Development of System for on-site dosimetry audit of High Dose Rate Brachytherapy

doi:10.21203/rs.3.rs-5063848/v1

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Development of System for on-site dosimetry audit of High Dose Rate Brachytherapy

https://doi.org/10.21203/rs.3.rs-5063848/v1

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Background: Various international institutions established key quality assurance guidelines for radiation therapy, focusing on treatment accuracy and equipment reliability. Despite international standards, the lack of an independent audit system in South Korea has led to quality management challenges in high-dose-rate brachytherapy, prompting this study to develop a precise source activity measurement system and regulatory MATLAB code.

Methods: Variations in Remote After-Loading System (RALS) models across institutions are considered, with Standard Imaging source holders and a CDX2000B electrometer used for accurate source strength measurements, focusing on maximum point accuracy. Mechanical accuracy checks involve assessing dwell position accuracy, reproducibility, and offset using self-developed phantoms with in-house MATLAB code, supported by EBT3 film for precise measurement and verification of radiation source positions and offsets.

Results: Measured data present the discrepancy between measured and calculated values, with uncertainty observed below 0.14%. Additionally, The developed in-house MATLAB code for film data analysis, utilizes grayscale window analysis for accurate identification of irradiated points, demonstrating minimal position and step size errors, affirming the precision of the method.

Conclusion: In this study, we developed an audit system integrating a source strength measurement system and mechanical accuracy check phantoms, utilizing a well-type chamber – electrometer setup and in-house MATLAB code. We achieved audit suitability with appropriate uncertainty, compatible across various Remote After-Loading Systems (RALS). Uncertainty assessments resulted in proposed expanded uncertainties (k = 2) of 2.72% and 2.71% for models 70010 and 72280. Based on these results, we plan to implement this audit system nationwide to comply with domestic regulations.

HDR Brachytherapy

On-site dosimetry audit

ISO/IEC17025

Audit Procedure

In 1984, AAPM TG-24 established comprehensive quality assurance guidelines for radiation therapy. It

covered methods, frequencies, and tolerances, focusing on the accuracy of treatment console positioning for intracavitary and high-dose-rate therapy. Daily tests ensured the proper functioning of the indicator gauge, with a reproducibility error set at ± 1 mm. Simple measurements using radiation-sensitive films were recommended. [1]

After the incident involving wire breakage of the source during patient treatment at an Indiana cancer center, led to the documentation of radiation incidents/accidents in NUREG-1480 in 1992. [2] The NRC investigated the quality management procedures for high-dose-rate brachytherapy through the Idaho National Engineering Laboratory in 1994. NUREG/CR-6276, compiled by the NRC through INEL, serves as a report on quality assurance concerning the physical aspects related to safety in the use of remote after-loading high-dose-rate brachytherapy equipment. As radiation therapy and measurement equipment transitioned to digitalization and complex control systems in 1993, AAPM TG-40 issued a comprehensive quality assurance report for advanced equipment in radiation oncology. [3] In the interest of the patient safety, periodic and appropriate monitoring for radiation therapy equipment is recommended by internationally accredited organizations. [4] In the Republic of Korea, regulations similar to aforementioned recommendations are introduced in "Article 52, Paragraph 1 of the Radiation Regulations (Quality Management of Medical Radiation)."

However, the absence of an accredited audit system leads to confusion regarding the independent assessment of medical radiation quality management, which is essential for ensuring the safety of patient treatment. For high-dose-rate brachytherapy, verification of the positional accuracy of the moving radiation source through catheter guides is crucial and source strength measurements are conducted using an electrometer and well-type chamber. Considering certificate from vendor represents source strength value with uncertainty of \(\:\pm\:\)5%, end-user should compare with their own measurement systems. Research by Nair, M.T.B. et al., and Nappan et al. has shown potential discrepancies between vendor-provided values and actual source strength. [5, 6] To address this situation, various studies have been conducted to ensure the reliability of brachytherapy equipment. This includes attempts that comparing reference air kerma rate values using a well type chamber and audits using multiple dosimeter like optically stimulated luminescence dosimeter and radiochromic film. Additionally Okamoto H et al. focused on methods for mechanical accuracy with radiochromic film. [7–9]

But neither considered mechanical accuracy with source activity in the context of audits, may correlated each other. Therefore in this study, we aim to establish and evaluate the characteristics of the source activity measurement system, design a mechanical accuracy phantom, and develop in-house MATLAB code for application in regulations within our nation.

