A PRELIMINARY STUDY OF RADIATION QUALITY CORRECTION FACTOR (KQ) DETERMINATION FOR WELL-TYPE IONIZATION CHAMBER

This paper deals with the determination of the radiation quality correction factor (kQ) well-type ionization chamber for the measurement of Co-60 brachytherapy. The measurement of the brachytherapy source has been done in the hospital brachytherapy facilities. The measurement of air Kerma has been done using three different ionization chambers 0.6 cm3, 30 cm3, 80 cm3, which have calibration factor traceability for Co-60 and one well-type ionization chamber which calibrated for Ir-192. The determination of the radiation quality correction factor (kQ) was determined based on the results of the air Kerma ratio between measurements using ionization chambers, which have traceability to Co-60 and Ir-192 sources. The results of the measurement of the reference air Kerma rate (RAKR) obtained from the three chambers were 21.36, 19.87, 19.34 mGy.m2.h-1, while the results of measurements with ionization chambers get a value of 19.01 mGy.m2.h-1. The kQCal results from 0.6 cm3 ionization chamber get a value of 1.07. The kQCal value was compared with the value of Andreas Schuller’s et al. kQreff of 1.05 and get a deviation of 2.2%. Implementation of the booth kQ value on the results of the RAKR from the HDR1000Plus well-type ionization chamber in the measurement of brachytherapy in two different facilities gets maximal deviation 1.7% with dose value from Treatment Planning System (TPS). The deviation was in the acceptable range of ±5%. Based on this, the use of radiation quality correction factor (kQ) value can be implemented as one method if it does not have the traceability factor calibration of the Co-60 brachytherapy source.


INTRODUCTION
Brachytherapy is one of the modalities of radiation therapy in addition to external radiotherapy. The advantage of brachytherapy compared to external radiotherapy is the ability to deliver the right dose to the target, whereas the drawbacks can only be used for tumor cases that are located (for example, cervix and prostate) and relatively small [1]. Therapeutic modalities using brachytherapy are divided into two, namely, the high dose rate (HDR) modality and the low dose rate (LDR). However, for a low dose rate, brachytherapy is no longer used in Indonesia.
Initially, the radiation source used for brachytherapy was Ra-226. As artificial sources were discovered, the use of radium was replaced by Co-60, Cs-137, and Ir-192 [2]. Until now, the source of brachytherapy radiation that is often used by hospitals is Ir-192 and Co-60 [3]. The source of the Ir-192 brachytherapy has a half-life of 74 days with an average energy of 350 keV, while Co-60 has a half-life of 5.27 years with an average energy of 1250 keV.
In Indonesia, as of December 2019, eleven hospitals had brachytherapy with a source of Ir-192 and three hospitals had brachytherapy with a source of Co-60. The brachytherapy dosimetry equipment must be calibrated once a year by applicable regulations. This calibration activity is also part of the quality assurance activities of the brachytherapy because the dosimeter will be used to measure the brachytherapy radiation source [4,5].
Institutions that have brachytherapy with an Ir-192 source usually replace the radionuclide source with an interval of about 3 months. The new source is accompanied by a manufacturing certificate or measurement results by the vendor with an uncertainty of ± 5% [6,7]. As for brachytherapy with a Co-60 source, it can usually replace the radionuclide source at intervals of one and a half times of the half-life source (5-7 years).
Related to the problem of replacing Ir-192 sources with a short amount of time, for some institutions, this is a crucial problem because it relates to the availability of funds for the procurement of sources, as well as licensing and transportation of Ir-192 sources [3]. Delays in licensing and clearance services at the customs services within a few days have a significant impact on the decay of the Ir-192 source activity, which then affects clinical irradiation because the source half-life is relatively short. In contrast to brachytherapy with a Co-60 source which, if there is a delay of some time, does not have a significant impact on the activity of the source.
The International Atomic Energy Agency (IAEA) has published protocols that describe in detail the calibration equipment, measurement techniques, stability checking, and recalibration intervals [8]. There were several methods in measuring the source of brachytherapy according to IAEA protocol, namely measurement in the air (free in-air measurement) and using a well-type ionization chamber. Measurement in the air has the disadvantage of contributing to scattering in a large room, thus making the uncertainty of the measurement also large [9]. Unlike direct measurements in the air, well-type ionization chambers were specifically designed to measure the source of brachytherapy and open to the atmosphere. Well-type chambers were more recommended in measuring the source of the brachytherapy in terms of air Kerma strength (AKS) [10][11][12].
At present, the availability of calibration services for Ir-192 brachytherapy sources at the Primary Standard Dosimetry Laboratory (PSDL) level was available at several National Metrology Institute (NMI's) [13,14], but for Co-60 source brachytherapy only available at German NMI, Physikalisch-Technische Bundesanstalt (PTB) [15].
Andreas Schűller, et al. [15], in his research, investigated a value of the radiation quality correction factor (kQ) that can be used for measurement of Co-60 source brachytherapy using a well-type ionization chamber which has a calibration factor at the Ir-192 source. In that study, 35 well-type ionization chambers with two different types were used, which were tested for measurements at the source of the Ir-192 and Co-60 brachytherapy.
On the other hand, the reference paper [15] used the ionization chamber volume 1000 cm 3 to measure the reference air Kerma rate from the PTB's brachytherapy primary standard source. Not only using the 1000 cm 3 ionization chamber, but the other reference also used the other volume of the ionization chamber, i.e., 0.6 cm 3 and well-type chamber with volume 245 cm 3 or 116 cm 3 [8]. For the reason of Co-60 brachytherapy measurement, the ionization chamber will be suitable because, generally, the chamber has the traceability calibration factor for Co-60.
This paper describes the determination of the radiation quality correction factor (kQ) for the Co-60 brachytherapy source using an HDR 1000Plus well-type ionization chamber. Verification of the measurement results of a well-type ionization chamber was done by comparing the measurements with a 0.6 cm 3 , 30 cm 3 , and 800 cm 3 ionization chamber using a free in-air measurement method. Implementation of the results of the kQ value was carried out on two measurements of Co-60 brachytherapy source using a 1000Plus HDR well-type ionization chamber compared to the TPS value in different brachytherapy facilities. The use of four different ionization chamber volumes has a purpose of ensuring that the measurement is in a good agreement. Besides, the 0.6 cm 3 , 30 cm 3 , and 800 cm 3 ionization chamber have the traceability of the Co-60 calibration factor.

