Recommendations for Quality Assurance Guidelines for Gamma Knife Radiosurgery in Republic of Korea : A Multi-institutional Survey

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J Korean Neurosurg Soc. 2026;69(2):299-306

Publication date (electronic) : 2025 September 1

doi : https://doi.org/10.3340/jkns.2025.0118

Hyeong Cheol Moon^1,*, Jong Hyeok Kwak^2,*, Seong Jin Jin³, Byungmok Kim⁴, Yong-Seok Im⁵, Gyeong Rip Kim^,6

¹Department of Neurosurgery, Chungbuk National University Hospital, Chungbuk National University College of Medicine, Cheongju, Korea

²Department of Radiology, Pusan National University Yangsan Hospital, Yangsan, Korea

³Gamma Knife Center, Haeundae Paik Hospital, Inje University, Busan, Korea

⁴Department of Neurosurgery, Gamma Knife Center, Kyungpook National University Hospital, Daegu, Korea

⁵Department of Neurosurgery, Samsung Medical Center, Seoul, Korea

⁶Department of Neurosurgery, Gamma Knife Center, Pusan National University Yangsan Hospital, Yangsan, Korea

Address for correspondence : Gyeong Rip Kim Department of Neurosurgery, Gamma Knife Center, Pusan National University Yangsan Hospital, 20 Geumo-ro, Mulgeum-eup, Yangsan 50612, Korea Tel : +82-55-360-1522, Fax : +82-55-360-1901, E-mail : sjkim76@pusan.ac.kr

*Hyeong Cheol Moon and Jong-Hyeok Kwak contributed equally to this work.

Received 2025 May 29; Revised 2025 August 22; Accepted 2025 August 25.

Abstract

INTRODUCTION

The Leksell Gamma Knife (LGK; Elekta Instruments AB, Stockholm, Sweden) is a widely used modality in stereotactic radiosurgery for the treatment of various benign and malignant intracranial disorders [5-7]. As a leading technology specifically designed for cranial indications, LGK has revolutionized the management of brain tumors, vascular malformations, and functional neurological disorders. Given the high radiation doses delivered during Gamma Knife radiosurgery (GKRS), rigorous quality assurance (QA) is critical to ensure patient safety and treatment accuracy.

In the Republic of Korea, the clinical adoption of LGK systems has increased significantly, reflecting a global trend toward advanced medical technologies. As of June 2025, 12 institutions operate LGK Icon^TM, five institutions use LGK Esprit^TM, three institutions use LGK Perfexion^TM, and three institutions were equipped with the older C-type model. The majority of institutions now used Icon^TM model. And they are in the process of upgrading to the Esprit^TM model. The Gamma Knife Icon^TM and Esprit^TM are built upon the same main hardware platform, with variations confined to supplementary features such as workflow enhancements and integrated imaging options. The Gamma Knife Icon^TM and Esprit^TM follow an identical set of QA items, reflecting their shared hardware architecture and performance specifications.

Owing to the different specifications of each model, QA procedures are often tailored accordingly, resulting in notable variations across institutions. This highlights the need for standardized QA protocols specific to each LGK model. Historically, QA practices in GKRS have been guided by the American Association of Physicists in Medicine (AAPM) Report 54, published in 1995 [9]. This report addresses the commissioning, dosimetry, and QA of LGK systems. Since then, protocols such as the AAPM Task Group 51 [1] and the International Atomic Energy Agency (IAEA) Technical Report Series-398 have provided dosimetric standards for clinical reference dosimetry using liquid water [3]. However, the limitations of standard ionization chambers in small-field environments have prompted the development of IAEA’s Technical Report Series-483 (TRS-483), a code of practice for small-field dosimetry that includes essential correction factors [4]. A key feature of TRS-483 is the inclusion of detector-specific output correction factors, which are critical for achieving accurate dose measurements under non-standard and small-field conditions. These correction factors account for differences in detector response caused by variations in field size, geometry, and beam quality, which conditions particularly relevant to LGK. Building on this, the AAPM Task Group 178 (TG-178) subsequently proposed QA and calibration guidelines optimized for small static beams, including those used in Gamma Knife systems [8].

In the Republic of Korea, the Gamma Knife Quality Management Association generally follows AAPM’s and IAEA’s recommendations, while also incorporating vendor-specific maintenance protocols provided by Elekta. However, these adaptations have led to inconsistencies in QA implementation across institutions. Additionally, the QA items mandated by the Korea Institute of Nuclear Safety (KINS) differ in scope from those of international bodies, emphasizing the need for harmonization within the national regulatory context. Despite increasing oversight by KINS, the Republic of Korea still lacks a definitive regulatory framework specific to Gamma Knife QA. The clinical application of the TRS-483 small-field dosimetry protocol also remains underexplored. Recent efforts have begun to explore specific aspects, such as dose rate consistency following the application of machine-specific reference correction factors for the Gamma Knife 16-mm collimator field [2]. Nonetheless, comprehensive evaluation and adaptation of this protocol are crucial to further enhance domestic QA standards.