2.1. Well type chamber – electrometer measurement

In account to field investigation, dosage of High-Dose-Rate (HDR) brachytherapy is determined by source strength refer to certification of source vendors and A-Point dose. When external audit results do not satisfy criteria, it is may inherent that incorrect activity value or inept manipulation of Treatment Planning System (TPS). So, we establish a on-site dosimetry audit system based on that scenario. Simultaneously, the on-site audit system should consider the difference of Remote After-Loading System (RALS) model among each institution. For that purpose, we used Standard Imaging source holder model 70010 and 72280 (Standard Imaging, USA) and a comparable electrometer (CDX2000B, Standard Imaging, USA).

Source strength measurement should be taken at the maximum point to minimize measurement uncertainty and maintain an objective position during the audit. [10] To determine the maximum position of the well-type chamber, current measurements are first conducted at every 2.5 mm step. Then more detailed measurements are taken at a 1.0 mm step size around the maximum point.

Considering that the minimum dwell step of domestic brachytherapy devices varies depending on the manufacturer or version, measurements for the maximum position were conducted based on two criteria: a 2.5 mm step size for less fine version and a 1.0 mm step size for finer version. This enables conducting audits that take into account the characteristics of each institution.

Following the determination of the maximum point, an uncertainty check should be conducted on the measurement system. The first day of measurements for each maximum result were considered as the reference, and subsequent measurements were conducted to acquire four types of data varied by the maximum point and source holders. Each data was acquired by averaging five repeated measurements with the source positioned at the maximum point for 45 seconds, and this process was repeated ten times for each measurement. And those all compared with the calculation value calculated from Eqs. (1) and 1st measurement.

\(\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{C}_{i}=\:{M}_{0}\bullet\:{e}^{-\mu\:t}\)			Eq. (1)
Where,
	\(\:{M}_{0}\): 1st Measurement current
	\(\:\mu\:\): 192-Ir, decay constant

Many researchers organized quality assurance (QA) and quality check (QC) items and methodologies including Ion recombination effect, leakage current and so on. [11–13] Following those previous research, we considered Ion recombination effects as follow equation. [14]

\(\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{k}_{ion}=\:\left(\frac{4}{3}-\frac{{I}_{300}}{3\bullet\:{I}_{150}}\right)\)			Eq. (2)
Where,
	\(\:{I}_{300}\): Measured current with 300V
	\(\:{I}_{150}\): Measured current with 150V

2.2. Mechanical accuracy check

In HDR treatment methods, the distribution of radiation doses depends on the location of the RI source which is confirmed by the attending physician through TPS tool. [15] At this time, the decision regarding treatment is determined by confirming the dose distribution of OAR (Organs at Risk) such as the bladder, rectum, etc around treatment area. Two factors influence the accuracy of RI source positioning: dwell position reproducibility and offset. The dwell position denotes the designated point of radiation source, serving the purpose of prescribed dosage. Meanwhile, the offset refers to the distance between catheter end and source midpoint, ensuring collision prevention during its movement to the first dwell position. [7] Taking these factors into consideration, we designed and evaluated a phantom that can verify dwell position and offset: Lumencath phantom and offset phantom.

2.2.1. Lumencath phantom

Figure 1 Here

Lumencath catheter (Elekta Stockholm, Sweden), widely recognized and adopted by numerous institutions, is selected for the insertion of mechanical accuracy check phantom due to its commendable versatile. For inspection of dwell position, in-house-MATLAB code and Lumencath phantom are developed as we can see in Fig. 1. EBT Strip film (Elekta, Stockholm, Sweden) is used to measure radiation source position. After irradiation, first dwell position: 150cm, 1.0cm step size, and 11 dwell point, the analog film data is converted to digital data through EPSON-10000 XL film scanner (EPSON, NAGANO, Japan). Various analysis method and scanning conditions were tested to find the optimal method. Measurements were performed a total of 5 times, using a 3.17 Ci 192-Ir source. RI source is designated at each dwell position of 10 seconds to evaluate position and step size using our institution's RALS and the developed phantom.

2.2.2. Offset phantom

Figure 2 Here

To ensure the availability of offset data from the vendor, an in-house MALTAB code also developed. The built-in date in the Oncentra software (Elekta, Stockholm, Sweden) serves as a reference for comparison. Film scan data was obtained using 1.5 cm × 2.6 cm EBT3 film (Ashland Specialty Ingredients, Bridgewater, USA) and EPSON-10000 XL. By precisely aligning each end of the catheter and film, the first dwell position, representing the furthest position of the catheter, was irradiated (Fig. 2 (a)). Then offset was measured by taking a distance between “no signal region (in view of gray scale ratio: 0) and maximum point (in view of gray scale ratio: maximum), (Fig. 2 (b)).