METHOD
By documents published by the International Atomic Energy Agency (IAEA), namely IAEA TEC-DOC 1274: Calibration of photon and beta ray sources used in brachytherapy, there were several methods to calibrate dosimeters used in the measurement of brachytherapy sources.
The strength of the brachytherapy radiation source was determined by the amount of the reference air Kerma rate (RAKR). RAKR is the rate of air Kerma in the air at a distance of 1 meter (dref) corrected by attenuation and scattering [16]. Besides RAKR, another recommendation concerning the strength of the brachytherapy radiation source is in the amount of water kerma | 132 SPEKTRA: Jurnal Fisika dan Aplikasinya Volume 5 Issue 2, August 2020 strength (sK) i.e the rate of air kerma in the air at a distance d from the radiation source is corrected by attenuation and scattering and multiplied by the square of the distance (d 2 ) [17].

FREE-IN-AIR MEASUREMENT METHOD
Based on IAEA TEC-DOC 1274 RAKR measurements for brachytherapy were carried out at a distance of 1 meter. The determination of the RAKR using an ionization chamber can be calculated using the equation below.
Here the parameter description of EQUATION (1), KR is the air Kerma rate, NK is the air Kerma calibration factor, MU/t is the charge reading per time (60 seconds), kair is the attenuation correction factor of the beam in the air, kscatt is a factor scattering correction, kn is the non-uniformity correction factor, d is the measurement point distance to the ionization chamber and dref is the measurement reference distance (1 meter).
NK was obtained from the chamber calibration results against the standard reference chamber. The attenuation correction factor in the air (kair), scattering correction factor (kscatt), and the beam uniformity correction factor (kn) were obtained from the calculation results.
The parameter kn is obtained from the calculation using the equation below [8].
Here the description of the parameters used in EQUATION (2), Apn(d) is a calculation parameter for anisotropic electron fluence between the air cavity and the degree of anisotropy given by radiation energy and material dependency factor (ω). The parameter value ω can be seen in Table VI IAEA TEC-DOC 1274. The parameters (d) and A'pn(d) were form function factors of cylindrical ionization chambers, σ=Rc/Lc, and distance factor α=Rc/d. Rc is the internal radius of the chamber, and Lc is the dimension of half of the internal length of the ionization chamber, while (d) is the measurement distance. Based on the information on the radius and width of the chamber, conformity can be found in Table VII and Table VIII of IAEA TEC-DOC 1274. Kn parameter obtained after Apn(d) get from EQUATION (3).
Some publications [18][19][20] state that to calculate kscatt can use the 7 distance method published by Goestch et al. [21]. The 7 distance method uses an ionization chamber to measure the charge reading at several distances. The purpose of this method was to evaluate the scattering that occurs at each charge reading at each measurement point. The method was still used and developed for the measurement of brachytherapy sources.
Here the description of the constant used in EQUATION (4)

