TURKISH JOURNAL OF ONCOLOGY

Summary

Surface-guided radiation therapy (SGRT) has gained wide popularity across radiation oncology community due to its non-radiographic characteristic and real-time motion monitoring capability. Nevertheless, it has not yet gained its full potential in routine clinical practice. Implementing SGRT system into the clinical practice requires not only the definition of steps in clinical workflow, but also establishment of the comprehensive quality assurance (QA) program including commissioning, acceptance and periodic QA test to facilitate a safe, and efficient use of SGRT system in clinical settings. This review focuses on the latest recommendation of American Association of Physicists in Medicine and European Society for Radiotherapy and Oncology guidelines about the implementation of comprehensive QA program for SGRT system.

Summary

Introduction

Surface-guided radiation therapy (SGRT) has emerged as a special form of image-guided radiation therapy (IGRT). Since its first introduction as a useful IGRT tool, many researcher have explored the feasibility of SGRT system for patient positioning, realtime motion management, four-dimensional imaging for motion tracking and threshold gating.[1-8] In several studies, it was proved to improve initial patient positioning by correcting posture differences before online imaging.[9,10] Nevertheless, after initial patient setup with SGRT, online imaging modalities such as planar imaging (kV or MV) and cone-beam computed tomography still needs to be performed in many anatomical sites especially located in thorax and abdomen since the changes in internal motion remain undetected through surface scanning with current technology.[9] However, up to that time, limited guidelines and the complexity of the clinical settings have led to diverse patterns of practice between the clinics. In 2019, Padilla et al.[11] conducted an electronic survey under the auspices of American Association of Physicists in Medicine (AAPM) Task Group Report 302 (TG-302) to identify the necessity of formal guidance and to gain more insight on prevalence of the SGRT system in USA, length of its use, existing recommendation for commissioning procedures and clinical implementation. According to questionnaire, 36% of the users (n=86) only followed the vendor's guidelines and 49.1% of the respondents (n=115) used more than one reference during commissioning. In terms of the question about the use of any end-toend (E2E) test verification approaches, 12% of the users (n=28) response this question as "No" and 14.1% of the respondents (n=33) do not know whether they performed any E2E test, or not. Similarly, in 2022, another international survey was conducted with the collaboration of European Society for Radiotherapy and Oncology (ESTRO) and AAPM to provide an overview about the current status of SGRT in clinical practice with a focus on the user's experience in terms of implementation, commissioning, periodical quality assurance (QA), training, and clinical workflow.[12] According to results of the survey, clinical implementation of the SGRT systems was predominantly based on the vendor's recommendation. Indeed, 94% of the respondents (n=132) primarily followed vendor's guidelines during clinical implementation, commissioning and periodical QA. About 42% of the participant (n=59) used two different sources and only 19% (n=27) used at least three different sources including vendor's guidelines and published studies in the literature or peer-to-peer consultation. In addition, 54% of the respondents (n=76) exclusively used QA tools provided by the vendors during the commissioning and periodical QA tests. About 44% of the users (n=62) preferred to use vendor-provided phantom in combination with the third party commercially available phantoms (n=34) and/or with in-house phantoms (n=28). However, 8% of the respondents (n=12) reported the use of only either third-party commercial phantoms (n=6) or adapted in-house phantoms (n=6) instead of vendor-provided phantoms. According to results of both surveys, it was strongly emphasized that consensus guidelines on SGRT are needed for standardization in clinical practice since the use of different techniques during implementation, commissioning and periodical QA test may cause a systematic errors in patient setup and monitoring. Recently, two different guidelines were published by AAPM, called as TG-302[1] building on the TG-147[13] report, and ESTRO-ACROP[2] to expedite its safe adaptation in clinical practice. AAPM TG-302 also referred other guidelines such as AAPM TG-76[14] and AAPM TG- 142[15] for several QA tests. Although both of these guidelines (AAPM TG-302 and ESTRO-ACROP) were comprehensive and informative, there are still several differences in terms of suggested parameters and tolerance values that need to be considered during QA program including acceptance, commissioning, and periodical QA tests. We aimed to compare both guidelines in terms of recommended parameters based on system specification (simulation room vs. treatment rom, C-arm vs. ring gantry, photon vs. particle etc.), type of tests and tolerances/specifications during the acceptance, commissioning, and periodical QA tests. In addition, phantom selection criteria for SGRT QA and current challenges in SGRT QA were discussed in detail.

QA PROGRAM FOR SGRT
Acceptance Test
The acceptance process need to include all required tests including static/dynamic localization accuracy, spatial reproducibility and drift to illustrate the safe operation and proper functionality of the SGRT system with the integrated treatment or simulation platform. In most cases, the acceptance test document is provided by the vendor and it may not include all necessary tests that need to be checked. However, it is important to keep in mind that the acceptance procedure is an integral part of the purchasing process to ensure whether the product or solution meet the clinical need, or not. Therefore, primary responsible person, generally qualified medical physicist expert, needs to be familiar with the fundamental or basic tests recommended in the commissioning and if these tests are not included in the vendor's acceptance documents, it is generally recommended to negotiate with the vendor to perform these tests during acceptance. According to AAPM TG-302, vendor's recommendation and other AAPM reports such as TG- 142, TG-147, and TG-76 need to be followed together for checking the localization accuracy and reproducibility of the system. In addition, safe operation and proper functionality of the system with all other unit interface, including imaging system (if necessary), treatment machine, treatment planning system, data transfer and information system, and need to be validated as described in Table 1. In contrast to TG-302, ESTRO-ACROP guideline provides a more detailed information about the description of each parameters.[2] Moreover, all tests are categorized with respect to importance level (x- mandatory, o-optional, pass-within Vendor's system specifications), type of systems (computer tomography [CT], closed-bore linac, C-arm linac and particle therapy), and subgroups for each suggested parameters.

