The International Atomic Energy Agency has mentioned that the dose of therapeutic radiation administered to the patient’s tumor, or the target volume, should be within the uncertainty of 5%, while the International Commission on Radiation Units and Measurements (ICRU) has stated that the radiation therapies require the uncertainty of 2%. Despite various opinions as to whether the mentioned above uncertainty is within a 95% confidence level, the conclusion drawn from several studies is that the combined standard uncertainty of the administered dose should be less than 3.3% to 3.5% for some tumors [1].

We expected that the dosimetry quality for the therapeutic radiation must be maintained systematically and consistently to deliver the precise dose to the patient as prescribed within the acceptable uncertainty. By article 5, paragraph 1 in the Nuclear Safety Commission Notice No. 2019-6 “Technical standards for radiation safety management in the medical field,” a medical institution shall establish the documented dosimetry quality management procedures to confirm that the dose delivery to the patient shall be maintained as prescribed and perform the quality management process accordingly. The quality management components in the documented procedures are listed in Article 5 Paragraph 2 and included the procedure for undergoing the independent dosimetry audit every three years. The dosimetry audit ensures that the dosimetry quality management system (QMS) of the medical institution set up under the Nuclear Safety Commission Notice is operated properly. The dosimetry audit shall be undertaken independently, impartially, and competently as intended. However, no requirement is imposed on the dosimetry audit service provider in Notice No. 2019-6, so anyone can provide the dosimetry audit service under the current legal framework.

The Korea Research Institute of Standards and Science (KRISS), as a primary standard dosimetry laboratory and as the representative institute of national measurement standards designated by Article 13 in the Framework Act on National Standards, has established the national quality system for the independent dosimetry audit service on the therapeutic radiation and determined to conduct a proficiency test (PT) for the dosimetry audit service provider. Since one of the missions of the KRISS is to establish the national QMS, we have discussed with the dosimetry audit service provider how to set up the QMS. The KRISS dosimetry team and the dosimetry audit service provider have agreed that the audit service provider would prepare the technical procedures required for the dosimetry audit service items based on the QMS of ISO/IEC 17025 [2], and the KRISS dosimetry team would run the PT program regularly for the dosimetry audit service items. The result of the PT can be used as evidence to support the competency claim of the dosimetry service provider.

The purpose of a PT includes evaluating laboratory performance, identifying problems within the laboratory, and the effectiveness of methods of measurement, establishing comparability, providing additional confidence to customers, confirming uncertainty claims, and training personnel in the participating laboratories [3]. According to the definition in the National Institute of Technology and Standards Notice No. 2022-0047 (KOLAS-R-007) entitled “the proficiency test operation guidelines,” the PT in the form of a one-to-one comparison run by the KRISS is denoted as the “measurement audit” (MA). The MA is different from the ordinary PT in that the reference value directly traceable to the primary standard is presented to the MA, whereas the “assigned value” replaces the reference value in the PT. On request, the KRISS provides the reference value to any PT. Here, we keep using the term PT even for the MA because PT is a more general term and MA is defined only for legal metrology.

In 2021, we conducted the PT for the dosimetry audit service provider, who agreed upon the need for the PT to gain public confidence and applied for the PT voluntarily. In this paper, we present the result of the PT and describe the PT procedure prepared for the dosimetry audit service provider in detail, followed by a discussion about the PT.

1. KRISS primary standard absorbed dose to water

The primary standard of absorbed dose to water at linear accelerator (LINAC) high-energy X-ray beams at the KRISS is the KRISS graphite calorimeter (C1505-4). This calorimeter was constructed in 2015 [4] with a small graphite core (16 mm diameter, 3 mm thickness) and two layers of thermal jackets to isolate the core from environmental temperature changes. The core has three thermistors to sense the core’s temperature and another thermistor for electrical heating to the core. The calorimeter was operated in the quasi-adiabatic mode, in which electric heat was feedbacked to the jackets to maintain the constant temperature difference between the core and the inner jacket with minimal loss of the absorbed energy by the core. The temperature rise of the core was measured under both irradiation and electric heating, and the absorbed radiation energy was obtained by comparing the temperature rises by irradiation against those by electric heating. The absorbed radiation energy was then divided by the mass of the core and multiplied by the graphite to the water dose conversion factor [5] to determine the absorbed dose to water finally. The established standard was confirmed by comparison studies performed by the National Metrology Institutes of (NMIJ) Japan in 2016 [6] and by the International Bureau of Weights and Measures (BIPM) in 2017 (BIPM.RI(I)-K6 [7]).