3.1. On-site dosimetry audit system measurement

Figure 3 and Table 1 show measured data. there was a difference of 2.0 and 2.5 mm between the two source holder models, with measurements of 52 mm and 54 mm from the bottom for the finer step size, and 52.5 mm and 55 mm for the less fine step size, respectively.

Table 1

A portion of the measured data identified the maximum points (bold) at 52 mm and 54 mm from the bottom. The 3rd and 5th columns represent the percentage difference from each maximum point.
Distance form bottom	Model 70010	Percentage difference	Model 72280	Percentage difference
57.5 mm	\(\:-87.844\pm\:0.047\) nA	0.279%	\(\:-88.041\pm\:0.013\) nA	0.060%
55 mm	\(\:-87.985\pm\:0.048\) nA	0.119%	\(\:-88.091\pm\:0.016\) nA	0.003%
54 mm	N/A	N/A	\(\:-88.094\pm\:0.010\) nA	0.000%
53 mm	N/A	N/A	\(\:-88.065\pm\:0.006\) nA	0.033%
52.5 mm	\(\:-88.061\pm\:0.043\) nA	0.033%	\(\:-88.075\pm\:0.011\) nA	0.022%
52 mm	\(\:-88.090\pm\:0.002\) nA	0.000%	N/A	N/A
51 mm	\(\:-88.055\pm\:0.005\) nA	0.040%	N/A	N/A
50 mm	\(\:-88.058\pm\:0.042\) nA	-0.036%	\(\:-87.989\pm\:0.012\) nA	0.119%
47.5 mm	\(\:-87.987\pm\:0.037\) nA	0.117%	\(\:-87.830\pm\:0.012\) nA	0.300%

Figure 4 and Table 2 show the result of the difference between measured and calculated values. Uncertainty has been observed only under 0.14%. Additionally, the factor, k_ion, considering the ion recombination phenomenon based on the voltage of the well-type ionization chamber, was measured and calculated as shown in Table 3.

Table 2

Source strength measurement variations in maximum point and source holders compared with calculation value
	Model 70010 (less fine)	Model 72280 (less fine)	Model 70010 (finer)	Model 72280 (finer)
2024-02-18	0.07%	0.13%	0.04%	0.03%
2024-03-03	0.17%	0.08%	0.02%	0.09%
2024-03-16	0.08%	0.06%	0.06%	0.06%
2024-03-30	0.16%	0.17%	0.00%	0.16%
2024-04-13	0.17%	0.09%	0.02%	0.04%
2024-05-06	0.20%	0.10%	0.02%	0.11%

Table 3

Quantification of k_ion factor considering ion recombination phenomenon in well-type Chamber
	Model 70010	Model 72280
150 V	\(\:-67.733\pm\:\) 0.01 nA	\(\:-67.864\pm\:\) 0.02 nA
300 V	\(\:-67.746\pm\:\) 0.02 nA	\(\:-67.886\pm\:\) 0.01 nA
k_ion	1.001	1.001
k_ion.avg	1.001

3.1. Mechanical accuracy

Figure 5 illustrates the developed in-house MATLAB code for analyzing scanned film data. Two options were considered: 'space-frequency reconstruction' and 'grayscale window analysis'. The former automatically identifies average step sizes as shown in Fig. 5 (a) and (b), reconstructing sine waves as depicted in Fig. 5 (c). However, this method proved unsuitable for RALS due to its separated dwell positions, causing non-consistent frequencies. Therefore, the latter option, grayscale window analysis, was pursued. This method smoothens the original scan data to remove unexpected noise. Then appropriate gray scale window identifies irradiated points, analyzing on the x-axis. After analyzing the acquired images and method, it was confirmed that applying the 800dpi gray scale window method demonstrates excellent reproducibility and offers advantages in terms of processing time.

The analysis results are presented in Table 4. For position error, we confirmed an error of \(\:0.339\pm\:\) 0.169 mm while for step size, an error of \(\:0.033\pm\:\) 0.023 mm was observed. However, the experimenter's analysis suggests that there may be position errors under 0.5 mm with the naked eye.

