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| ORIGINAL ARTICLE |
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| Year : 2015 | Volume
: 38
| Issue : 4 | Page : 144-150 |
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Brachytherapy source calibration, reviews, and consistency of 192Ir high-dose rate afterloading sources supplied over the period of 10 years: A retrospective analysis
Balasubramanian Nagappan, Yogesh Kumar, Narayan P Patel, Anil Kumar Dhull, Vivek Kaushal
Department of Radiotherapy, Regional Cancer Centre, Pt. B.D. Sharma Post Graduate Institute of Medical Sciences, Rohtak, Haryana, India
| Date of Web Publication | 11-Feb-2016 |
Correspondence Address:
Balasubramanian Nagappan
Department of Radiotherapy, Regional Cancer Centre, Pt. B.D. Sharma Post Graduate Institute of Medical Sciences, Rohtak, Haryana
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0972-0464.176158
Measurement and verification of strength of monomodal high-dose rate (mHDR)192 Ir source supplied by the vendor is a major part of quality assurance program. Reference air kerma rate (RAKR) or air kerma strength (AKS) is the recommended quantity to specify the strength of gamma emitting brachytherapy sources. Physicist in our institution performed the source calibration as soon as each 192 Ir new source was loaded on the mHDR afterloading machine. The AKS accurately measured using a physikalisch technische werkstatten (PTW) re-entrant chamber-electrometer system in a scatter-free geometry was used to compute the air kerma rate (AKR) at one-meter distance in the air. To ensure accurate dose delivery to brachytherapy patients, measured AKS or RAKR should be entered correctly in both HDR treatment console station (TCS) as well as treatment planning system (TPS) associated with it. The clinical outcome mainly depends not only on the accuracy of the source strength measurement in the hospital but also on the correct source strength entered into both TCS and TPS software. A retrospective study on 22 mHDR V2 sources supplied by the vendor for the period of 10 years was taken up to access the accuracy of source strength supplied to the Radiotherapy department. The results are analyzed and reported. The accuracy in measured RAKR of all 22 sources supplied by vendor was well within the tolerance limits set by the national regulatory body and international recommendations. The deviations observed between measured RAKR versus manufacturer's quoted RAKR were in the range from −1.71% to +1.15%. In conclusion, the measured RAKR have good agreement with vendor quoted RAKR values. Keywords: Air kerma strength, brachytherapy, high-dose rate, quality assurance, reference air kerma rate, source calibration, treatment console station, treatment planning system
How to cite this article:
Nagappan B, Kumar Y, Patel NP, Dhull AK, Kaushal V. Brachytherapy source calibration, reviews, and consistency of 192Ir high-dose rate afterloading sources supplied over the period of 10 years: A retrospective analysis. Radiat Prot Environ 2015;38:144-50 |
How to cite this URL:
Nagappan B, Kumar Y, Patel NP, Dhull AK, Kaushal V. Brachytherapy source calibration, reviews, and consistency of 192Ir high-dose rate afterloading sources supplied over the period of 10 years: A retrospective analysis. Radiat Prot Environ [serial online] 2015 [cited 2022 Jul 5];38:144-50. Available from: https://www.rpe.org.in/text.asp?2015/38/4/144/176158 |
| Introduction | |  |
Brachytherapy is one of the modalities for the treatment of cancer almost in all sites either alone or in combination with external beam radiotherapy. High-dose rate (HDR) remote after loading brachytherapy units are becoming more common not only worldwide but in India also. HDR brachytherapy treatment has an extensive track record of efficacy.[1],[2] Brachytherapy is done by using encapsulated radioactive sources say 137 Cs,192 Ir, and 60 Co to deliver a high dose to tissue near the source.[3] For brachytherapy HDR application,192 Ir HDR sources are commonly being used. More than 150 hospitals in India are presently using HDR afterloading treatment systems. The quantity used to specify brachytherapy source strength of gamma emitting sources is the air kerma strength (AKS) defined by the American Association of Physicists in Medicine (AAPM) as “the product of air kerma rate in free space and the square of the distance of the calibration point from the source center along the perpendicular bisector.” The unit recommended for source strength by AAPM is µGym 2/h. The International Commission on Radiation Units Report 38[4] defined the reference air kerma rate (RAKR) as an emission specification quantity for brachytherapy sources emitting gamma rays. This was first recommended by the French committee for the measurement of ionizing radiation in 1983[5] then by the British Committee on Radiation Units and Measurements in 1984[6] and has been widely established in Europe.[7] The reference quantity recommended for the specification of brachytherapy gamma source is RAKR or AKS defined as the kerma rate to air, in air, at a reference distance of one meter, corrected for air attenuation and scattering.