The degree of equivalence (DOE) of the KRISS primary standard absorbed dose to water at LINAC high-energy X-ray beams was 0.85% [7], where the reference value was that of the BIPM as agreed upon among the members of the Consultative Committee for Ionizing Radiation, section I. The DOE was within the expanded uncertainty of 1.0% of the irradiated dose in the KRISS.

2. Standard irradiation procedure at the KRISS

To determine the delivered amount of the absorbed dose to water in the postal dose audit package, the dose rate of the KRISS LINAC high-energy X-ray beam was finely measured. The KRISS LINAC had two beam monitors: the internal monitor providing the monitor unit (MU) and the external monitor made of a transparent chamber (7862, PTW). Reading of the external monitor chamber was corrected for changes in air density with the temperature and pressure. Long term stability of these monitors was guaranteed by using a stable farmer-type chamber (NE2571; Phoenix Dosimetry Ltd, NE2571; Phoenix Dosimetry Ltd, Sandhurst, Berkshire, United Kingdom), named as a reference chamber. At the beginning and end of every set of irradiations, the reference chamber was mounted downstream of the external monitor chamber, and the ratio of the readings of the monitors to the reference was obtained. Then, every dose delivery was managed at the dose rate to the reference chamber.

The X-ray irradiation direction was set vertically downward. Vertical irradiation is detrimental to the precise setup of phantoms because the water surface is ambiguous for source-to-surface distance (SSD) measurement. However, in this study, it was set vertically following the audit provider protocol. The beam energy of the X-rays was set at 6 MV with tissue phantom ratio at 20 and 10 (TPR20,10) at 0.684.

The 6 MV X-ray beams were irradiated at 279 MU/min (200 pulses per second). Readings of monitors were normalized to the reference chamber at (2.411±0.012) MU/nC and (6.500±0.005) nC/nC, respectively, where values following the symbol “±” represent the standard deviation of repeated measurements.

The dose rate of the 6 MV X-ray beams was measured using two farmer chambers (PTW30013, S/N 9304, and S/N 9305). These chambers had been calibrated to the absorbed dose to water under the same 6 MV X-ray beam against the KRISS graphite calorimeter [7]. The calibration coefficients of these chambers are shown in Table 1. A water phantom was set up at an SSD of 100 cm. The chambers were positioned where their center electrodes were 10 cm deep in the water. The dose per pulse rate at 10 cm deep in water was 0.16 mGy/pulse, and the ion recombination was corrected by (1.0021±0.0004) for both chambers. Polarity and lateral non-uniformity of the beam was corrected by (1.0005±0.0010) and (0.9995±0.0005), respectively. Results of the two chambers were within 0.1%, and the dose rate at 10 cm deep in water was determined against the internal and external monitors at (16.41±0.13) mGy/nC, (16.63±0.13) mGy/nC, respectively. These results corresponded to 10.06 mGy/MU at d_max, the depth where the dose rate was maximum. The standard uncertainty of the determined dose rate was evaluated (Table 2).

Table 1 . Calibration coefficients of the ionization chambers used to measure the absorbed dose to water.

Beam quality	Calibration coefficient ND,W, mGy/nC		Remarks
	PTW30013 (S/N9304)	PTW30013 (S/N9305)
60Co	53.79±0.22	53.73±0.22	Without sleeve, 2021
TPR20,10=0.686	53.21±0.25	53.14±0.25	Without sleeve, 2017

Values are presented as mean ± standard uncertainty..

Table 2 . Uncertainty components of the dose rate measurement.

Parameter	Relative standard uncertainty, %
	Type A	Type B
Dose measurement
ND,W, calibration coefficient		0.50
M, chamber reading	0.03
PTP, air density correction		0.04
PH, humidity correction		0.05
Pion, ion recombination correction		0.04
Ppol, polarity correction		0.10
Prp, lateral non-uniformity correction		0.05
Pdepth, chamber positioning		0.17
Pstb, chamber stability		0.20
Beam monitoring
Normalization ratio to the reference chamber	0.50
Long term stability of the reference chamber		0.20
Square root quadratic sum	0.50	0.61
Relative combined standard uncertainty	0.79

3. Amount of dose delivered to the dosimetry audit service package

Considering the postal dose audit program of the dosimetry audit service provider requested attendees to deliver the dose at 2 Gy within the tolerance level of 5%, we designed this PT to deliver 2.2 Gy to the postal dose audit service package, which is close to 2 Gy and an appropriate amount of dose to evaluate the performance of the audit service provider.

The 6 MV X-ray beam was irradiated onto the postal dose audit package (Fig. 1). The postal dose audit package was set up at an SSD of 100 cm. This audit package consisted of a plastic phantom (20×20×20 cm³) and three glass dosimeters centered at 10 cm depth in the phantom from the upper surface. The plastic phantom was made of the acrylonitrile butadiene styrene copolymer (1.07 g/cm³). The code, lot number, and ID number of the glass dosimeters are shown in Table 3. One of the glass dosimeters (code No. 485) was provided for background correction purposes.

