Comparison of absorbed dose to air calibration factors for a parallel plate ionization chamber

Abstracts

OBJETIVO: O objetivo deste trabalho foi realizar uma comparação entre os fatores de calibração em termos de dose absorvida no ar determinados em feixes gama (60Co) e de elétrons. MATERIAIS E MÉTODOS: Foram utilizados um irradiador de 60Co e um acelerador linear Varian, modelo Clinac 2100C, com feixes de fótons e de elétrons. Foram testadas uma câmara de ionização cilíndrica e três de placas paralelas. RESULTADOS: Os sistemas de medidas foram submetidos aos testes preliminares (estabilidade de resposta e corrente de fuga), com resultados muito bons. Os fatores de calibração em termos de dose absorvida no ar foram determinados utilizando-se quatro sistemas de medidas e dois tipos de objetos simuladores, com a obtenção de resultados dentro das recomendações internacionais. CONCLUSÃO: Os resultados mostraram que os fatores de calibração em termos de dose absorvida no ar obtidos para câmaras de ionização de placas paralelas, determinados em feixes de 60Co, são no máximo 1,2% mais altos que os valores obtidos em feixes de elétrons de altas energias.

Câmaras de ionização; Feixes de elétrons; Calibração de instrumentos

ORIGINAL ARTICLE

Comparison of absorbed dose to air calibration factors for a parallel plate ionization chamber^*

Roseli T. Bulla^I; Linda V.E. Caldas^II

^IMaster in Sciences, Nuclear Technology

^IIDoctor in Science, Nuclear Physics

^{Mailing address}

ABSTRACT

OBJECTIVE: The objective of this study was to compare the absorbed dose to air calibration factors determined in gamma (⁶⁰Co) and electron beams.

MATERIALS AND METHODS: An irradiator with a ⁶⁰Co source and a Varian, Clinac 2100C linear accelerator with photon and electron beams were utilized. One thimble-type and three parallel-plate ionization chambers were tested.

RESULTS: The measurement systems were submitted to preliminary tests (response stability and leakage current), with quite good results. The absorbed dose to air calibration factors were determined using four measurement systems and two types of phantoms. Results were obtained in compliance with the international recommendations.

CONCLUSION: Absorbed dose to air calibration factors obtained for parallel plate ionization chambers, determined in ⁶⁰Co beams, at maximum, are 1.2% higher than the values obtained in high energy electron beams.

Keywords: Ionization chambers; Electron beams; Calibration of instruments.

INTRODUCTION

In measurements of absorbed dose in photon and electron beams, the most used dosimeter is the ionization chamber recommended by international protocols^(1-11), due to its precision. However, this type of chamber frequently does not have the calibration factor in terms of absorbed dose to air, N_D,ar, that relates the dose in the chamber gas and the collected charge. As a result, there is a need for a calibration aiming at having an indication of the most precise possible absorbed dose.

The determination of a N_D,ar calibration factor for a parallel plate ionization chamber in ⁶⁰Co and electron beams, has not an assured traceability, but, in terms of N_K; the N_D,ar value comes from this term and different procedures are recommended by dosimetry protocols considering that the international recommendation is the utilization of the TRS 381 protocol⁽¹⁰⁾. In this case, the N_D,ar calibration factor for the parallel plate ionization chamber is obtained from a comparison of the absorbed dose to water value D_W determined in a high energy electron beam with a cylindrical reference chamber that has a known N_D,ar value. A similar intercomparison with a phantom in a ⁶⁰Co gamma radiation beam also allows the N_D,ar determination for this type of chamber, provided the appropriate correction for the difference between the chamber composition and the simulator (phantom) material is taken into consideration^(12–16).

Several studies have estimated N_D,ar values for several parallel plate ionization chambers and some of them show that the N_D,ar value is higher in calibration with ⁶⁰Co beams in phantoms than with high energy electron beams^(12,13). So, this study was performed aiming at analyzing some clinical dosimeters calibration techniques, determining the calibration factor N_D,ar for parallel plate ionization chambers in ⁶⁰Co gamma radiation beams of the laboratório de Calibração de Instrumentos (Laboratory of Instruments Calibration) of Instituto de Pesquisas Energéticas e Nucleares (LCI-IPEN/São Paulo) and with high energy electron beams of Hospital Israelita Albert Einstein (HIAE).

MATERIALS AND METHODS

The calibration factors for parallel plate ionization chambers were determined employing four measurement systems: one cylindrical chamber (System A) as calibration factor in terms of air kerma and, consequently, as a calibration factor in terms of known absorbed dose to air, in ⁶⁰Co beams as a reference chamber, and three parallel plate chambers (systems B, C and D). All the chambers coupled with their respective electrometers as well as their specifications are shown in Table 1.