This study aimed to identify and prioritize essential QA items for LGK Perfexion^TM and Icon^TM/Esprit^TM models based on a nationwide, multi-institutional survey. We also evaluated the clinical implementation of TRS-483 in the Republic of Korea and sought to establish practical, standardized QA guidelines tailored to the Korean healthcare context. These guidelines include recommended tolerance values for both general QA procedures and items required by Korean regulatory authorities. This initiative is intended to enhance the consistency, accuracy, and safety of GKRS nationwide.

MATERIALS AND METHODS

This study did not involve human data but focused on Gamma Knife dose measurement methods using a survey-based approach among participating institutions. Therefore, Institutional Review Board (IRB) approval was not required. The survey did not include sensitive topics or collect identifiable personal information, and participation posed no risk to any individuals.

A structured survey was conducted across 16 institutions in the Republic of Korea to establish a prioritized list of QA procedures for the LGK. These institutions were selected based on their active clinical use of either the LGK Perfexion^TM or Icon^TM/Esprit^TM models for GKRS.

The survey was divided into four categories corresponding to the frequency of the QA tasks (daily, monthly, semiannual, and annual), following the recommendations of the AAPM TG-178. Each category included a comprehensive list of QA items. Participants were asked to evaluate the importance of each item using a Likert scale ranging from 0 (not important) to 5 (extremely important). Items that received scores of 4 or 5 from most respondents were classified as high priority and were included in the final QA checklist. These high-priority items were then grouped according to their corresponding QA frequencies.

Descriptive statistics were used to analyze the distribution of scores and identify items with strong consensus. The resultant list of prioritized QA procedures was reviewed by medical physics experts to ensure consistency with the current clinical practices and recent technological developments in GKRS. This expert review also included a comparison of the identified QA item tolerance values with the TG-178 guidelines, facilitating the establishment of Korea-specific GKRS tolerance criteria. In this study, when the TG-178 guidelines specified both tolerance and action levels, the action levels were used as the primary reference for comparison, where appropriate.

RESULTS

Priority daily QA procedures

Based on the responses from the 16 participating institutions, 11 high-priority daily QA items were identified for the LGK systems (Table 1). Among these, three items (the emergency stop sequence, Patient Position System [PPS] focus precision test, and cone-beam computed tomography [CBCT] Precision test) are typically performed monthly in Korean clinical practice. These items apply to both the Perfexion^TM and Icon^TM/Esprit^TM models, except for the CBCT precision test, which is exclusive to the Icon/Esprit system. The remaining eight QA tasks are primarily functional checks, including system emergency alarms, warning indicators, door interlocks, video and audio communication systems, time synchronization, guard rail interlocks, and proper functioning of the treatment pause. These checks are qualitative and must be confirmed as operational every workday before patient treatment. These functional checks can be confirmed during system initiation and through visual inspection.

Table 1.

Daily priority QA items identified through a multi-institutional survey of Gamma Knife centers in Korea

Among the items with monthly frequency in Korean practice, the Emergency stop sequence is a functional check confirmed by a pass/fail outcome. The PPS focus precision test and the CBCT geometric accuracy test involve quantitative tolerance- based assessments. In the absence of any physical defects in the machine, the test can be performed on a monthly basis, as also recommended by the ELEKTA vendor. Therefore, daily testing is not mandatory, and a monthly interval is recommended in Korean practice. The focus accuracy of the PPS must be verified using the Elekta QA tool, ensuring radial deviation remains within 0.4 mm in Korean practice, as recommended by the ELEKTA vendor. For LGK Icon^TM/Esprit^TM systems, the CBCT geometric accuracy test must confirm that image volume deviations remain within 0.4 mm in order to pass QA. Fig. 1 illustrates two representative daily QA procedures. Fig. 1A shows the setup for PPS focal precision testing using a dedicated Elekta QA tool. Fig. 1B displays the CBCT accuracy verification interface on the LGK Icon^TM/Esprit system, indicating a “pass” status when all geometric tolerance thresholds are satisfied. These tests are typically performed in the QA mode before patient treatment to ensure high-precision targeting and system readiness.

Fig. 1.