Table 3 . ID number of the glass dosimeters.

Code	Lot number	ID number	Goal	History
37	FD7131213-2	221	6 MV	KRISS sample 1
38	FD7131213-2	224	6 MV	KRISS sample 2
146	FD7160725-1	162	6 MV	KRISS sample 3
Unknown	Unknown	485	Background	KRISS Bkg

KRISS, Korea Research Institute of Standards and Science..

Figure 1. Irradiation of the 6 MV X-ray beam on the postal audit package. (Left) before irradiation onto the postal audit package in which the glass dosimeters were not located, and (right) after irradiation onto the glass dosimeters located in the plastic phantom. (a) external monitoring chamber of the beam, (b) aluminum block for mounting the reference chamber, (c) plastic phantom of the postal audit package.

Onto the postal dose audit package, the 6 MV beam was irradiated at 220 MU. Then, the postal dose audit package and the glass dosimeters were returned to the dosimetry audit service provider. Percent depth dose data measured from 0 to 250 mm and lateral non-uniformity data were also provided to the dosimetry audit service provider.

4. Performance evaluation method

The statistical evaluation of the performance from the PT result of the participant is shown in five ways in Annex B.3 Calculation of performance statistics of ISO/IEC 17043 “Conformity assessment - General requirements for proficiency testing” [3]. In the PT program run by the KRISS, the E_nnumber is used:

where x_lab is the result of the dosimetry audit service provider, X_KRISS is the reference value of the KRISS, U_lab is the expanded uncertainty of x_lab and U_KRISS is the expanded uncertainty of X_KRISS. Eq. (1) is correct when x_lab and X_KRISS are independent. In this study, we assumed that they were independent. According to the criteria for performance evaluation of ISO/IEC 17043, if |E_n| ≤1.0, then it indicates “satisfactory” performance, and if |E_n| >1.0 then it indicates “unsatisfactory” performance and generates a corrective action signal. The performance evaluation method employing the E_n number is applicable only when the reference value is given.

1. Data analysis and evaluation of E_n number

When the 6 MV beam was irradiated onto the postal dose audit package, the monitor readings were 91.240 nC and 90.553 nC for the internal and external monitors, respectively. Based on the determined dose rate, the absorbed dose to water at 10 cm deep in the water and at the d_max were determined at (1.501±0.012) Gy and (2.221±0.019) Gy, respectively (Table 4). Therefore, X_KRISS was determined at 2.221. The effective degree of freedom of X_KRISS was larger than 10. Thus coverage factor 2 was multiplied by the standard uncertainty and U_KRISS was determined at 0.037.

Table 4 . Determination of the absorbed dose delivered to water.

Parameter	Monitor		Relative standard uncertainty, %
	Internal	External
Dose at 10 cm depth
Dose rate, mGy/nC	16.405	16.627	0.79
Monitor reading, nC	91.240	90.533	-
Dose, Gy			-
Each	1.497	1.506
Mean	1.501		0.26
Dose at 10 cm depth, Gy	1.501		0.83
Dose at dmax
Dose at 10 cm depth, Gy	1.501		0.83
Percent depth dose at 10 cm depth	0.675 9		0.10
Dose delivered
Dose delivered to the dosimetry audit service package, Gy	2.221		0.84

The values of x_lab and U_lab were reported by the dosimetry audit service provider at 2.092 Gy and 0.130 Gy, respectively. The uncertainty of x_lab was evaluated (Table 5). Based on the determined X_KRISS, x_lab, U_KRISS, and U_lab, E_n number was evaluated at −0.954. The PT result is summarized in Table 6 and Figs. 2 and 3.

Table 5 . Uncertainty budget for determination of x_lab which is provided by the dosimetry audit service provider.

Parameter	Relative standard uncertainty, %
MID, reading of the glass dosimeters	1.1
kID, calibration factor to the absorbed dose to water under the 60Co gamma-rays	1.1
kpw, phantom to water conversion factor	1.8
kE, correction for energy dependency of glass dosimeters	1.8
PDD10, percent depth dose at 10 cm deep in water provided by an audit participant	0.05
Relative combined standard uncertainty	3.0
Relative expanded uncertainty in approximately 95% confidence level (k=2)	6.0

Table 6 . Results of the proficiency test for the dosimetry audit service provider.