Table 4

Assessment of discrepancy of position and step size with developed phantom and MATLAB code
	1st	2nd	3rd	4th	5th
Position	0.416 mm	0.246 mm	0.597 mm	0.173 mm	0.264 mm
Step size	0.031 mm	0.006 mm	0.069 mm	0.031 mm	0.028 mm

Table 5

Assessment of vendor-offering offset accuracy with developed phantom and MATLAB code
	Cylindrical	Fletcher_tandem	Fletcher_ovoid_R	Fletcher_Ovoid_L
MATLAB	\(\:5.57\pm\:\) 0.21 mm	\(\:6.67\pm\:\) 0.17 mm	\(\:6.31\pm\:\) 0.38 mm	\(\:7.32\pm\:\) 0.28 mm
Elekta	N/A	7.0 mm	6.0 mm	6.0 mm

Table 5 presents the results of our instrument’s offset analysis that satisfies the criteria of 1mm. Except for the Fletcher_ovoid_L, which exhibited a maximum deviation of 1.6 mm compared to the manufacturer-provided data. This confirms the necessity of applying offset analysis to audit all instruments within the system, ensuring the accuracy of patient treatment. We also analyzed the results of five repeated measurements of offset for Lumencath, which is used for source strength measurement and position determination. Specifically, for the purpose of this study, we aimed to calculate the average offset and composite uncertainty of Lumencath using the following Eq. (3) for offset, to derive the composite uncertainty.


\(\:{\sigma\:}_{LUMEN}^{2}=\sum\:_{i=1}^{N}{\left(\frac{\delta\:\overline{x}}{\delta\:{x}_{i}}\right)}^{2}\times\:{\sigma\:}_{{x}_{i}}^{2}\)	Eq. (3)


Where,
\(\:N\)	: Evaluated number of Lumencath:
\(\:\overline{x}\)	: Mean offset of N Lumencath
\(\:{x}_{i}\)	: Mean offset of the i-th Lumencath
\(\:{\sigma\:}_{{x}_{i}}^{2}\)	: Variance of the offset of the i-th Lumencath

Table 6 presents the offset values of four sample Lumencaths, all close to the vendor-certified 10 mm (< 0.5 mm). Considering the results from Tables 4 and 6, we can conclude that uncertainties stemming from mechanical issues are negligible when utilizing Lumencath.

Table 6

Assessment of offset value with developed phantom and MATLAB code
	Lumencath (1)	Lumencath (2)	Lumencath (3)	Lumencath (4)
MATLAB	9.86\(\:\pm\:\) 0.26 mm	9.91\(\:\pm\:\) 0.42 mm	\(\:10.00\pm\:\) 0.37 mm	\(\:10.21\pm\:\) 0.23 mm

4.1. Analysis and uncertainty assessment

In this study, we established an audit system comprising a source strength measurement system and mechanical accuracy check phantoms. Through various measurements, we determined that a well-type chamber – electrometer system is suitable for auditing with around 0.1% uncertainty and is compatible with various institutions' RALS. To directly compare the acknowledged source strength (vendor-certified) with our audit system, Eq. (4) was introduced, taking into account for current-radiation constant, temperature-pressure correction, and ion recombination factor.

\(\:{A}_{measured}\left[Ci\right]=1.146\:\bullet\:{10}^{-1}\bullet\:\left(\frac{273+T}{293}\right)\bullet\:\left(\frac{101.3}{P}\right)\:\bullet\:\:{R}_{avg}\:\bullet\:\:{k}_{ion\bullet\:avg}\)			Eq. (4)
Where,
\(\:T\)	: Surrounding temperature during measurement [K]
\(\:P\)	: Surrounding atmospheric pressure during measurement [kPa]
\(\:{R}_{avg}\)	: Average of three measurements from the electrometer [nA]
\(\:{k}_{ion\bullet\:avg}\)	: Ion recombination correction factor for the well-type ionization chamber

In this process, the current-radiation constant was determined using the values employed for chamber calibration. Also, uncertainty assessment is needed to adapt to regulation in the field. Here, two kinds of evaluation of uncertainty: A Type; certification of chamber, electrometer and B Type, measurement data for current and film data. Ultimately, we aim to propose expanded uncertainty (k = 2) based on A type and B type error. Factors influencing the uncertainty regarding the maximum dose and the assessed uncertainty through experiments to evaluate the uncertainty related to the well type chamber - electrometer measurement system are summarized in Tables 7 and 8.

