This study presents the results of the preliminary tests performed using a kerma-area product (KAP) meter in order to verify its usefulness as a monitor chamber in a clinical X ray system, which will be used in ionization chambers’ quality control program. The tests performed were repeatability, surface measurement uniformity, linearity with tube current, measurement variation changing the distance and field size, KAP chamber radiation attenuation, and energy dependence. The KAP meter has presented a good repeatability (a maximum variation of 0.4%); its surface has a homogeneous response (a maximum variation of 1.0% among the different irradiated regions); the response is linear with the tube current and the variation of the PKA along the distance is less than 3.43%. On the other hand, the results obtained for chamber attenuation and energy dependence show that some corrections must be applied when the measurements are made. The results have shown a good performance of the KAP meter, confirming that it is possible to use this instrument as a monitor chamber. However, it is important to highlight that some correction factors for energy dependence and chamber radiation attenuation must be considered.
In radiation meters calibration laboratories are common to find a monitor chamber in calibration setup, which is used for a real-time radiation beam monitoring. When it is plugged to appropriate software, it is possible to obtain information about exposure time, accumulated air-kerma and air-kerma rate, and also identify a possible beam intensity variation. This equipment is important in standardization dosimetry laboratories, which must always work with reduced uncertainties.
The Instruments Calibration Laboratory (LCI) of the Instituto de Pesquisas Energéticas e Nucleares (IPEN/ CNEN-SP) has a PTW monitor chamber (reference chamber) installed on its X ray calibration system.
The radiation physics and hygiene coordination of the Federal University of São Paulo (CFHR-UNIFESP) have a laboratory (LEMADIM) with a clinical X ray system used to perform dosimetry and image techniques research. Due to the importance of this equipment for the CFHR researches, and the financial difficulty in buying a specific monitor chamber, it is proposed to use the KAP meter as a monitor chamber, since it is already used to monitor the X ray beam in interventional equipments in order to estimate the patient dose [
The KAP performance tests were made at LCI and were based on the International Eletrotechnical Commission (IEC) standard IEC 60580 [
The aim of this study was to verify if the KAP meter could be used in radiation beam monitoring at the LEMADIM clinical X ray system in order to achieve the uncertainties required by IEC.
In this study, the recommended nomenclatures were used, presented by the International Atomic Energy Agency (IAEA) [
A PTW KAP meter, Diamentor E2 model (
Due to the fact that this is a non-sealed chamber, all the measurements must be corrected for environmental conditions (temperature and pressure), using Equation (1):
where P and T are, respectively, the measured pressure (in kPa) and temperature (in ˚C).
The tests were made using an industrial X ray system Pantak/Seifert (
The air-kerma was obtained using Equation (2).
where LC is the ionization chamber measurement, corrected for environmental conditions, Nk is the chamber calibration coefficient and kQ is the radiation quality coefficient [
The tests were made using the reference radiation quality for radiation diagnostic, RQR-5 (70 kV), except repeatability, linearity and energy dependence.
The International Vocabulary of Metrology [
A 90Sr + 90Y source was positioned at the center of the KAP meter. Ten measurements were taken, and the procedure was repeated for both chamber sides.
The objective of this test was to verify if KAP response depends or not on the surface region that is being irradiated. The chamber was placed about 55 cm from the focal spot, its surface was divided into five regions (
This test was made to verify if KAP meter measurement increases linearly with the tube current. A voltage value was established and current was gradually increased. The four voltages recommended by the standard IEC 60580 [
The major differential of the KAP meter is its capacity to measure the kerma-area product, which should be constant along the distance between the chamber and the X ray tube. In order to verify this characteristic the KAP chamber has been moved from 50 cm to 100 cm away from the X ray focal spot, keeping the apertures. This test was made using two circular apertures: 50 mm and 17 mm diameter.
This test was made to verify how much the KAP chamber attenuates the radiation beam. The beam intensity has been measured using the reference ionization chamber RC6, which was placed one meter away the focal spot with and without the KAP chamber as monitor chamber.
