ESTRO-ACROP guideline on surface guided radiation therapy

Surface guidance systems enable patient positioning and motion monitoring without using ionising radi- ation. Surface Guided Radiation Therapy (SGRT) has therefore been widely adopted in radiation therapy in recent years, but guidelines on workﬂows and speciﬁc quality assurance (QA) are lacking. This ESTRO- ACROP guideline aims to give recommendations concerning SGRT roles and responsibilities and high-lights common challenges and potential errors. Comprehensive guidelines for procurement, acceptance, commissioning, and QA of SGRT systems installed on computed tomography (CT) simulators, C-arm linacs, closed-bore linacs, and particle therapy treatment systems are presented that will help move to a consensus among SGRT users and facilitate a safe and efﬁcient implementation and clinical application of SGRT. (cid:1) 2022 The Author(s). Published by Elsevier B.V


a b s t r a c t
Surface guidance systems enable patient positioning and motion monitoring without using ionising radiation. Surface Guided Radiation Therapy (SGRT) has therefore been widely adopted in radiation therapy in recent years, but guidelines on workflows and specific quality assurance (QA) are lacking. This ESTRO-ACROP guideline aims to give recommendations concerning SGRT roles and responsibilities and highlights common challenges and potential errors. Comprehensive guidelines for procurement, acceptance, commissioning, and QA of SGRT systems installed on computed tomography (CT) simulators, C-arm linacs, closed-bore linacs, and particle therapy treatment systems are presented that will help move to a consensus among SGRT users and facilitate a safe and efficient implementation and clinical application of SGRT. Surface Guided Radiation Therapy (SGRT) is a relatively recent technology that was added to existing image guidance systems. SGRT typically uses visible structured light, stereo-vision systems, time-of-flight systems, or laser scanners to image the surface of a patient with high temporal and spatial resolution without additional radiation dose [1,2].
Generally, the systems employ multiple cameras and through the principle of triangulation, they can reconstruct a threedimensional representation of the patient's surface (referred to as surface) that is related to the treatment coordinate system and, in some systems, also calculate the treatment isocentre location within that surface. The surface is updated in real-time by the system during patient setup and treatment delivery. For setup this can improve accuracy and workflow. During delivery any inconsistencies between the surface ('live' surface) and the reference surface can be detected in real-time [3]. The reference surface is based on the computed tomography (CT) scan acquired for treatment planning or a surface image obtained earlier with the SGRT system.
The main technical advantages of SGRT systems are that they use non-ionizing radiation and that they offer near real-time monitoring with a large field-of-view (FOV) compared to other in-room imaging systems, with additional possibilities of introducing tattoo-free and open-mask or mask-free workflows. The visual information provided by SGRT is intuitive and may increase patient safety [4]. One limitation of surface-based systems is that they only image the patient's external surface and the correlation to the internal anatomy remains uncertain. Additionally, the patient's skin needs to be exposed, which can provide challenges in terms of privacy and comfort, but also in the presence of fixation devices such as masks. Abbreviations: 2D, 2-dimensional; 3D, 3-dimensional; 4D, 4-dimensional; 4DCT, respiratory-correlated computed tomography; 6DoF, six degrees of freedom; CBCT, cone beam computed tomography; CT, computed tomography; DIBH, deep inspiration breath hold; FB, free-breathing; FMEA, Failure modes and effects analysis; FOV, field-of-view; IGRT, Image-guided radiotherapy; kV, kilovoltage; MDT, multidisciplinary team; MPE, Medical Physics Expert; MV, megavoltage; OIS, oncology information system; QA, quality assurance; R&V, record and verify; RO, Radiation Oncologist; ROI, region-of-interest; RT, radiation therapy; RTT, radiation therapist; SBRT, stereotactic body radiation therapy; SGRT, Surface Guided Radiation Therapy; SOP, standard operating procedure; SRS, stereotactic radiosurgery. SGRT is established in many modern radiation therapy (RT) departments. Several vendors offer SGRT systems for patient setup and monitoring for a wide range of treatment simulation and delivery systems, while some institutions have developed inhouse SGRT solutions [5,6]. The European Society for Radiotherapy and Oncology (ESTRO) SGRT working group was initiated at the 3rd ESTRO physics workshop 2019 in Budapest. As confirmed by the survey carried out by this working group in 2020 [7], one major concern shared by many prospective users is the lack of consensus-based clinical guidelines including specific quality assurance (QA) recommendations. This led to the need for a comprehensive up-to-date European consensus on the clinical use and QA of SGRT and prompted the development of these recommendations. Independently, the American Association of Physicists in Medicine (AAPM) published a SGRT-specific task group report (TG 302) [8], building on the TG 147 report which covered QA for non-radiographic localisation and positioning systems [9].
This ESTRO-ACROP guideline is designed to guide current and/ or prospective SGRT users through the clinical workflow with specific comments on staff responsibilities, emphasising key points for the optimisation of workflows, and highlighting the most common challenges and potential failure events. It details considerations for acceptance testing and describes important aspects of commissioning and routine QA methods. Many vendors offer additional functionalities as part of their commercial SGRT system, such as patient identification, accessory verification, initial whole-body setup, etc. These additional functionalities are out of the scope of this report as it only considers the SGRT functionality of these systems. The consensus for the guidelines was reached in multiple online discussions of thematic subgroups and approval of the final draft by all group members.