X-ray energy (TPR20,10)	Unit	XKRISS	UKRISS (k=2)	Uncertainty in the CMC of the lab	xlab	Ulab (k=2)	En number
6 MV (0.684)	Gy	2.221	0.037	6% (k=2)	2.092	0.130	−0.954

CMC, calibration and measurement capability claimed by the dosimetry audit service provider..

Figure 2. The evaluated values of the absorbed dose to water, X_KRISS and x_lab with their uncertainties of U_KRISS and U_lab in 95% confidence level (k=2). KRISS, Korea Research Institute of Standards and Science.

Figure 3. E_n number of the dosimetry audit service provider. If the absolute value of E_n number is less than or equal to 1, the performance shall be decided to be “satisfactory” according to the decision rule of conformity given in ISO 17043 [3].

2. Decision of conformity

The E_n number of the dosimetry audit service provider in the present PT is −0.954. We decided that the performance of the dosimetry audit service provider was “satisfactory” in the specific dosimetry audit service item under the PT, which was the postal dose audit service using the radio-photoluminescence (RPL) glass dosimeters for the measurement of the absorbed dose to water from the LINAC high-energy X-ray beams.

According to the Nuclear Safety Commission Notice No. 2019-6, the dosimetry audit service provider shall be “external” and “independent,” which means the audit service provider shall be a legal entity responsible for all its audit service activities and free from all the interest of the medical institution. The dosimetry audit service provider undergone the PT shall not provide the audit service to itself to prevent the audit from being done impartially.

All external radiation therapy uses the LINAC beams, but no medical institutions or hospitals calibrate the ionization chambers in the LINAC beams. Instead, they have mainly calibrated the chambers in the Co-60 gamma-ray beam because the secondary standard dosimetry laboratories accredited by the Korea Laboratory Accreditation Scheme (KOLAS) only provide the calibration service in the Co-60 beam. To have traceability to the primary standard, at least the following requirements should be satisfied: all the critical equipment is calibrated, the procedures are verified and valid, the personnel are well educated and trained, the environment is maintained suitably, and the QMS is to be set up to organize all these components to run properly. Being aware of the current situation, the dosimetry audit is the least qualification of the acceptable radiation therapy for securing the patient’s safety from over- and under-exposure during treatment by ensuring the traceability of the dose delivered to the cancer patient. Likewise, the PT for the dosimetry audit service provider is an essential element in assessing the competence of the audit service provider.

As a part of the conformity assessment, we carried out a PT for the dosimetry audit service provider. The RPL glass dosimeters in the postal dose audit package used for the dose audit service to the medical institutions were irradiated in the KRISS LINAC high-energy X-ray beams in the same way as done in the medical institutions. The dosimetry audit service provider read the irradiated dose by the KRISS and reported the value with the uncertainty to the KRISS. We evaluated the performance of the dosimetry audit service provider from the PT result. The E_n number of the audit service provider was estimated at –0.954 from Eq. (1) and it can be concluded from the criteria of ISO/IEC 17043 [3] that the performance of the dosimetry audit service provider is “satisfactory.” We expect the PT program will be repeated yearly. Through all these efforts, we evaluated the performance of the voluntary-based dosimetry audit service provider by running the PT program and giving confidence to the medical institutions undertaking the audit service and, consequently, to the cancer patient.

The result of this PT is useful for the dosimetry audit service provider, medical institutions, and cancer patients. However, there are limitations to using this PT result to assure the competence of the dosimetry audit service provider. It is essential to note that the successful performance in a particular PT scheme may represent the evidence of the competence of the dosimetry audit service provider on that test item but not ensure continual competence.

Other than the dosimetry audit service to LINAC high-energy X-ray beams, the audit service to the high dose-rate (HDR) brachytherapy using Ir-192 is considered necessary, and we expect to offer a PT program to the dosimetry audit service provider for the HDR brachytherapy in the near future.

The KRISS has supported the present work under the project “Development of Measurement Standards for Medical Radiation (2022).” We thank the dosimetry audit service provider for allowing us to report the PT result.

The authors have nothing to disclose.

All relevant data are within the paper and its Supporting Information files.

Conceptualization: Chul-Young Yi. Data curation: In Jung Kim. Formal analysis: In Jung Kim and Chul-Young Yi. Funding acquisition: none. Investigation: Chul-Young Yi and In Jung Kim. Methodology: Chul-Young Yi. Project administration: In Jung Kim. Resources: Chul-Young Yi, In Jung Kim, and Jong In Park. Software: In Jung Kim. Supervision: Chul-Young Yi. Validation: In Jung Kim. Writing-original draft: Chul-Young Yi and In Jung Kim. Writing-review and editing: Chul-Young Yi, In Jung Kim, Yun Ho Kim, Jong In Park, and Young Min Seong.