9

doi:10.1093/rpd/ncv366

INFLUENCE OF THE PHANTOM SHAPE (SLAB, CYLINDER OR ALDERSON) ON THE PERFORMANCE OF AN Hp(3) EYE DOSEMETER R. Behrens* and O. Hupe Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany *Corresponding author: [email protected]

In the past, the operational quantity Hp(3) was defined for calibration purposes in a slab phantom. Recently, an additional phantom in the form of a cylinder has been suggested for eye lens dosimetry, as a cylinder much better approximates the shape of a human head. Therefore, this work investigates which of the two phantoms, slab or cylinder, is more suitable for calibrations and type tests of eye dosemeters. For that purpose, a typical Hp(3) eye dosemeter was irradiated on a slab, a cylinder and on a human-like Alderson phantom. It turned out that the response on the three phantoms is nearly equal for angles of radiation incidence up to 4588 and deviates only at larger angles of incidence. Thus, calibrations (usually performed at 088 radiation incidence) are practically equivalent on both the slab and the cylinder phantoms. However, type tests (up to 7588 or even 9088 radiation incidence) should be carried out on a cylinder phantom, as also for large angles of incidence the response on the cylinder and the Alderson phantoms is rather similar, whereas the response on the slab significantly deviates from the one on the Alderson phantom.

INTRODUCTION

MEASUREMENTS

Monitoring the eye lens may become more important than it has been in the past in order to make sure that the new annual dose limit of 20 mSv recommended by the International Commission on Radiological Protection (ICRP)(1) is not exceeded. The most appropriate operational dose quantities to monitor the eye lens are the personal and directional dose equivalent at 3 mm depth, Hp(3) and H 0 (3;V), respectively(2, 3), although in well-known radiation fields, other quantities such as Hp(0.07) and H 0 (0.07;V) or Hp(10) and H*(10) can be appropriate as well(4 – 6). In the past, for Hp(3), a slab phantom (30`  30`  15 cm, made of ICRU tissue) was recommended by the International Commission on Radiation Units and Measurements (ICRU) for the calculation of conversion coefficients(2, 3), i.e. for the determination of the conventional quantity value (true value), and a slab of the same size but made of polymethyl methacrylate (PMMA) filled with water was suggested for calibrations(7). However, a short time ago, a cylinder phantom (20 cm in diameter and 20 cm high) was suggested as it much better approximates the shape of a human head—again made of ICRU tissue for the determination of the conventional quantity value, and made of PMMA filled with water for calibrations(8 – 11). In this work, the differences when using the slab or cylinder phantom compared with a human-like Alderson phantom were investigated by performing irradiations of a typical Hp(3) eye dosemeter on all three phantoms in several photon and beta radiation fields.

Dosemeter A commercially available dosemeter designed for the measurement of Hp(3), Eye-DTM (Radcard), was used for the investigations(12). The dosemeter utilises MCPN (LiF:Mg,Cu,P) thermoluminescence detectors (TLDs) with a thickness of 0.9 mm. The TLD’s cycle was as follows: (1) (2) (3) (4)

annealing at 2408C for 10 min, irradiation inside the badge, preheating at 1008C for 10 min, readout with a RISØ TL/OSL-DA-15 reader with planchet heating: 3 K s21 up to 2408C.

The thermoluminescence signal up to the glow curve’s maximum was used for the evaluation of the dose. Unirradiated TLDs served for the background subtraction. Irradiations The dosemeters were irradiated in photon and beta reference radiation fields according to ISO 4037-1(13) and ISO 6980-1(14), respectively. Figure 1 shows the dosemeter’s mounting on the three different phantoms. Irradiations were performed for angles of radiation incidence of a ¼ 0, 45, 60, 75 and 908 (the latter only for photons). As the influence of the different phantoms were to be investigated, for photons the air kerma, Ka, and for betas the absorbed dose in a tissue slab, Dt, were chosen to be the same for all three phantoms. In