The parameters employed in the N_D,ar value calculation are those included in the TRS 381 protocol (Table 2). Considering that a difference in calibration factors N_D,ar values is expected between the two measurement methods (i.e., in ⁶⁰Co and electron beams), maximum attention should be paid aiming at minimizing errors and reproducing the recommended calibration conditions (Table 3) in compliance with the TRS 381 protocol.

The cambers positioning during procedure calibration in the LCI-IPEN and HIAE, was achieved with the assistance of laser beam systems lined up with the geometrical center of collimation systems. The chambers were positioned paralleling the beams and, aiming at reducing the random uncertainty in the charge measurement, this was done by means of ten consecutive readings corresponding to a measurement (taking the average value) in each voltage.

1. Radiation systems

Devices utilized were: a Philips Model XR2000 irradiator with a ⁶⁰Co source owned by LCI-IPEN; a Varian Clinac 2100C linear accelerator owned by the HIAE, with two photon beams with nominal energies of 6 and 18 MeV and five electron beams with nominal energies of 4, 6, 9, 12 and 16 MeV.

Environmental conditions, both in the LCI-IPEN and in the HIAE Department of Radiotherapy were controlled by means of air conditioning systems and dehumidifiers, with the support of a portable barometer, a digital thermometer and a hygrometer.

2. Measurement systems

Measurement systems were employed with ionization chambers coupled with their respective electrometers – Keithley, model 35614 EBS and Physikalisch-Technische Werkstätten (PTW) model 0002, whose specifications are found in Table 1. The reference system employed was a thimble-type model 2505/3A Nuclear Enterprises (NE) chamber series 2080, with traceability to the Laboratório Nacional de Metrologia das Radiações Ionizantes (LNMRI) (Rio de Janeiro).

The stabilization time of the systems constituted by chambers and their electrometers was 30 minutes before the measurements start.

3. Phantoms

The following simulators (phantoms) were utilized:

a) Water phantom produced by International Atomic Energy Agency (IAEA) measuring 30 x 30 x 30 cm³, with acrylic (PMMA) walls and supports, owned by IPEN.

b) Solid phantom projected and produced in the IPEN, measuring 30 x 30 x 20 cm³, with acrylic (PMMA) walls and supports, owned by IPEN.

c) PTW water phantom, measuring 40 x 40 x 40 cm³, with acrylic (PMMA) walls and supports, owned by HIAE.

4. Electron beam parameters

The dosimetric properties of the clinical electron beams depend significantly on the energy spectrum (or energy distribution) This spectrum can be characterized by parameters like those for nominal energy of 16 MeV:

(E_p)₀ = 16.70 MeV, the most probable energy on the phantom surface;

= 15.85 MeV, the average energy on the phantom surface;

(E_p)_z = 11.19 MeV, the most probable energy in a reference depth;

E_z/E₀ = 0.706.

RESULTS

1. Calibration in a phantom in ⁶⁰Co beams

The parallel plate ionization chambers calibration was performed in ⁶⁰Co beams in the LCI-IPEN Philips irradiator.

Parallel plate chamber was calibrated in comparison with a cylindrical ionization chamber previously calibrated in a water phantom. The chambers were alternately positioned at a reference depth in a phantom, the N_D,ar factor being a result from the comparison of the absorbed dose obtained with both chambers.

In this method, the effective point of measurement for the chambers is positioned at a 5 cm reference depth, i.e., the center of the frontal surface of the parallel plate chamber air cavity is defined in an effective point of the cylindrical chamber that is equal to 0.6 r in front of the chamber center (r is the cavity radius). However, for practical reasons, the center of the cylindrical chamber is placed at a depth of 5 cm and the correction for the displacement effect is made with a () factor. This displacement factor guarantees that the center of any cylindrical ionization chamber used in a phantom is at a same depth, independently of the chamber diameter.

Figure 1 presents an experimental mounting diagram employed for measurements in ⁶⁰Co.

By means of the expression (1), N_D,ar was obtained for the parallel plate ionization chamber.

where:

= the chamber calibration factor in terms absorbed dose to air;

M^Refand M^pp: (M =

. f_T,p. k_h. P_S) – readings of the cylindrical and parallel plate ionization chambers, respectively, for environmental reference corrections: pressure and temperature, (F_T,p), and air relative humidity (k_h); and for recombination correction (P_s);

: correction factor for attenuation of the reference cylindrical chamber wall;

: factor that takes into consideration the non-air equivalence of the material in the central electrode of an ionization chamber;

: 1 – 0,004.r, where r is the internal radius of the reference chamber in mm, for a ⁶⁰Co beam, according to Johansson et al.⁽¹⁴⁾, an article on which the TRS 381 publication⁽¹⁰⁾ is based.

: parallel plate chamber wall attenuation correction factor.