Table 1 Cont.

Commissioning
The commissioning of the SGRT system is a substantial part of the comprehensive QA program before implementing it into clinical practice. This part also includes measuring the system accuracy/precision and determining system limitations for all clinically relevant scenarios. Since the commissioning data are accepted as a reference for future measurement, all tests need to be reproducible to assess the consistency of the system performance over the period of time for periodical QA tests or for later measurements after maintenance and repair of the system. In addition, according to AAPM TG-147 recommendation, commissioning test need to be repeated in case of special situation, ranging from major upgrade and power outages to earthquake and building vibration, to check the stability of the system before using it in clinical practice. All suggested parameters for system commissioning in AAPM TG-302 and ESTRO-ACROP guidelines are summarized in Table 2. Some specification and tolerance values were tightened in ESTRO-ACROP guidelines and new tests were described based on the availability of new technologies and updated clinical needs.

Table 2 Cont.

Periodic QA Program
The main goal of the periodical QA program is to ensure about the stability of the system over a period of time (e.g., daily, weekly, monthly, and annually) and to catch the unexpected errors or changes in system performance due to the many factors such as component failure, machine malfunction or aging of the system component. ESRTO-ACROP guideline also recommended to start with a higher frequency and higher number of tests until the RT team feel more comfortable about the stability of the system based on the test outcome preferably including a failure modes and effective analysis specific to the clinic. In addition, ESTRO-ACROP guideline reported the list of failure modes and potential errors in SGRT workflows with possible solutions. Similar to acceptance and commissioning part, ESTRO-ACROP guideline provides more comprehensive periodic QA program compared to AAPM TG-302 and TG-147 recommendations as presented in Table 3. Detailed information and description of each test are also provided in both AAPM TG- 147 and supplement of ESTRO-ACROP guidelines.

Table 3 Periodical QA tests for SGRT

QA Phantoms for SGRT
SGRT requires dedicated QA phantoms with specific properties (e.g., color, reflectivity, texture, and topography) that make it accurately trackable. Although some commercially available SGRT systems allow the user to change imaging parameters (e.g., camera light and exposure time) for capturing surface information from the bodies/phantoms with variety skin/surface tones, opaque/matte and light colored phantoms yields the best monitoring results during QA due to the better reflection characteristic for the projected light pattern. In fact, the use of SGRT system in variety skin tones, especially in case of dark skin, is still one of the challenging issues to consider in clinical practice. However, ESTROACROP guidelines recommended to check localization accuracy of the SGRT system with both light- and darktoned phantoms when it is possible, especially in clinics where a larger proportion of patients with darker skin tones are treated. In addition, it needs to be taken in to account that if the surface of the phantom is shiny, it may also cause numerous or unwanted reflection pattern of the projected light. Therefore, in case of necessity, it is generally recommended to cover the phantom surface with a paint coat or light colored tape. In addition to color and reflectivity properties, topography and texture of the QA phantom may significantly affect the result of the QA tests. Indeed, in case of insufficient topography, it is difficult to discern position or motion of the phantom during the check of localization accuracy of SGRT system. To overcome this issue, vendors provide dedicated phantoms that mimic anatomical surfaces such as the head, leg, or breast. In many clinics, homemade Styrofoam phantom with a different topography is also used as an inexpensive way of 3D surface phantom for SGRT. However, we need to be careful that Styrofoam with expanded polystyrene beads may cause uncertainties due to the abundance of texture and the projected light pattern may not be identified correctly. Therefore, smoot foam phantoms satisfying the outlined recommendation in both ESTRO-ACROP and AAPM guidelines can be also good alternative to commercially available phantoms. Several types of commercially available phantoms were also demonstrated in AAPM TG-302 and ESTRO-ACROP guidelines.

Challenges in SGRT QA
As also defined in AAPM TG-302, there are still several major issues that cause in uncertainties during both QA and clinical practice of SGRT. For instance, the use of DICOM based surface structure generated from CT imaging is considered as the one of the challenging issue for accurate localization of the phantom/body. In fact, many parameters (e.g., CT voxel size, scan speed, respiratory phase effect for moving phantom/surface, Hounsfield unit threshold for surface segmentation, and image quality) can significantly affect the topography of reference body surface ant it may cause a systematic bias during localization. Similarly, the size and the shape of the selected region-of-interest for surface tracking can also affect the response of the system during QA. In addition to these parameters, the tracking accuracy of the SGRT system can decreases when the component of treatment unit (e.g. gantry head and kV imaging arms) occlude the SGRT cameras, especially in non-coplanar treatment techniques with couch angle. Therefore, all these parameters need to be checked for different scenarios to evaluate the impact of defined issues on the tracking and localization accuracy of the implemented SGRT system before using in clinical practice.

Introduction

Conclusion

AAPM TG-302 mainly focused on the implementation of SGRT in C-arm linac. However, the use of SGRT system is also getting widespread in other platforms (like closed-bore linac, robotic gantry system, particle therapy, and CT simulator). Therefore, as also emphasized ESTRO-ACROP guidelines, each system need to have a different parameters and corresponding tolerance values for acceptance, commissioning, and routine QA. In terms of this aspect, an ESTRO-ACROP guideline is more comprehensive than AAPM TG-302. Nevertheless, AAPM TG-302 provides more detailed information about the phantom selection criteria and QA issue unique to SGRT and possible solution for these issues. Therefore, both of these reports need to be used together during the implementation of QA program in clinical settings.

Peer-review: Externally peer-reviewed.

Conflict of Interest: I have no conflict of interest.

Financial Support: None declared.

Conclusion