MEASUREMENT USING WELL-TYPE IONIZATION CHAMBERS
The principle of measurement with this method was that the charge reading collected by the well-type ionization chamber was corrected and multiplied by the chamber calibration factor. The value of the chamber calibration factor used was traced to the University of Wisconsin Accredited Dosimetry Calibration Laboratory traced to the National Institute of Standards and Technology (NIST). Here the equation for calculating air Kerma rate using an ionization chamber. = . .
Here the description of the parameters in EQUATION (5), RAKR is the air kerma rate, which in the brachytherapy modality is called air Kerma strength, Mstd is the reading of a standard well type chamber (nC), NAKRSTD is the value of the calibration factor of a standard well-type ionizing chamber (Gy m 2 h -1 A -1 ), KPT is a correction factor for temperature and room air pressure.
Here the description of the parameters in EQUATION (6), KPT is the correction factor for temperature and room air pressure, T is the measured temperature ( o C), T0 is the reference temperature (20 o C), P is the measured pressure (kPa), P0 is the reference pressure (kPa).

MATERIALS
The radiation source used in this research was the Co-60 brachytherapy Saginova afterloading machine from the manufacturer Eckert & Zeigler BEBIG GmbH. This machine was equipped with 25 channels for loading the brachytherapy source. For Co-60 source brachytherapy seed from the Saginova machine has dimensions of 3.5 mm x 1.0 mm, while for seed source Ir-192 has dimensions of 3.5 mm x 0.9 mm.
There were several ionization chambers used in this study. Ionization chambers 0.6 cm 3 , 30 cm 3 , 800 cm 3 and well-type ionization chambers. The 0.6 cm 3 ionization chamber used was the PTW chamber TW30013 serial number 6367 made by the German PTW manufacturer Freiburg. The 30 cm 3 ionization chamber was also used from the German Freiburg PTW manufacturer with a type 23361 cylindrical stem chamber. For ionizing chambers with a volume of 800 cm 3 was used an ionization chamber from the manufacturer of Standard Imaging, USA, with Exradin A6 type. These three chambers have traceability factor calibration to the Co-60 source. The use of ionization chamber volume 800 cm 3 was based on the reference [15], which used the LS-01 volume 1000 cm 3 for measurement of the brachytherapy source.
The well-type ionization chamber used was the HDR1000Plus well-type ionization chamber from the manufacturer of Standard Imaging, USA. This well-type ionization chamber has a traceability factor calibration to the source Ir-192. This well-type ionization chamber was calibrated in terms The electrometer used to read the charge reading from the ionization chamber was the PTW Unidos Webline electrometer.

EXPERIMENTAL SETUP
Measurement of air Kerma using an ionization chamber was done using a custom jig for positioning of the ionization chamber. A custom jig image can be seen in FIGURE 1 below.
(a) (b) (c) After the chamber was positioned parallel to the source of the brachytherapy, a scanning step was performed to determine the maximum response position of the reading of each ionization chamber. At the maximum point of the response, the measurement of air Kerma was carried out with five repetitions of data.
Measurement using a well-type ionization chamber was done by setting the chamber positioning on a wooden table. It is with a minimum distance of 1 meter from the floor and the wall. It aims to reduce the effect of scattering [12]. The placement of the well-type ionization chamber can be seen in Same with before, the first step was scanning the maximum response at a depth of the welltype ionization chamber [11]. Scans were performed in a well-type ionization chamber at each step according to the brachytherapy machine. After getting the maximum response position, measurements were taken at that position with five repetitions. Temperature and pressure conditions in the room were also calculated as a correction of temperature and pressure (kPT).