[4],[8],[9] This quantity RAKR is defined differently but numerically equals the AKS defined by AAPM. The British Commission on Radiation Units and Measurements (1984),[6] the Netherlands Commission on Radiation Dosimetry,[10] National Institute of Standards and Technology USA and Bureau International des Poids et Mesures have recommended the practice of specifying the source strength in terms of air kerma rate at the reference distance. The commercially available 192 Ir source supplied to us comes with the certified calibration from the manufacturer. The manufacture's certificate states the RAKR in mGy per hour at 1 m with the uncertainty of ±5%. The SI unit for the RAKR is Gray per se cond, but it is not practical for the sources used in brachytherapy. For practical purposes, RAKR should be expressed either in mGy per hour at 1 m or cGy per hour at 1 m.[9],[11] According to national regulations [12] and international recommendation [11] on the quality assurance test in HDR brachytherapy, the determination of RAKR of a new source shall be carried out prior to its use for the first clinical application.[9],[13],[14],[15],[16],[17] Our clinical physicists carried out the source calibration measurements after each loading of new 192 Ir brachytherapy source in the monomodal HDR (mHDR) after loading machine. Many measurements have been reported at international level where the deviations are more than ±5%.[18],[19],[20],[21] Nowadays, well-type chambers are preferred for the routine calibration of HDR Ir-192 sources because of its ease, precision in use, fastness, and reproducibility of source positioning. The independent verification, as well as measurement of vendor, supplied 192 Ir source strength is quite important to ensure the delivery of intended dose to brachytherapy patients. The retrospective study was carried out on 22 mHDR V2 192 Ir sources supplied over the period from 2005 to 2014 to assess the accuracy and consistency of mHDR 192 Ir source supplied to the Department of Radiotherapy of our Institution.
Radiation protection
Before commissioning the mHDR machine for patient treatment, we ensured that patient, staff members and public are well-protected from the radiation safety point of view. The brachytherapy room was approved by the competent authority, i.e. Atomic Energy Regulatory Board (AERB). Stringent radiation safety procedures recommended by the AERB are followed for radiation protection of the patient, staff, public, and other personnel during measurements and treatment of patients. The source ON position radiological survey of the brachytherapy installation and OFF position radiological survey around the mHDR unit were carried out initially before the starting the acceptance tests of the machine. To check the integrity of the room and machine shielding, the ON position and OFF position radiological surveys were conducted initially and followed by at regular intervals. ON position dose rate measurements at various locations around the installation are well within the limits prescribed by competent authority (AERB). OFF position dose rate measurements around the machine are also well within the limits prescribed by the competent authority. No staff members received over exposure during the last 10 years because no radiological emergency occurred in the Department of Radiotherapy of this Institution (e.g., source getting stuck during treatment or measurement).
| Materials and Methods | | |
A Microselectron HDR brachytherapy machine (Nucletron, Mallinckrodt Medical B.V., The Netherland) having a maximum source capacity of Iridium-192 of 442GBq (12Ci) was installed at our Institution and commissioned for patient treatment on January 11, 2005. The half-life of 192 Ir is 73.83 days,[22],[23],[24] requiring relatively frequent source replacement after 4–5 months to maintain short treatment times. The accuracy of the brachytherapy dose delivery depends upon the dose measuring device, calibration factor of the device provided by national standard laboratory, measurement set-up and the methodology adopted to measure the RAKR. There are three principal methods of calibrating brachytherapy sources; among these, either the RAKR or AKS measured at a distance of 1 m is recommended.[8],[25] The most frequently used method employs a calibrated re-entrant type (well-type) ionization chamber. The second method makes use of an ionization chamber [22], 23, [26],[27],[28] (e.g., 0.6 cc farmer type chamber) to measure the air kerma rate at a known distance from the source. The third method uses a solid phantom into which sources and ion chamber can be introduced in a convenient and reproducible way. Among these, satisfactory results can be obtained from re-entrant ionization chamber and solid phantom methods.[3],[29]
The Nucletron Source Dosimetry System comprises of vented well-type ionization chamber, electrometer, coaxial cable, and source holder specially designed for calibration of the 192 Ir source of microSelectron HDR after loader. Source calibration measurement was performed using calibrated well-type ion chamber/electrometer (Type TM33004, Serial Number 0049 with electrometer PTW UNIDOS E/10008–80233) from PTW, Germany. The chamber is specially designed for brachytherapy source calibration and is meant for measuring the RAKR of HDR sources. The chamber with electrometer has a calibration factor traceable to National Standard Dosimetry Laboratory (NSDL), Radiation Standards Section, BARC, Mumbai. Calibration was done routinely, i.e., once in 3 years as per national regulations. [Table 1] shows the detailed specifications of ion chamber used. The calibration factor supplied by NSDL has a standard uncertainty of ±3% (k = 2) at the confidence level of 95%. The certificate supplied by the manufacturer states the value of RAKR in mGy per hour at 1 m with an expanded uncertainty of ±5% (k = 3) at the confidence level of 99.7%.