To determine the KAP meter energy dependence it was necessary to obtain the calibration coefficient
where
Reference air-kerma rate
Calibration coefficients were normalized for the reference quality RQR-5, thereby obtaining the kQ value.
In repeatability tests the KAP meter presented a maximum variation of 0.37% (
It is possible to note that the KAP meter is a very stable instrument, but there is a measurement difference of around 2.5% between the two sides of the chamber. There is no technical information about this phenomenon in the KAP meter user manual suggesting that it might be a characteristic of this instrument.
Results for the uniformity test are shown in
Results show that the KAP meter presents a linear response with the current (
Results for this test are shown in
SIDE 1 | SIDE 2 | |
---|---|---|
Mean PKA (μGy×m²) | 2.27 (4) | 2.33 (5) |
KAP meter Irradiated area | PKA (mGy×m²) | Variation from the mean PKA (10−2 %) |
---|---|---|
1 | 14.44 (30) | 97.90 (99) |
2 | 14.33 (25) | 20.98 (33) |
3 | 14.16 (15) | −97.90 (99) |
4 | 14.29 (29) | −6.99 (10) |
5 | 14.28 (20) | −14.99 (11) |
Mean PKA (mGy×m²) | 14.30 (24) |
Aperture diameter (mm) | 50.8 (1) | 17.0 (1) | ||
---|---|---|---|---|
KAP chamber distance from the focal spot (cm) | 100.0 (5) | 50.0 (3) | 100.0 (5) | 50.0 (3) |
Kerma-area product, PKA (μGy×m²) | 125.84 (3.10) | 127.83 (2.98) | 13.76 (40) | 14.25 (42) |
Variation (%) | 1.57 (5) | 3.43 (11) |
The maximum variation for the PKA was of 3.43%, when the distance was increased from 50 cm to 100 cm. The standard IEC 60580 [
When the monitor chamber was replaced by the KAP chamber it was noticed an attenuation that varied from 11.0% to 17.7%, depending on the quality (
Results show a correction is necessary for each quality, since the attenuation caused by the KAP chamber is not constant.
Result for the KAP meter energy dependence, for
The uncertainty for the product Nk ´ kQ is 3.5%. Results show that the KAP meter energy dependence reached values of up to 25%, which is greater than the 8% required by IEC 60580 [
It is also important to highlight that these tests were made using the new RQR qualities [
Results showed a good performance of the KAP meter. This equipment has presented stable and accurate measurements, surface uniformity, linearity with tube current and small PKA variation along the distance.
The energy dependence verified was much higher than the 8% required by international standards. Previous studies have shown that this behavior is common in this kind of instrument. However, an IEC 60580 standard review, considering the new RQR qualities, may be necessary.
Radiation beam attenuation caused by the KAP chamber was of up to 17.7 %, and this value was different for each quality, which indicated that a correction factor must be applied to each measurement.
Radiation quality | Voltage (kV) | Additional filtration (mmAl) | Attenuation (%) |
---|---|---|---|
RQR 3 | 50 | 2.4 | 17.7(5) |
RQR 5 | 70 | 2.8 | 15.3(4) |
RQR 8 | 100 | 3.2 | 13.1(4) |
RQR 10 | 150 | 4.2 | 11.0(3) |
Radiation | Tube voltage | kQ |
---|---|---|
Quality | (kV) | |
RQR 3 | 50 | 0.971 |
RQR 5 | 70 | 1.000 |
RQR 8 | 100 | 1.147 |
RQR 10 | 150 | 1.252 |
In general, it is possible to use the KAP meter as a monitor chamber since the energy dependence and attenuation for different beam qualities are well known and re-checked periodically.
The authorsacknowledgethepartial financial supportoftheInternationalAtomic Energy Agency (IAEA), Vienna, the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Ministério da Ciência e Tecnologia (MCT, Project: Instituto Nacional de Ciência e Tecnologia (INCT) em Metrologia das Radiações na Medicina), Brazil.