Daily quality assurance (QA) procedures performed to ensure geometric and imaging accuracy. A : Evaluation of focus precision using the patient positioning system (PPS) with the Elekta QA tool. B : Verification of cone beam computed tomography (CBCT) accuracy using the dedicated Icon QA interface.

Priority monthly QA procedures

Monthly QA procedures were evaluated across the 16 institutions to identify the high-priority items, with six items received a priority scores of 4 or 5 (Table 2). These procedures should be performed at least once a month to maintain consistent performance and treatment safety.

Table 2.

Monthly priority QA items identified through a multi-institutional survey of Gamma Knife centers in Korea

Dosimetric evaluations included measuring dose rate with the largest collimator (16 mm) using an ionization chamber, and the AAPM TG-178 recommended tolerance is within 1.5% of the treatment planning system (TPS) reference value, and the IAEA TRS-398 recommends less than 3% total uncertainty for dose calibration. The Korean GKRS also recommends less than 3%, which value includes 1.5% radiation measurement uncertainty from the Korean calibration institute. Timing-related checks assessed linearity (TG-178 : ±1%; Korean GKRS : ±1%) and timer on–off error (TG-178 : <0.01 minutes; Korean GKRS : <0.01 minutes).

Additional image-based QA metrics, specific to the Icon^TM/Esprit^TM model, evaluate CBCT performance, including spatial resolution (TG-178 : ≥7 lp/cm for both low and high Computed tomography dose index [CTDI] settings; Korean GKRS : ≥6 lp/cm for both low and high CTDI settings), contrast-to-noise ratio (CNR) (TG-178 : >0.6 for low and >0.9 for high CTDI settings; Korean GKRS : >0.5 for low and >0.8 for high CTDI settings), and uniformity (TG-178 : <12%; Korean GKRS : <21%). In Korean clinical practice, institutions generally adhere to Elekta’s vendor-accepted uniformity threshold of <21%. In Korean GKRS practice, CBCT image quality checks are typically performed every 2 months.

Fig. 2 illustrates the key monthly QA procedures. The first image shows a dose rate measurement using a 16-mm collimator and a solid water phantom. The second image shows the electrometer used to record the absorbed dose. The third image highlights the use of CBCT of an Icon/Esprit system to confirm accurate positioning of the ionization chamber at the isocenter. This CBCT-guided verification is critical for maintaining precision in dose delivery and ensuring reproducibility across QA sessions.

Fig. 2.

Monthly quality assurance (QA) procedures to validate dosimetric accuracy and chamber setup. A : Dose rate measurement using an ionization chamber with a 16-mm collimator. B : Dose reading obtained using an electrometer following measurements. C : Target alignment confirmation of the ion chamber using cone beam computed tomography (CBCT).

Priority biannual and annual QA procedures

In addition to daily and monthly QA routines, selected procedures are designated for biannual and annual implementation to ensure long-term mechanical accuracy and radiological consistency of the Gamma Knife system. Table 3 summarizes the QA items identified as high priority for these extended intervals.

Table 3.

Biannual and annual QA items identified through a multi-institutional survey of Gamma Knife centers in Korea

A wipe test, performed annually in Korean practice, was conducted on external components, including the collimating helmets (where applicable), the outer surface of the collimator cap, and the sector drive shafts. Although AAPM TG-178 recommends a contamination threshold of 185 Bq (0.005 μCi), Korean GKRS adopts a slightly higher tolerance of 200 Bq (0.0054 μCi), consistent with local regulatory practice. This test is designed to detect potential radioactive contamination, and the measured activity must remain below the specified threshold to ensure radiation safety compliance. The annual QA procedures primarily involve film-based dosimetric verification. A critical component of this process is the irradiation and analysis of radiochromic films to verify spatial alignment between the unit center point and radiation focal point across all collimator sizes. Both the AAPM TG-178 and Korean GKRS guidelines specify identical tolerances for this test : Δx, Δy, and Δz must each be ≤0.3 mm, and Δr must be <0.5 mm. A second annual film-based test is performed to evaluate the dose profiles for each collimator size and compare them with the reference profiles generated by the TPS. Agreement was confirmed if the full width at half maximum (FWHM) of the measured profile was within ±1.0 mm of the TPS reference value as per TG-178, and within ±1.0 mm in Korean GKRS. These biannual and annual QA procedures are essential for ensuring the continued mechanical precision and dosimetric fidelity of the Gamma Knife system over time.