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Received 13 March 2015; revised 11 June 2015; accepted 13 June 2015

R. BEHRENS AND O. HUPE

the photon fields, two dosemeters were irradiated simultaneously, in the beta fields only one at a time. The following radiation qualities were used for the irradiations: for photons, N-15, N-20, N-40, N-80, N-150, N-300, S-Cs and S-Co and for betas, 90Sr/90Y. Conventional quantity values (true values) Photon radiation Air kerma, Ka, is the basic quantity in photon dosimetry. Conversion coefficients from Ka to Hp(3) were used for the slab, hpK(3)slab,(16) and the cylinder, hpK(3)cyl,(17) to obtain Hp(3)slab and Hp(3)cyl, respectively, and accordingly the responses Rslab and Rcyl. The indices ‘cyl’ and ‘slab’ denote the conventional quantity value defined in the cylinder and slab, respectively, made of ICRU tissue. The response on the Alderson phantom is assumed to be representative for the dosemeter’s behaviour when worn by a person. Therefore, it was determined using the conversion coefficient hlensK from Ka to equivalent dose to the lens of the eye, Hlens. To obtain hlensK for the photon radiation qualities used in this work, the corresponding fluence spectra were folded with the monoenergetic conversion coefficients—as described earlier for the two other phantoms(16, 17). Mono-energetic conversion coefficients for the equivalent dose in organs (and for the effective dose) are usually obtained by the ICRP and are based on their reference voxel phantoms defined in ICRP 110(18). However, as the voxel size of these phantoms is too large for a precise simulation of the lens of the eye, ICRP adopted for that purpose, in addition, a stylised eye model(19); for details, see Annex F of ICRP 116(20). In ICRP 116, data for hlensK are only available for a ¼ 0 and 908 (among the values of a covered in this work). Therefore, the mono-energetic

values for 45, 60 and 758 for hlensK were taken from Behrens and Dietze(19), which are for a ¼ 0 and 908 equivalent to the ones stated in ICRP 116(20). As mentioned earlier, hlensK was determined in the reference phantoms adopted by the ICRP(18, 19), whereas the corresponding irradiations for this work were performed on the Alderson phantom. In any case, it is appropriate to use the values of hlensK for the irradiations on the Alderson phantom. The reason is as follows: the difference in the amount of radiation scattered back from the reference phantoms and the Alderson phantom is assumed to be negligible, as both the reference phantoms and the Alderson phantom are of course similar in size and material. Beta radiation For beta radiation, the quantity Hp(3) in the slab phantom has recently been implemented in the beta secondary standard BSS 2(21). The correction factor to account for the cylinder instead of the slab, kcyl ¼ Hp(3)cyl/Hp(3)slab, has become available as of late(22). Thus, also for beta radiation, the dosemeter’s response can be determined for both Hp(3)slab and Hp(3)cyl. However, the correction factor to account for the reference phantoms, klens ¼ Hlens/Hp(3)slab, is not available. Therefore, the response on the Alderson phantom cannot be determined for beta radiation. RESULTS Indication on the slab and the cylinder phantoms relative to the indication on the Alderson phantom Photon radiation Figures 2 and 3 show the measured value (indication) on the slab and the cylinder phantoms, respectively,

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Figure 1. Left to right: Dosemeters on an ISO slab phantom (water-filled PMMA, b`  h`  d ¼ 30`  30`  15 cm; front wall 2.5 mm thick, other walls 10 mm thick(15)), cylinder phantom (water-filled PMMA, Ø ¼ 20 cm, h ¼ 20 cm; side wall, top and bottom 5 mm thick(10)) and Alderson phantom (b ¼ 15 cm, h ¼ 24 cm and d ¼ 21 cm).