The source type supplied by Nucletron is Microselectron V2, encased in stainless steel. This miniature source has capsule dimensions of 0.9 mm diameter, 4.5 mm length and source pellet dimensions of 0.65 mm diameter, 3.60 mm length. The material and dimensional details of the Ir-192 HDR brachytherapy source pellets are given in [Table 2]. [Figure 1] shows the well-type ionization chamber and the dedicated source holder/adaptor used for source positioning/calibration. When this holder is kept inside the chamber, the source will be accurately centered in the middle of the measuring volume.[26] The measured source strength of 22 Ir-192 sources over the period of 10 years were reviewed retrospectively and compared with stated values of the manufacturer. | Figure 1: The electrometer and well-type chamber along with dedicated source holder (insert)
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The measurement procedures for the estimation of RAKR using the chamber have been described by several authors. As recommended, we have used chamber vented to atmosphere.[8],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26] The chamber with electrometer were kept energized at one corner of the brachytherapy room and maintained overnight to attain thermal equilibrium and electronic stabilization. Next day, a polarizing voltage of +400 V was applied to the chamber as recommended by NSDL and the dosimetry system allowed sufficient minimum warm-up time of 45 min. After the installation of the new source in the microSelectron HDR afterloading system, the chamber was placed on the treatment table in such a way that its distance from the floor and all side walls of the brachytherapy room was at least 1 m, so that the contribution to the chamber current from room scatter was negligible. One end of the gynecological transfer tube was connected to source holder that was kept inside the well of the ion chamber, and other end of the transfer tube was connected to the first index of mHDR unit. [Figure 2] shows the routine measurement set-up. The dosimeter was switched to the current mode, and high range was selected. According to the supplier and from our experience with the system, the maximum response would occur at 21st dwell position. The source was programmed to dwell in this position for a period of 300 s and HDR treatment simulation executed before taking any measurements. This execution helps the source and dosimetry system to settle down. The polarizing voltage used was the same as used by NSDL. The sensitivity varies along the long axis of the chamber. As recommended by NSDL, the point of maximum response of the chamber was found by moving the source in small steps of 2.5 mm in the region of maximum sensitivity along the axis of the chamber. In practice, ionization currents for various dwell positions from 16th to 26th were recorded. A set of three measurements was taken at each of these dwell positions and the readings were tabulated. [Figure 3] shows the response curve of the chamber at various dwell positions. The maximum response point (21st position) can be seen in [Figure 3]. With the source positioned at this 'sweet spot', the chamber current is maximized and the uncertainty in the RAKR determination, due to positional variations, is minimized. The measurements were therefore carried out at this position for a dwell time of 60 s, and current readings in nanoamperes (nA) were recorded. The measurements were repeated for 5 times and each time the readings were tabulated. These five readings should be within ±0.3%[3] of the average reading. The average of two sets of readings measured was found to be within ±0.5%.[3] The average ionization current has been used to find the RAKR of the source using equation 1 as mentioned below. The temperature and pressure of the treatment room before and after measurement were recorded. To maintain the same geometry, we have positioned each of all 22 sources at this point during source calibration during the past 10 years. It should be noted that we chose the current mode in preference to the charge mode to avoid extraneous currents generated in the chamber during source transit [3] (if charge mode is used, these currents will be integrated). The manufacturer's RAKR value were corrected backward for the initial source decay and were compared with measured values. The deviations observed were typically within ±2%. | Figure 2: Measurement set up for routine source calibration of monomodal high-dose rate Ir-192 source
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 | Figure 3: Response curve of Nucletron Source Dosimetry System well-type chamber with respect to source dwell positions from the bottom of ionization chamber
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Reference air kerma rate computation
This quantity was calculated using the following formulae used by numerous other investigators [13],[14], [15,[30] and subsequently adopted by various institutions, including ours.
