# Evaluation of the calibration procedure of active personal dosemeters for interventional radiology

J.-M. Bordy, Josiane Daures, Isabelle Clairand, Marc Denoziere, L. Donadille, F. d'Errico, Jean Gouriou, C. Itie, L. Struelens

► To cite this version:

J.-M. Bordy, Josiane Daures, Isabelle Clairand, Marc Denoziere, L. Donadille, et al., Evaluation of the calibration procedure of active personal dosemeters for interventional radiology. Radiation Protection Dosimetry, 2008, 131 (1), pp.87-92. 10.1093/rpd/ncn225. cea-05466982v2

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# EVALUATION OF THE CALIBRATION PROCEDURE OF ACTIVE PERSONAL DOSEMETERS FOR INTERVENTIONAL RADIOLOGY

J.-M. Bordy<fnref n="1" />*, J. Daures<fnref n="1" />, I. Clairand<fnref n="2" />, M. Denozière<fnref n="1" />, L. Donadille<fnref n="2" />, F. d'Errico<fnref n="3" />, J. Gouriou<fnref n="1" />, C. Itié<fnref n="2" /> and L. Struelens<fnref n="4" />

An overview of the use of active personal dosemeters (APD) in interventional radiology is presented. It is based on the work done by the working package 7 of the CONRAD coordinated action supported by the EC within the frame of the 6th FP. This study was done in collaboration with the working package 4 of CONRAD to deal with the calculations required for studying the new calibration facility. The main requirements of the standard for the APD and the difficulties caused by the use of pulsed radiations are presented through the results of an intercomparison organised in a realistic calibration facility similar to the workplace situation in interventional radiology. The main characteristics of this facility are presented.

## INTRODUCTION

Active personal dosemeters (APD) are increasingly used and their development is fast. They allow the assessment of the doses in real time and provide audible alarms (for pre-defined dose and dose-rate levels). They are useful tools for the optimisation of doses and for the application of the ALARA principle (as low as reasonably achievable) in agreement with the 96/29 European Council Directive(1) which is based on the ICRP 60 publication(2). The optimisation of doses and the ALARA principle are updated concepts restated in the new recommendations recently published by the International Commission on Radiological Protection, ICRP publication 103(3). In general, it is recommended that APD are worn by workers with a higher risk of exposure (category A workers) and more generally by those working in controlled areas. APD are used in addition to the legal dosemeter based on a monthly issued passive dosemeter or depending on the country as the legal dosemeter itself(4). These principles apply to any worker whatever the workplace in nuclear or non nuclear industry, research laboratories and for medical use of radiations.

Taking as an example the 2005 French statistical data on occupational exposure(5): from 273 886 monitored workers, 58% work in the medical sector, a situation representative of the industrialised countries. It was also reported that 26 out of 33 workers with annual effective doses between 20 and 50 mSv, and that 4 out of 7 workers with annual effective doses higher than 50 mSv, representing 79 and 57%, respectively, belonged to the medical sector. These figures show that the medical exposures are among the most important and that it is among them that the risk for exceeding the exposure limits is the highest. Hence, more work on the optimisation of practices and on the implementation of the ALARA principle is still needed, at least in the medical sector.

In medical field, the ionising radiations are almost exclusively photons and electrons. The use of neutrons and protons is much less frequent. Looking briefly at the medical practices using ionising radiations, one has: the radiotherapy, the brachytherapy, the diagnostic (including the mammography and the dental exams), the interventional radiology and the nuclear medicine. This paper is focused on the issues met in interventional radiology. In this field, the high voltage of the X-ray tubes can be up to 150 kV. The medical staff, remaining near the primary source of radiation during the intervention, is exposed, except in exceptional cases, only with the radiations scattered by the patient, the room (walls, floor and ceiling) and the medical material (patient table, image intensifier, etc.)—see Figure 1. The collective protections (shielding of the generator to avoid leakage, lead shield under and above the patient table), and individual protections (protective clothing such as lead equivalent apron, thyroid collar and lead loaded glasses) allow to

reduce in an appreciable way the exposure of the medical staff. However, the individual protections are not often really ergonomic and therefore difficult to use even if they are mandatory (i.e. apron and thyroid collar in some countries). Even attenuated by the collective and individual protection, the exposures to the ionising radiation remain notable and one has to limit the number of interventions per practitioner to respect the annual limits of exposure.