In this procedure, the ionization chambers calibration factors are obtained in terms of air kerma and, consequently, the calibration factors in terms of absorbed dose to air (N_D,ar), are determined in ⁶⁰Co gamma radiation beams. The measurements performed in the IPEN Laboratory of Clinical Dosimeters Calibration for N_D,ar (mGy/nC) determination employing PMMA water phantoms (as per Table 3), and the NE 2505/3A reference chamber of the system A with the systems B and C of LCI-IPEN and system D of HIAE are shown in Table 4.

For the measurements performed in the solid phantom, it is necessary to make a correction in the measure M_plást reading, by means of the expression: M^pp =

. h_m, where h_m = 1,00975 for a maximum reference depth. In the case of the ⁶⁰Co, 5 cm of water are necessary for obtaining the calibration factor in the reference conditions.

In Table 4 it is possible to observe that, when we simultaneously compare the two calibration methods (in water and in PMMA), the behavior of the system C parallel plate ionization chamber may be considered as excellent, with a variation between methods of only 0.05%. In comparison, system D presents a 2.1% variation. Each value corresponds to the average of several factors obtained in different dates, with an uncertainty rate not exceeding 1.4%.

In the calculation of the associated uncertainties, one has taken into consideration the equipment uncertainty in the system calibration in standard laboratory, environmental factors (temperature, pressure and air relative humidity), the uncertainties in the experimental measurement instrument, chambers stabilization time and perturbation factors for each type of radiation.

2. Calibration with electron beams

The parallel plate ionization chambers calibration was performed in electron beams with nominal energy of 16 MeV in a model Clinac 2100C linear accelerator owned by HIAE.

In this method, the measures were obtained in a solid phantom with the same methodology applied in water phantom, where parallel plate ionization chambers were calibrated in comparison with a previously calibrated cylindrical chamber and with a known N_D,arat a reference depth (for electron of 16 MeV nominal energy, 2 g/cm² in water). Corrections were made in M_plást measures, with the solid plate phantom that should be of the same material as the parallel plate chamber and in reference conditions included in Table 3.

The Figure 2 presents an experimental mounting diagram for measurements in the linear accelerator.

The calibration factor N_D,ar is calculated by means of the expression (1); the correction parameters employed in calculations, in compliance with the TRS 381, protocol are shown in Table 2.

In this procedure, the ionization chambers calibration factors were obtained in terms of absorbed dose to air, determined in electron beams with nominal energy of 16 MeV. Results of measurements performed in HIAE, N_D,ar (mGy/nC), with the use of water and PMMA phantoms (Table 3), the system A NE 2505/3A reference chamber, the LCI-IPEN systems B and C and the HIAE system D, are shown in Table 4.

In this table, comparing the two calibration methods (in water and PMMA phantoms), the system C presents a percentage difference < 0.1% and the system D a maximum difference of 0.8%. The maximum uncertainty rate associated with the calibration factor is 1.8% in calibrations with parallel plate chambers, which is within the limit recommended by IAEA protocols ^(3,11).

DISCUSSION AND CONCLUSIONS

Results show that the N_D,ar values for the parallel plate chambers determined in ⁶⁰Co beams are 1.2% higher than the value obtained in high energy electron beams. This difference in measurements series may be related, like in some published studies^(12,13,17), but this hypothesis is promptly discarded, since, in the charge measurement, the maximum uncertainty between measurements is ±0.15% for each voltage. So, this discrepancy is assigned to parallel plate chambers walls attenuation correction factors supplied by the protocol, which should not be coherent when calibration in photon beams is performed.

McEwen et al.⁽¹⁸⁾ have shown that Markus-type parallel plate ionization chambers responses are not very reliable in relation to other chambers in electron beams, due to the perturbation factor great variation of this type of chamber as a result of the E_Z energy, i.e., a great P_u variation may occur as a function of E.

The system C parallel plate ionization chamber behavior can be considered as excellent, with a percentage difference of only 0.05% between the two calibration methods employing two different phantoms.

This study results are perfectly in compliance with international recommendations suggested for calibration of this type of chamber in relation to the total uncertainty associated with the chamber calibration factor, in terms of absorbed dose to air in both ⁶⁰Co gamma radiation beams and electron beams.

Acknowledgements

The authors express their gratitude to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the partial financial support to the development of this project; to Mr. Marcos Xavier for the technical support; to Dr. Laura Natal Rodrigues for her important suggestions on this text; to Hospital Israelita Albert Einstein for the opportunity to utilize the linear accelerator; and especially to Dr. José Carlos Cruz, for the profitable discussions.

REFERENCES

Received May 5, 2005.

Accepted after revision August 27, 2005.

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*

Study developed at Instituto de Pesquisas Energéticas e Nucleares (IPEN), Comissão Nacional de Energia Nuclear, São Paulo, SP, Brazil.

Publication Dates

• Publication in this collection
  17 Aug 2006
• Date of issue
  June 2006

History

• Accepted
  27 Aug 2005
• Received
  10 May 2005