Clinical workflows and responsibilities
Roles, responsibilities and training requirements SGRT implementation should be led by a core multidisciplinary team (MDT), usually consisting of radiation therapists (RTTs), medical physics experts (MPEs) and radiation oncologists (ROs). Each institution should establish guidelines for staff responsibilities, which may vary between institutions and countries, as national professional training for healthcare staff varies widely from country to country. Frequently, MPEs are involved in commissioning and monthly/yearly QA tests, while RTTs perform daily QA checks, patient treatment preparation, patient positioning and treatment delivery. All team members should principally be familiar with each step of the workflow (see Fig. 1) so that they can intervene in case of unforeseen events or the need for rapid troubleshooting. The core SGRT team, consisting of at least two MPEs and two RTTs, should be endorsed to receive extensive SGRT training.

Workflow optimization
Although a SGRT workflow is vendor-and clinic-specific, some concepts and steps are common in each SGRT application (see Fig. 1).
A typical setup and monitoring workflow uses one or several reference surfaces for positioning and monitoring the patient during RT. A reference surface is generated either using the SGRT system itself (at the CT simulator or in the treatment room) or by extracting the external contour from the DICOM-RT structure data set and the isocentre information from the DICOM-RT plan. Through calculating the deviation between the current (live) and the reference surface, in a region-of-interest (ROI)/point/whole surface, the SGRT system can support patient-positioning by providing information on the required translations and/or rotations of the treatment couch to the RTT on a screen and by other visual means. If so configured, the SGRT system can also send the information to the treatment system allowing for automatic couch movement. Depending on the treatment site and workflow a radiographic position verification can subsequently be performed. The SGRT system can monitor the position of the defined area of the patient's surface during radiographic imaging and treatment. The reference surface used for monitoring is dependent on vendor and/or treatment modality and strategy. Continuous motion monitoring can be carried out throughout the treatment fraction. If the patient moves out of the surface tolerance at any time, the system can stop the treatment, if this function is available and turned on for the combination of SGRT system and linac, or RTTs can interrupt the beam manually.
A generic workflow as described above can be found in more detail in several literature sources, with respective modifications for only-setup [10,11], breath-hold [12], stereotactic radiosurgery (SRS)/stereotactic body radiation therapy (SBRT) [13,14], and for different anatomic sites like head-and-neck, thorax and abdomen [15][16][17]. To take advantage of SGRT, a well-structured clinical implementation is recommended and therefore, a comprehensive and site-specific workflow should be clearly defined. A common SGRT application is breath-hold monitoring, which is most commonly, but not exclusively, used for left-sided breast cancer treatments. Breath-hold treatments require special workflows often involving multiple reference images in addition to a breathing signal as a surrogate for inspiration level. Table 1 summarises some recommendations for each workflow step aimed to streamline the process, highlighting the essential parameters to be considered/improved, saving resources (time and staff), increasing patient safety and treatment accuracy. A separate column is shown for the particular case of a breath-hold workflow.
The combination of Image-guided RT (IGRT) and SGRT should be considered as standard, however periodicity of radiographic imaging and action levels should be site-and workflow-specific adapted.