RAKR (mGy/h) = MR × NRAKR × Kion × KTP × KP (1)
Where:
MR is the average ionization current in nA of all five dosimeter readings, corrected for the leakage current of the electrometer.
NRAKR is the calibration factor of the ionization chamber provided by the NSDL, (expressed in Gym 2/h/nA) at a reference temperature and pressure (20°C and 1013.2 mbar, respectively).
Kion is the ion recombination correction factor (correction factor for ion recombination losses in the ionization chamber), computed using the two-voltage technique.[31],[32] as follows:
Where I400 is the average maximum ionization current measured at the polarizing voltage of +400 V.
And I200 is the average maximum ionization current at a voltage of +200V.
KTP, the air density correction factor (for temperature and pressure) as determined using the following formula as per technical reports series (TRS) 277.[32]
Where T and p, are average room temperature (degree centigrade) and atmospheric pressure (expressed in millibars) measured before, the middle of the measurement and after measurement respectively; To is the reference temperature, and 1013.2 mbar is the standard pressure.
kpol is the correction factor for polarization effect of the ionization chamber as determined using the following formula in TRS 277.[32]
Where modulus of M+ is average current reading when the bias voltage applied is +400 V and modulus of M- is average current reading when the bias voltage applied is negative 400 V.
Evaluation of uncertainties
The evaluation of uncertainty in calibration measurements in the present study was carried out as per the EQ-4/02 1999 draft.[33] The sources of uncertainty in this measurement may arise from the chamber, electrometer, positioning error, variations in temperature and pressure and primary calibration of the chamber. The overall uncertainty in the calibration at NSDL has been quoted as ±3% at the 95% confidence limit with a coverage factor of k = 2 with a standard deviation 1.5%. Our measurement shows reproducible reading at the position of maximum response. We looked at all possible uncertainties and estimated the magnitude of each. [Table 3] shows the overall expanded uncertainty in our measurement is ±3.12% at the 95% confidence limit with a coverage factor of k = 2. It is worth mentioning here that, NIST, USA offers calibration for well-type ion chamber, with the uncertainty estimated to 0.8% based on standard uncertainty multiplied by a coverage factor k = 2, at 95% confidence level.[34] [Table 3] overall expanded uncertainties in our measurement using well-type chamber. | Table 3: Overall expanded uncertainties in our measurement using well-type chamber
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| Results | | |
As per AAPM recommendations, the source calibration measurements were carried out every time the HDR 192 Ir source was exchanged/replaced. The axial response curve of the chamber at various source dwell positions were investigated each time the source was supplied under the same conditions of the scatter-free environment. It was also confirmed that the point of maximum response of the curve obtained from our measurements also tallies with that given by the manufacturer. From our measurements, the point of maximum occurs 84.5 mm below from the chamber top. [Table 4] shows that the percentage of variation between the measured RAKR versus quoted RAKR values ranges from −1.71% to +1.15%. This is also shown in the graph below the table. Among 22 mHDR sources, it can be observed that only 7 sources have deviations more than 1% and all other 15 sources have deviations within ±1%; further, all deviations were well within the tolerance limits prescribed by national competent authority and international recommendation. As per the source supplier, the recommendation is that either their certificate value or the user's measured value may be used if they agree within ±2%.[8] However, AAPM Report 56 recommends that calibration be performed by a qualified medical physicist and that user measured source strength in RAKR should be used as the basis for treatment planning and prescription. | Table 4: Measured reference air kerma rate, quoted reference air kerma rate by vendor and the respective percentage variations
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| Conclusion | | |
Our studies show that the deviations between measured and quoted AKS values of 22 sources of brachytherapy Ir-192 sources are well within the limits (±3%) prescribed by competent authority of India (AERB).[12] It has been strongly emphasized by various national and international bodies that the mHDR 192 Ir supplied by the vendor must be evaluated prior to clinical use. Based on our experience, either vendor's RAKR or measured RAKR may be used in our HDR treatment console station and the Plato treatment planning system since the deviations are less than ±2%. Hence, this retrospective study serves not only to verify the source strength supplied by the manufacturer but also ensures quality in brachytherapy treatment. The procedure used to find the RAKR is traceable to NSDL. To improve the brachytherapy treatment accuracy, it is strongly recommended that the user of mHDR after loading machine should carry out independent source calibration at the time of source replacement. Besides, for routine calibration of these remote afterloading systems, well-type ionization chamber is recommended because it is easy to use, safe, reproducible, and reliable.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]
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