PHANTOM INFLUENCE ON AN Hp(3) EYE DOSEMETER

Figure 3. Measured value (indication) on the cylinder phantom divided by the measured value on the Alderson phantom, depending on the mean photon energy. The uncertainty bars are the standard deviation of the mean values of about five TLDs.

divided by the measured value on the Alderson phantom, and depending on the mean photon energy. It is obvious that the measured value of both the slab and the cylinder phantoms is, for nearly all photon energies and angles of radiation incidence, larger than the measured values on the Alderson phantom— all irradiated with the same air kerma, such that Hp(3)cyl ¼ 1 mSv resulted. The reason is that both the slab and the cylinder have a larger volume and, even more importantly, a larger cross-sectional area at the dosemeter’s level than the Alderson phantom,

resulting in more backscatter. The maximum ratios are larger for the slab than for the cylinder, as the slab’s cross-sectional area at the dosemeter’s level is about 80 % larger than that of the Alderson phantom, whereas the cylinder’s cross-sectional area is only about 30 % larger than that of the Alderson phantom. As expected, the ratios take their maximum in the energy region around 65 keV photon energy according to the maximum of the Compton cross section in that energy region in materials of rather low atomic number

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Figure 2. Measured value (indication) on the slab phantom divided by the measured value on the Alderson phantom, depending on the mean photon energy. The uncertainty bars are the standard deviation of the mean values of about five TLDs.

R. BEHRENS AND O. HUPE

Beta radiation Figure 6 shows the measured value (indication) on the slab and the cylinder phantoms divided by the measured value on the Alderson phantom for beta radiation of 90Sr/90Y, depending on the angle of radiation incidence a. The ratios do not significantly deviate from unity. This was expected as the range of betas from 90Sr/90Y (with an endpoint energy of 2.3 MeV)

is at maximum about 10 mm in tissue (and slightly smaller in materials with a density larger than unity, such as polyamides—of which the dosemeter badge is made—or PMMA and water). Thus, the beta radiation does not penetrate deeply into the phantoms, and, therefore, the different dimensions of the three phantoms have practically no influence on the dosemeter’s measured value. Particularly for beta radiation, the large uncertainty bars are noticeable, see Figure 6. A reason might be the slightly varying form of and blowholes in the caps of different dosemeter badges, see Figure 7: the left one has a deep scratch and a dimple (not used for irradiations), the middle one is quite homogeneous and the right one has a blowhole. As the scattering and absorption of beta radiation strongly depends on the amount of material the radiation penetrates, the measured values of different badges can vary significantly, depending on the dosemeter’s cap and resulting in larger uncertainties. Response on the slab and the cylinder phantoms relative to the response on the Alderson phantom Photon radiation For calibrations and type tests, the response (measured dose divided by irradiated dose) of a dosemeter is of importance. In order to quantify the influence of the phantom shape, the response of the dosemeter irradiated on the slab and the cylinder phantoms relative to the response on the Alderson phantom was calculated: Rslab RAlderson

Mslab Mslab Hp ð3Þslab Ka  hpK ð3Þslab ¼ ¼ MAlderson MAlderson Hlens Ka  hlensK ¼

Figure 4. Geometry at an angle of incidence of a ¼ 758.

Rcyl RAlderson

Figure 5. Geometry at an angle of incidence of a ¼ 908.

Mslab  hlensK MAlderson  hpK ð3Þslab

Mcyl Mcyl Hp ð3Þcyl Ka  hpK ð3Þcyl ¼ ¼ MAlderson MAlderson Hlens Ka  hlensK ¼

ð1Þ

ð2Þ

Mcyl  hlensK MAlderson  hpK ð3Þcyl

where Rslab is the response on the slab, Rcyl is the response on the cylinder, RAlderson is the response on the Alderson phantom, Mslab is the measured dose on the slab, Mcyl is the measured dose on the cylinder, MAlderson is the measured dose on the Alderson phantom, Hp(3)slab is the irradiated dose on the slab, Hp(3)cyl is the irradiated dose on the cylinder, Hlens is the irradiated dose on the Alderson phantom, Ka is the air kerma (the same for the irradiations on all three