<figure><img src="image_2.png" /><figcaption>Figure 1. Side and top views of the workplace configuration.</figcaption></figure>

## CHARACTERISTICS REQUIRED FOR AN APD

Historically, APD were firstly used in nuclear industry and research laboratories for whole-body dosimetry. They consist of light and small-size devices designed to be worn at the level of the chest to match the calibration procedures described in the ISO (International Organization for Standardization) standards(6,7). The sensors used in the dosemeter are mainly based on Geiger-Muller (GM) tubes, silicon diodes and direct ion storage (DIS) detectors(5,8).

<footnote n="1">¹ CEA-LIST Laboratoire National Henri Becquerel, CEA Saclay, 91191 Gif sur Yvette Cedex, France</footnote>

<footnote n="2">² Institut de Radioprotection et de Sûreté Nucléaire (IRSN), BP 17, 92224 Fontenay-aux-Roses Cedex, France</footnote>

<footnote n="3">³ Università di Pisa (UNIPI), Facolta' di Ingegneria, 2 via Diotisalvi, 56126 Pisa, Italy and Yale University, PO Box 208043, New Haven, CT 06520-8043, USA</footnote>

<footnote n="4">⁴ Belgian Nuclear Research Centre (SCKCEN), Boeretang 200, 2400 Mol, Belgium</footnote>
Independently of the sensor used in the dosemeters, the required characteristics for the ADPs dedicated to whole-body dosimetry are described in IEC 61526 standard(9). Hereafter, the main requirements in terms of radiation measurements are summarised (without going into detail) for the dimensions, weight, electric, electromagnetic and mechanical characteristics.

Concerning Hp(0.07), the limit of variation of the relative response of the dosemeters is 229-67% over the energy range (from 30 to 250 keV for X and gamma radiation, from 0.2 to 0.8 MeV for mean beta radiation energy) as well as for angles of incidence from reference direction (from 08 to 608). For Hp(10), the limit of variation of the relative response of the dosemeters is 229-67% over the energy range (from 80 keV to 1.5 MeV or from 20 to 150 keV) for X and gamma radiations as well as for angles of incidence from reference direction (from 08 to 608). The limit of the relative response of the dosemeters with the dose rate is +20% from 0.5 to 1 mSv h21 both for Hp(0.07) and Hp(10).

Among these parameters, the most important for APD in interventional radiology is the energy detection threshold which must be at least 20 keV.

## CALIBRATION PROCEDURE: PRINCIPLE OF THE REALISTIC RADIATION FIELD FOR THE CALIBRATION OF APD

The radiation fields used for the calibration and the type tests of the dosemeters are those described in the ISO standards 4037 series(6) for photon and 6980 series(7) for beta. The very large choice of radiation fields defined in these standards, in particular for photons, allows, in theory, to cover the range of energies found in the medical applications but the methods of production are based on industrial X-ray generators producing continuous beams with different filtrations than those actually used. To obviate this difficulty, the calibration of the dosemeters for the medical sector is thus sometimes carried out by using radiation qualities named RQR and RQA, defined for the quality controls of the generators in standard IEC 61267 (10), representative of the medical conditions of use. One has to notice that, although pulsed radiations are used in interventional radiology, they are not included in the requirements of the standard. This lack is harmful for the validation of APD because pulsed radiations can be associated with very high instantaneous dose rates. In such a case, a GM-based dosemeter tested in the comparison reported here after was not able to measure the single pulsed radiation field. It was also found that many dosemeters did not warn the worker when they are overloaded. In such a case, the reading can be anything else including zero!

For medical applications, the energy and angle distributions at work place depend on the position of the medical staff next to the patient and can be calculated. It is thus possible to know with a good precision the radiation fields met at workplace. Hence it can be possible and useful to define some particular conditions of calibration corresponding to the situations met at workplace.

The idea is to propose an irradiation assembly which could be set up easily in any primary or secondary calibration laboratory. From the workplace situation described in Figure 1, a few simplifications has been introduced. At first, it was found that the image intensifier and the table on which the patient lies down do not contribute significantly to the scattered radiation field at the surgeon's location(11). The patient was replaced by an ISO slab phantom commonly used for calibration purposes. Other material such as lung tissue equivalent material could also be used(11). For the calibration of personal dosemeters, an ISO slab phantom was used as it is recommended in the ISO standards for whole body dosemeters. Figure 2 shows a scheme of the facility (side and top view). It can be seen that the ISO water slab phantom for calibration has been shifted from the level of the hip of the patient to be set in front of the side face of the 'patient phantom'. Calculations showed that the energy distributions at both locations are similar (see

Figure 3) but the total fluence is multiplied by a factor of 3.7 when moving the calibration point from the level of the chest of the surgeon located at the hip of the patient to the level of the hip of the surgeon, located at the chest of the patient(11). Figure 3 also shows that simulating the patient with Rando phantom(12,13) instead of an ISO water slab phantom does not significantly change the shape of the scattered spectrum.