DISCUSSION

Medical institutions in the Republic of Korea currently operate LGK systems using independently developed QA protocols. These internal QA procedures vary across the 16 domestic centers utilizing Gamma Knife systems. This study aimed to develop practical and standardized QA guidelines for GKRS, based on existing procedures in clinical practice. A survey was conducted across 16 Korean institutions using the LGK Perfexion^TM and Icon^TM/Esprit^TM models. Following the AAPM TG-178 guidelines, QA items were categorized by frequency : daily, monthly, biannual, and annual. Through the survey results, 20 high- priority QA items were identified : 11 daily, six monthly, and three annually. In the Republic of Korea, medical physicists do not need to perform every TG-178 QA task, as many maintenance procedures are supported by Elekta’s embedded systems and service engineers.

Daily QA items primarily ensure the functional performance and geometric accuracy. Most checks such as system emergency alarms, interlock indicators, and audio-visual systems, are performed during system initiation and verified through visual inspection. Positional accuracy checks, including PPS and CBCT alignment using Elekta QA tools, are recommended every workday before patient treatment. However, in Korean GKRS practice, certain tests, such as the emergency stop sequence, PPS focus precision test, and CBCT precision test, are typically performed monthly. Daily QA typically requires approximately 30 minutes to complete.

Monthly QA focuses on dosimetric and imaging system evaluations. Due to system design constraints, Gamma Knife systems cannot use water phantom; instead, solid water phantoms are employed to simulate tissue-equivalent dose absorption. To ensure accurate dose measurements, appropriate correction methods and stable environmental conditions are necessary during monthly QA procedures. Key tasks include dose rate measurements using a 16-mm collimator and ionization chamber, timer accuracy, linearity, and spatial and image quality assessments (e.g., CNR and uniformity). Measurement results must fall within the specified tolerance limits. Dose error is generally limited to ±3% in Korean GKRS, considering the uncertainty of the ionization chambers (1–2%) and the predictable decay of cobalt-60 sources. Although the TG-178 report recommends a CBCT uniformity tolerance of 12%, clinical practice in Korea GKRS adheres to Elekta’s vendor-accepted threshold of 21%, which reflects system-specific reconstruction characteristics. Therefore, image quality is generally considered acceptable unless the uniformity exceeds this level. Furthermore, routine CBCT image quality checks—including uniformity, CNR, and spatial resolution—are typically performed monthly or bi-monthly depending on institutional policy. A 2-month interval is regarded as clinically acceptable in Korean GKRS unless a noticeable degradation in image quality is observed.

The biannual wipe test, which checks for radioactive contamination on helmets, collimator caps, and sector drive shafts, is typically performed by Elekta or other authorized service providers. It was rated as highly important by 15 of the 16 institutions surveyed. Although TG-178 lists the biannual wipe test as an optional recommendation, the cobalt-60 sources used in Gamma Knife systems are securely sealed in pellet form, making radioactive contamination highly unlikely except in cases of significant mechanical damage. In the Korean GKRS, routine wipe tests are performed annually, as mandated by the Korean Atomic Safety regulations. This annual frequency is considered both legally sufficient and clinically appropriate in domestic practice.

Annual QA procedures include film-based mechanical and dosimetric verifications. These tests ensure alignment between the radiation and mechanical isocenters, with spatial accuracy required to be within Δx, Δy, Δz ≤0.25 mm and Δr <0.4 mm as specified by TG-178, and Δx, Δy, Δz ≤0.3 mm and Δr <0.5 mm in Korean GKRS protocols. In addition, dose profiles for all collimators must match the reference values of the TPS within ±1.0 mm FWHM in both TG-178 and Korean GKRS. All 16 institutions identified these procedures as the most critical components of QA.

This study complements AAPM Reports 54 and TG-178 by adapting the recommendations to the Korean clinical environment. The proposed checklist standardizes the daily, monthly, and annual QA protocols for the Gamma Knife Perfexion^TM and Icon^TM/Esprit^TM systems. The adoption of these recommendations by the Korean Society for Gamma Knife Radiosurgery is expected to enhance treatment safety, QA consistency, and national preparedness for implementing external QA programs.

CONCLUSION

In this study, we developed a prioritized list of QA procedures for Gamma Knife Perfexion^TM and Icon^TM/Esprit^TM models based on a comprehensive multi-institutional survey conducted in the Republic of Korea. By identifying 20 essential QA items and categorizing them by frequency, including specific Korean GKRS tolerances, we laid the foundation for standardized QA implementation tailored to Korean clinical practice. The findings underscore the need for harmonized QA protocols to ensure treatment accuracy, enhance patient safety, and support consistent maintenance practices across institutions. Overall, this study contributes to ongoing efforts to improve the quality and reliability of GKRS in the Republic of Korea.

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