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Z  6: water, PMMA, tissue and skull equivalent materials, the latter in the Alderson phantom. While the ratio of the measured values for the cylinder and the Alderson phantoms shown in Figure 3 has a rather smooth dependence on the angle of incidence, this is not the case for the slab phantom, see Figure 2. This behaviour can be explained by the form of the slab: the maximum at an angle of incidence of a ¼ 758 probably results from radiation scattered from the phantom surface directly into the dosemeter, see Figure 4. On the other hand, the rather small ratio for the slab at a ¼ 908 results from the fact that practically half of the radiation field is blocked by the phantom, see Figure 5.

PHANTOM INFLUENCE ON AN Hp(3) EYE DOSEMETER

Figure 7. Photograph of three dosemeter badges. The left one has a deep scratch and a dimple (not used for irradiations), the middle one is quite homogeneous and the right one has a blowhole.

phantoms), hpK(3)slab is the conversion coefficient from Ka to Hp(3)(16) slab, hpK(3)cyl is the conversion coefficient from Ka to Hp(3)(17) cyl and hlensK is the conversion coefficient from Ka to HlensK (obtained as explained in the subsection ‘Conventional quantity values (true values)’). The last ‘equals signs’ in equations (1) and (2) hold as the amount of air kerma was chosen to be the same for all three phantoms. Figures 8 and 9 show the response on the slab and the cylinder phantoms, respectively, divided by the response on the Alderson phantom, and depending on the angle of incidence, a, and the mean photon energy, E¯ph: †

For a  458, the ratio is nearly the same for both calibration phantoms (slab and cylinder), i.e. calibrations (usually performed at a ¼ 08) are equivalent on either phantom. Below 30 keV, the ratio is smaller than unity, i.e. the responses on the

calibration phantoms are smaller than on the Alderson phantom. In this region (a  458 and E¯ph  30 keV), the indication on all three phantoms is quite similar, see Figures 2 and 3. Therefore, the reason for the ratios smaller than unity in Figures 8 and 9 is that hlensK is smaller than hpK(3)slab and hpK(3)cyl, see equations (1) and (2); i.e. both quantities Hp(3)slab and Hp(3)cyl are conservative with respect to Hlens as was seen earlier(23). † However, for larger angles of incidence, the response on the slab compared with the Alderson phantom increases with increasing angle of incidence, see Figure 8. This is not the case for the cylinder phantom, see Figure 9. The main reason is that hpK(3)slab becomes much smaller than hlensK, which is—again—not the case for hpK(3)cyl. This effect is more dominant, the larger the angle of incidence and the smaller the photon energy(23). The reason is that hpK(3)slab is determined in a slab (having sharp edges) made of ICRU tissue, whereas hlensK and hpK(3)cyl are determined in phantoms without edges (the human-like reference phantoms and the cylinder made of ICRU tissue, respectively). Particularly for large angles of incidence, this results for the slab in much longer path lengths through ICRU tissue than for the cylinder and the reference phantoms. In Figure 10, the extreme case of a ¼ 908 is illustrated: the path length the radiation has to penetrate to reach the reference point (3 mm deep in the phantom) is 15 cm long in the slab (solid and dotted arrow), while it is only about 2.4 cm for the

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Figure 6. Measured value (indication) on the slab and the cylinder phantoms divided by the measured value on the Alderson phantom for beta radiation of 90Sr/90Y, depending on the angle of radiation incidence. The uncertainty bars are the standard deviation of the mean values of about seven TLDs.