<figure><img src="image_3.png" /><figcaption>Figure 2. Experimental realistic set up for the calibration of APD (producing a radiation field similar to those met at interventional radiology workplace).</figcaption></figure>

## DETERMINATION OF THE REFERENCE VALUES AT THE CALIBRATION POINT

The principle for establishing the reference value in terms of personal dose equivalent at the calibration point is to measure the air kerma using a calibrated cavity ion chamber and to compute the conversion coefficient from air kerma to personal dose equivalent using Monte-Carlo calculations.
<figure><img src="image_4.png" /><figcaption>Figure 3. Comparison of the spectra calculated at different location around the patient phantom (ISO water slab or Rando phantom).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Comparison of the calculated and measured spectra free in air.</figcaption></figure>

The initial photon spectrum of the X-ray generator was calculated using the software XCOMP5(14). The validation of the Monte-Carlo calculations was done by measuring and calculating the photon spectrum with a CdTe spectrometer. Figure 4 shows the good agreement attained between measurements and calculations when the spectrometer is included in the calculations so that the pulses due to the interaction of the incident photons with the spectrometer and its collimator are taken into account. The air kerma was calculated free in air and the personal dose equivalent was calculated 10 mm inside the four elements ICRU tissue slab phantom. Then the ratio between these two quantities was calculated. Five Monte-Carlo codes each associated with different cross-sectional libraries were used for this purpose, namely, MCNP4C, MCNP5(15) and Penelope 2006(16) at CEA-LIST-LNHB, MCNPx2.5f at IRSN and MCNPx2.5.0(17) at SCK†CEN. The results of the calculations are presented in Table 1.
As it is pointed out in ISO standard 12789-2(18) for realistic neutron radiation fields, that the phantom standing at the surgeon's location must not introduce a perturbation on the radiation field during calibration. This perturbation comes from the multiple scattering between the patient and surgeon phantoms. The closer these phantoms are, the higher the multiple scattering effects. Despite the small distance (17.5 cm) between the phantom faces in the configuration studied for this new calibration facility, the multiple scattering was found to be negligible. The results of the computations show that the increase in terms of fluence due to multiple scattering is 0.03%.

Figure 5 presents the method for determining the reference value of Ka at the point of test. First, a free air chamber was used to measure Ka in the direct radiation field. The standard uncertainty associated to this measurement was 0.45%. Secondly, a cavity chamber (30 cm³ cylindrical PTW TM 23361) was put at the same location as the free air chamber and

was calibrated. Taking into account uncertainties, like positioning, temperature, pressure and so on, the combined standard uncertainty on the calibration coefficient is 1.39%. This relatively high uncertainty compared with the one of the reference values of the air kerma takes into account the uncertainty not only the positioning, the temperature, the pressure and so on, but also a larger dispersion of the measured current than usual due to the very short duration of the pulses. Thirdly, Ka at the point of test was measured using this cavity ionisation chamber. The combined standard uncertainty on the measured Ka at the point of test is 2.09%. The response of the cavity

chamber in terms of air kerma as a function of the quality of the beam (RQR) is given in Table 2.

The radiation qualities at the point of calibration and in the direct beam are between RQR 4 and 7. So, a (1.3%/√3) uncertainty was added to the standard uncertainty budget of the air kerma at the point of calibration. Comparing these conversion coefficients to those of the ISO standard 4037 series(6) the deviation is quite large. At 08 for the N60 and L70 qualities, which does not seems too far from the spectra presented in this report; the conversion coefficients are 1.65 and 1.87, respectively. These conversion coefficients are the ratio of the average values of Hp(10) and Ka. The results achieved with the Tissue Equivalent lung phantom as patient are similar to those reported in this report and they can be found in ref.(14)

From Table 1 it can be seen that the 'absolute' value of Hp(10) and Ka depends on the code. Nevertheless, the ratio between these two quantities is stable and its final value, corresponding to the conversion coefficient from air kerma to personal dose equivalent, has been chosen to be equal to 1.41 Sv Gy⁻¹. The reference value in terms of air kerma at CEA-LIST LNHB was 3.21 mGy pulse⁻¹ and 3.03 mGy h⁻¹ at 20 mA at IRSN.

This method was used to set up two calibration facilities at CEA-LIST LNHB (French primary laboratory for ionising radiation) and at IRSN (France). These facilities were used for a comparison of APD(19).