RESULT AND DISCUSSION
The results of measuring the maximum response of the 0.6 cm 3 ionization detector and the well-type ionization detector can be seen in FIGURE 3. Based on these measurements, the maximum response of the 0.6 cm 3 ionization detector at step 85, while for the well-type ionization detector at step 54. Then, the measurement was done in this step for each chamber, perspectively. Based on FIGURE 3, the results of the normalization of responses obtained from the two chambers appear to be different. This was due to differences in the detector volume and the nominal response of each ionization chamber. Besides, the measurement of the response by the 0.6 cm 3 ionization chamber was carried out in the air using a custom jig whose reading results have not been corrected for radiation scattering. Different from the well-type ionization chamber, which has been designed in such a way as to measure the source of the brachytherapy.
The results of measurements of air Kerma using several ionization chambers can be seen in  In measuring the RAKR of the brachytherapy source, the important thing to consider was the scattering correction factor in the measurement. As explained earlier, this research used an approach method for calculating scattering correction factors using analytical methods from Liyun Chang et al. [22]. The room area of the brachytherapy installation used was 5. It was known that value of the RAKR in TABLE 1 has a deviation that varies with the value of the RAKR from the TPS. The deviations were obtained 1.7%, -5.4%, and -8.0% for the 0.6 cm 3 , 30 cm 3 and 800 cm 3 ionization chamber, respectively. The acquisition of the RAKR was measured by a well-type ionization chamber in TABLE 2 gets a deviation of -5.3% of the value of the RAKR from the TPS.
The result of RAKR measured by ionization chambers 30 cm 3 and 800 cm 3 obtained a deviation which was quite significant to the value of the RAKR from TPS, while for the deviation result 0.6 cm 3 ionization chamber to the value of RAKR from TPS was 1.7%, which was still within the acceptable range of ±5%. The radiation quality correction factor (kQ) was calculated from the ratio between the RAKR of the ionization detector having traceability of the Co-60 source calibration factor and the RAKR value of the detector having traceability of the Ir-192 source calibration factor. Based on the RAKR data from TABLE 1, the RAKR values of 0.6 cm 3 ionization chamber that were still within the acceptable range. Then these values were compared with the RAKR values of the well-type ionization detector (TABLE 2). We obtained the result of the comparison of 1.07. Henceforth, the calculated value of the radiation quality correction factor is called kQcal.
Andreas Schűller, et al. [15] in their research obtained the kQ value for a well-type ionization detector with the PTW brand Tx33004 was 1.19. While for the HDR 1000Plus ionization detector the kQ value was 1.05. The value of kQ 1.05 was only used for well-type ionization detectors with HDR 1000 Plus type made from Standard Imaging USA. Henceforth, the value of the radiation quality correction factor from the reference is called kQreff. If the kQcal value of 1.07 was compared to the kQreff value of 1.05, then there was a 2.2% deviation.
The implementation of the radiation quality correction factor (kQ) was done by multiplying the RAKR obtained by the HDR1000Plus/A152152 well-type ionization detector can be seen in TABLE 3. The implementation of the kQ value was carried out on the measurement of Co-60 brachytherapy RAKR in two different brachytherapy facilities.  Based on TABLE 3, it was known that the results of the kQCal implementation results obtained a maximum deviation of 1.7%. According to reference [15] in the use of kQ for determining the RAKR of the Co-60 brachytherapy source, it has an uncertain uncertainty of 3%. Based on these results, the use of kQ for measurement of Co-60 brachytherapy source with ionization chambers traced to the calibration factor of the Ir-192 brachytherapy source can be implemented. In addition, the RAKR reference value provided by the vendor has an expanded uncertainty (UEXP) of 5%. So with a maximum deviation of 1.7% was still in the acceptable range.
The measurement was held at RS-A and RS-B, which were they have the same facilities as Co-60 brachytherapy. Although the brachytherapy afterloading machine came from the same manufacture, the source activity and the dose rate will not same for the day measurement. Also, the geometry of the facilities (bunker). It will impact the performance of the measurement. So it became an excellent facility to take the measurement for research purposes.