R. BEHRENS AND O. HUPE

Figure 9. Response on the cylinder divided by the response on the Alderson phantom, depending on the mean photon energy. The uncertainty bars are the standard deviation of the mean values of about five values ( partly smaller than the symbols). It shall be noted that the ordinate is broken between 1.6 and 2 (dotted black line). For a better comparison, the ordinate was chosen to be the same as in Figure 8.

cylinder (only dotted arrow)—and similar to the reference phantoms. Therefore, hpK(3)slab is much smaller than hlensK and hpK(3)cyl resulting in large ratios of the responses, see equations (1) and (2). In conclusion, type tests at larger angles of incidence are only advisable using the cylinder phantom. Therefore, the corresponding standard

for passive dosimetry systems of the International Electrotechnical Commission (IEC)(24) should be revised by adopting the cylinder instead of the slab phantom. It is stressed that the extreme behaviour of the quantity Hp(3)slab for large angles of incidence, especially for a ¼ 908,

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Figure 8. Response on the slab divided by the response on the Alderson phantom, depending on the mean photon energy. The uncertainty bars are the standard deviation of the mean values of about five values (at 908 smaller than the symbols). It shall be noted that the ordinate is broken between 1.6 and 2 (dotted black line).

PHANTOM INFLUENCE ON AN Hp(3) EYE DOSEMETER is equivalent for the two other personal dose equivalents Hp(0.07)slab and Hp(10)slab. Therefore, for both of them, values for the conversion coefficients hpK(0.07; 908)slab and hpK(10; 908)slab are not stated in any documents issued by international organisations such as the ICRU(25) or International Organization for Standardization (ISO)(15).

and the cylinder phantoms could be compared: Mslab Mslab Hp ð3Þslab Rslab Hp ð3Þslab ¼ ¼ Mcyl Mcyl Rcyl Hp ð3Þcyl Hp ð3Þslab  kcyl ¼

Beta radiation

Figure 10. Geometry visualising the different path lengths through the slab and the cylinder phantoms at an angle of incidence of a ¼ 908.

Mslab  kcyl Mcyl

where Rslab is the response on the slab, Rcyl is the response on the cylinder, Mslab is the measured dose on the slab, Mcyl is the measured dose on the cylinder, Hp(3)slab is the irradiated dose on the slab, Hp(3)cyl is the irradiated dose on the cylinder and kcyl is the correction factor to account for the cylinder instead of the slab phantom(22). Figure 11 shows the response on the slab phantom divided by the response on the cylinder phantom for beta radiation of 90Sr/90Y, depending on the angle of radiation incidence. For comparison, the correction factor kcyl to account for the cylinder instead of the slab phantom is also shown(22). As the measured values on the two phantoms are equivalent, see Figure 6, the ratio Rslab/Rcyl follows kcyl. As mentioned earlier, the large uncertainties are, on the one hand, probably attributed to inhomogeneities in the dosemeter’s caps. Finally, it shall be noted that the response of the Eye-DTM for 90Sr/90Y relative to its response for S-Cs (137Cs) is only about 0.6, i.e. when calibrating with S-Cs, the response for 90Sr/90Y is only 60 %. The

Figure 11. Ratio of the response on the slab and the cylinder phantoms for beta radiation of 90Sr/90Y, depending on the angle of radiation incidence. For comparison, the correction factor to account for the cylinder instead of the slab phantom is also shown(22). The uncertainty bars are twice the standard deviation of the mean values of about five values.

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As the conventional quantity values of the equivalent dose to the lens of the eye, Hlens, are not available for beta radiation, see in section ‘Conventional quantity values (true values)’, only the response on the slab

ð3Þ

R. BEHRENS AND O. HUPE

CONCLUSIONS The influence of the phantom shape on the calibration and type test result of an Hp(3) eye dosemeter was investigated. The following turned out: † †