<figure><img src="image_6.png" /><figcaption>Figure 5. Synoptic of the method to measure the air kerma at the point of calibration.</figcaption></figure>

<table><caption>Table 2. Relative response of the 30 cm³ PTW cavity ion chamber in the direct radiation field.</caption><thead><tr><th></th><th>kV</th><th>Relative response<br>(normalised to RQR 9)</th><th>Standard<br>uncertainty</th></tr></thead><tbody><tr><td>RQR 4</td><td>60</td><td>1.021</td><td rowspan="3">1.2%</td></tr><tr><td>RQR 7</td><td>90</td><td>1.008</td></tr><tr><td>RQR 9</td><td>120</td><td>1.000</td></tr></tbody></table>

## FEEDBACK FROM THE COMPARISONS

In 2005, a comparison of the APD characteristics was organised by IAE (International Atomic Energy Agency) and EURADOS (European radiation Dosimetry Group)(20). APD were calibrated both in pulsed and continuous X-ray radiation fields, in the direct beams. The results of this comparison showed that dosemeters fulfilling the requirements of the standards are not always adapted to the medical field. Few dosemeters based on GM tubes were found to be blind to pulsed radiations. This means that it would be necessary to use beam qualities close or 'identical' to those of standard IEC 61267(10), pulsed radiations with high instantaneous dose rates and an important component of low energies to validate the use of the APD for radiation protection of medical workers. This validation could be left to the discretion of the users or could be integrated in the procedure of the type test before the marketing.

A new comparison has been organised by EURADOS in the framework of the CONRAD EC contract (2006–07)(19). Five different APD were selected for their capability to measure X-rays down to 20 keV, including silicon diodes, DIS and GM tube based APD. Those dosemeters were bought by the organisers. The realistic radiation field described above was used for this work.

This comparison confirmed(20) that (i) some GM tube based APD are blind even located at the position of the medical staff, in the scattered radiation field, (ii) the energy detection threshold of the APD must be sufficiently small to allow measuring radiation fields down to 10 keV. It highlighted also the need to check the calibration before using the APD.

## CONCLUSIONS

<table><caption>Table 1. Calculation of the conversion coefficient from $K_a$ (10⁻⁶ pGy history⁻¹) into $H_p(10)$ (pSv history⁻¹) using an ISO water slab phantom for the patient</caption><tbody><tr><td>K<sub>a</sub></td><td>MCNP4C—lib 02</td><td>4.40 (0.3%)</td></tr><tr><td></td><td>MCNP5—lib 04</td><td>4.38 (0.2%)</td></tr><tr><td></td><td>MCNPx2.5f—lib 04</td><td>4.85 (0.69%)</td></tr><tr><td></td><td>MCNPx2.5.0—lib 04</td><td>4.93 (0.71%)</td></tr><tr><td></td><td>Penelope 2006 (~F6)</td><td>4.87 (2.6%)</td></tr><tr><td rowspan="6">H<sub>p</sub>(10)</td><td>MCNP4C—lib 02</td><td>6.31 (0.6%)</td></tr><tr><td>MCNP5—lib 04</td><td>6.23 (0.4%)</td></tr><tr><td>MCNPx2.5f—lib 04</td><td>6.80 (0.59%)</td></tr><tr><td>MCNPx2.5.0—lib 04</td><td>6.96 (0.59%)</td></tr><tr><td>Penelope 2006 (~F6)</td><td>6.77 (0.54%)</td></tr><tr><td>Φ<sub>p</sub>/K<sub>a</sub></td><td>1.411</td></tr></tbody></table>

The values in parentheses represent the standard deviation on the results.
The use of ADP is of particular interest in situations where high doses and/or dose rates can occur and when a better accuracy in measurements is required, particularly in the medical field, such as in nuclear medicine, interventional radiology and brachytherapy, even when visiting the patient during the treatment. APD are suitable tools to answer the needs for the optimisation of (i) the medical practices and (ii) the collective and individual protections in order to reduce the effective doses following the ALARA principle.

However, the standards dealing with the characteristics of active dosemeters do not include criteria for pulsed radiation fields and it was found that some of the tested APD are blind. It would be useful to test the APD (under laboratory conditions or on site) before bringing it into service and sometimes to carry out a specific calibration for the radiation fields in which the APD will be used.

Other issues like the development of APD for the extremities and the lens of the eye are necessary. Specific values of the conversion coefficients to calculate the dose equivalent for the lens of the eye for X-rays do not exist so far, nor specific calibration procedures for eye lens dosimetry. Some of these concerns have been taken into account in the ORAMED ('Optimisation of Radiation Protection of Medical Staff') contract which begun in February 2008 for 3 y.

## FUNDING

This work was financially supported by the European Commission, contract number FP6-12684 (CONRAD).

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