Quality Assurance Program for Surface-guided Radiation Therapy: A Review of Guidelines

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.[] 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 Recommended parameters for acceptance tests in ESTRO-ACROP and AAPM guidelines

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 Suggested parameters in ESTRO-ACROP and AAPM guidelines to consider during system commissioning

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.