For beta radiation, there is no significant influence. For photon radiation, the most backscattered radiation is present from the slab, a little less from the cylinder and the least from the Alderson phantom. This behaviour corresponds to the phantom’s cross-sectional area at the dosemeter’s level and can, therefore, be simply understood. For both beta and photon radiations, the response is practically independent of the calibration phantom shape (slab or cylinder) up to angles of a ¼ 458. Thus, calibrations of dosemeters (usually performed at a ¼ 08) can be carried out on both phantom types (slab and cylinder). However, for both beta and photon radiations, the response is significantly different for a  608 for the two phantoms: the response on the slab phantom increases compared with the response on the cylinder phantom with increasing a. Finally, for photons, the response on the cylinder is quite similar to the response on the human-like Alderson phantom. This behaviour has its reason mainly in the strong decrease in the conventional quantity value (true value) in the slab phantom, Hp(3)slab, with increasing values of a. The corresponding decrease in the conventional quantity value in the cylinder phantom, Hp(3)cyl, is much more similar to the corresponding behaviour of the equivalent dose to the lens of the eye itself, Hlens,(23). Therefore, the use of the cylinder phantom is much more appropriate for type tests for a  608.

ACKNOWLEDGEMENTS The authors are grateful to Leonie Hoffman for optimising the temperature treatment of the TLDs and to Phil Bru¨ggemann and Christian Fuhg (both from PTB) for the several irradiations they performed.

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reason probably is that the dosemeter’s cap is made of 3 mm polyamide (r ¼ 1.13 g cm23)(12), whereas the detector is made of 0.9 mm lithium fluoride (r ¼ 2.64 g cm23)(12). This results in an effective tissue equivalent depth of about 4.6 mm (at the detector’s centre), which is much more than the reference depth of 3 mm ICRU tissue (r ¼ 1.0 g cm23) according to the definition of the quantity Hp(3).Consequently, the Eye-DTM is not appropriate for use at workplaces with beta contributions, which is especially the case in nuclear medicine where beta-emitting radionuclides are handled—at least not until correction factors specific for each single workplace are applied.

PHANTOM INFLUENCE ON AN Hp(3) EYE DOSEMETER

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(2011) and Erratum: J. Instrum. 7 E04001 (2012) and Addendum: J. Instrum. 7 A05001 (2012). A consolidated version is available. Behrens, R. Correction factors for the ISO rod phantom, a cylinder phantom, and the ICRU sphere for reference beta radiation fields of the BSS 2. J. Instrum. 10, P03014 (2015). Behrens, R. On the operational quantity Hp(3) for eye lens dosimetry. J. Radiol. Prot. 32, 455– 464 (2012). International Electrotechnical Commission. Radiation Protection Instrumentation – Passive Integrating Dosimetry Systems for Personal and Environmental Monitoring of Photon and Beta Radiation. IEC 62387, IEC (2012). International Commission on Radiation Units and Measurements (ICRU). Conversion Coefficients for use in Radiological Protection against External Radiation. ICRU Report 57, ICRU Publications (1998).

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16. Behrens, R. Air kerma to dose equivalent conversion coefficients not included in ISO 4037-3. Rad. Prot. Dosim. 147, 373– 379 (2011). 17. Behrens, R. Air kerma to Hp(3) conversion coefficients for a new cylinder phantom for photon reference radiation qualities. Rad. Prot. Dosim. 151, 450–455 (2012). 18. International Commission on Radiological Protection (ICRP). Adult Reference Computational Phantoms. ICRP Publication 110, Ann. ICRP 39 (2) (2009). 19. Behrens, R. and Dietze, G. Dose conversion coefficients for photon exposure of the human eye lens. Phys. Med. Biol. 56, 415 –437 (2011). 20. International Commission on Radiological Protection (ICRP). Conversion Coefficients for Radiological Protection Quantities for External Radiation Exposures. ICRP Publication 116, Ann. ICRP 40 (2–5) (2010). 21. Behrens, R. and Buchholz, G., Extensions to the PTB beta secondary standard BSS 2. J. Instrum. 6, P11007

Influence of the phantom shape (slab, cylinder or Alderson) on the performance of an Hp(3) eye dosemeter.

In the past, the operational quantity Hp(3) was defined for calibration purposes in a slab phantom. Recently, an additional phantom in the form of a c...
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