Table 7

uncertainty of well type chamber and electrometer for less finer case
	Positional reproducibility	Measurement reproducibility	Uncertainty of well type chamber	Uncertainty of electrometer
Model 70010	0.05%	0.14%	1.35%	0.10%
Model 72280	0.05%	0.11%	1.35%	0.10%
Uncertainty Type	A	A	B	B
Expanded uncertainty (k = 2)		Model 70010		2.72%
Expanded uncertainty (k = 2)		Model 72280		2.72%

Table 8

uncertainty of well type chamber and electrometer for finer case
	Positional reproducibility	Measurement reproducibility	Uncertainty of well type chamber	Uncertainty of electrometer
Model 70010	0.05%	0.03%	1.35%	0.10%
Model 72280	0.05%	0.08%	1.35%	0.10%
Uncertainty Type	A	A	B	B
Expanded uncertainty (k = 2)		Model 70010		2.71%
Expanded uncertainty (k = 2)		Model 72280		2.71%

For positional reproducibility, we adopted a conservative value of 0.05%. Table 1 illustrates that maintaining a constant measurement within a range of ± 5 mm at maximum value, yields a difference up to 0.04%. The combined expanded uncertainty (k = 2) calculated according to the law of uncertainty propagation is evaluated as 2.72% and 2.71% for each source holder model.

In terms of mechanical accuracy, the designated phantom accurately identifies the irradiated source position and calculates several intended features even more accurately than the experimenter's naked eye. Particularly, discrepancies were observed between the vendor-offered data and the measured offset. Based on these results, we confirmed the necessity of analysis certified by an accredited institution for each institution. For the purpose of audit criteria, we suggest the following specific values as Table 9.

Table 9

Mechanical accuracy tolerance
	Positional reproducibility	Step size	Offset
Tolerance	1 mm	0.1 mm	1 mm

The reproducibility of the source position and the accuracy of step size and offset were determined by considering experimental results, domestic regulation, and international organization recommendations [3, 16].

4.3. Future Directions and Utilization Plans

The measurement system, composed of a well-type ionization chamber and a mechanical accuracy measurement phantom has been developed and evaluated, adhering to the verification requirements outlined in Korea Nuclear Safety Commission Notification No. 2019-6, Article 5, for independent quality audits.

We have validated the on-site dosimetry audit systems' uncertainties. Based on these findings, we plan to implement the audit system for a nationwide survey of high-dose-rate brachytherapy institutions, contributing to the identification of equipment-specific errors for incorporation into treatment planning.

Secondly, through a comprehensive survey of institutions utilizing high-dose-rate brachytherapy, we aim to refine the established system and incorporate feedback from users and experts.

Annually, in collaboration with national standard organizations, primary standard laboratories (PSDL), and the International Atomic Energy Agency (IAEA), we plan to operate an independent quality audit system certified under KOLAS international standards and following IAEA/WHO SSDL guidelines.

Funding

This study was supported by a grant of the Korea Institute of Radiological and Medical Sciences (KIRAMS), funded by Ministry of Science and ICT (MSIT), Republic of Korea (No. 50572 − 2024) and by the Nuclear Safety Research Program through the Korea Foundation Of Nuclear Safety (KoFONS) using the financial resource granted by the Nuclear Safety and Security Commission (NSSC) of the Republic of Korea (No. 2103088).

Author Contribution

Data curation: Hyeok Jun GwonFormal analysis: Wonho Lee, Sang-Hyoun Choi, Soon Sung LeeFunding acquisition: Kum Bae KimMethodology: Kum Bae Kim, Gyu-Seok Cho, Soon Sung LeeResources: Gyu-Seok ChoSoftware: Hyeok Jun GwonSupervision: Kum Bae KimValidation: Kum Bae KimVisualization: Soon Sung Lee Writing – original draft: Hyeok Jun GwonWriting – review & editing: Wonho Lee, Sang-Hyoun Choi, Sun-Boong Hwang

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의료분야의 방사선안전관리에 관한 기술기준, 제 5조 3항, 원자력안전위원회고시 제2019-6호, 2019. 5. 10.

No competing interests reported.

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You are reading this latest preprint version

Development of System for on-site dosimetry audit of High Dose Rate Brachytherapy

Status:

Version 1

Abstract

Figures

1. INTRODUCTION

2. METHODS

2.1. Well type chamber – electrometer measurement

2.2. Mechanical accuracy check

2.2.1. Lumencath phantom

2.2.2. Offset phantom

3. RESULT

3.1. On-site dosimetry audit system measurement

3.1. Mechanical accuracy

4. DISCUSSION

4.1. Analysis and uncertainty assessment

4.3. Future Directions and Utilization Plans

Declarations

Funding

Author Contribution

References

Additional Declarations

Status:

Version 1