Challenges & potential failure events
Potential errors collected by the workgroup when using SGRT are summarised in Appendix I. In many cases, treatment errors can be avoided by commissioning and QA procedures. However, some failure modes and errors are the result of incorrect workflows and/or insufficient training. Common problems include using the wrong reference for treatment and not creating a proper ROI for treatment [18]. The positioning and monitoring parameters, such as a standard ROI for each treatment indication, action levels/tolerances, and acquisition settings, should be defined by the MDT and communicated to all staff involved in the SGRT programme. In general, training and continuous education of SGRT users are essential to prevent errors or non-optimal application of the technology. Regular safety notifications from vendors would be desirable to inform the user community of reported problems. IGRT is a very useful tool to detect SGRT inaccuracies. Therefore, IGRT and SGRT should be used in a complementary manner. Overall SGRT can increase the safety of radiation therapy treatments preventing severe errors but adds (less severe) failure modes [4]. A proper risk analysis [19,20] is an effective measure to minimise the risks of a SGRT workflow.

Procurement and installation of SGRT systems
For this phase, the core multidisciplinary SGRT project team of ROs, MPEs, and RTTs, should be expanded to also include in-house engineering, IT specialists, facility engineers and hospital management.
First, clinical priorities and indications need to be identified. Then to assess suitability, vendors should be asked to complete a questionnaire that describes the capabilities of their SGRT system concerning the needs of the clinic, considering existing equipment and special circumstances, and outlining installation timescales and conditions as well as staff training opportunities (see Appendix II). Invited vendor presentations can be used for further education on possibilities and options.
The MDT should conduct site visits to clinics with extensive experience with the SGRT system or systems considered for purchase. Ideally, clinics should be chosen that have similar patient management routines and radiation delivery systems. The MDT should consist of at least radiation therapists and MPEs, but preferably also include a radiation oncologist and hospital management. It is useful that the same team visits all clinics with considered SGRT systems, that questions are prepared in advance and that answers are collected at each visit and presented to the team. Vendors should be invited for a site visit and written confirmation of any additional work (costs) required should be obtained before a purchase decision is made.
Considerations include installation requirements like available space at the bunker ceiling, interfacing with linac and record & verify (R&V) system, interference with other systems, including videosurveillance cameras, ventilation vents and patient-support lifts, room lighting, cable channels between bunker and control room, power-supplies, wireless network availability, as well as additional requirements like additional licences needed for the envisioned workflow. For SGRT installations at particle-therapy systems, more considerations about requirements need to be made: the room configuration limits the available camera positions, the possibility of collision/blocking with other objects, and the possibility to prealign the patient out of the treatment isocentre. A close collaboration between the SGRT and particle therapy vendors is vital to find the best setup.
Based on all this information, the project team should make a recommendation on the purchase options for the clinic's decision makers. It should also perform a risk analysis and develop a realistic time plan for the next steps and an introduction of SGRT to the clinic, down to the changes in standard operating procedures (SOPs).

Systems specifications
The individual specifications of each kind of SGRT system, either simulation room or treatment room, dictate the level of the acceptance and commissioning protocol. Moreover, each system would have different parameters and respective tolerances to be assured.

CT simulators
An SGRT system at the CT simulator can be used for breath-hold monitoring during CT acquisition, for patient coaching and, with some systems, to create a SGRT reference surface. Additionally, some systems offer the possibility to use the respiratory motion signal as a surrogate to reconstruct a respiratory-correlated CT (4DCT) retrospectively or prospectively. Usually only one camera pod is installed in a CT room, resulting in a different FOV compared to a 3-camera pod installation. An interface with the CT simulator might be available with different levels of integration (import/export breathing signal, beam interlock).
Functions and main parameters to test during acceptance include the calibration and accuracy of the SGRT camera(s), tracking of the surface during couch movement, impact of the ROI selection and testing the connections with all integrated systems (see Table 2).

C-arm linacs
The application of SGRT in a C-arm linac setting is the most common [7,21], mainly due to the prevalence of these treatment systems, but also because of their multifaceted uses, ranging from patient setup, monitoring, motion management (free-breathing (FB)-gating, breath-hold to non-coplanar techniques). However, this diversity results in an extensive combination of parameters, tolerances and tests, which users need to adapt to their clinical practices. Issues that most affect the SGRT performance on C-arm linacs are camera pod occlusion (by gantry, imaging detectors or kilovoltage (kV)-source) in combination with the rotational irradiation techniques and reference image calculation for treatments with couch rotations.

Closed-bore linacs
Closed-bore linacs represent several challenges for standard ceiling mounted SGRT systems due to bore occlusions and patient self-occlusions which prevent sufficient surface coverage and six degrees of freedom (6DoF) surface tracking when the patient is ESTRO-ACROP guideline on surface guided radiation therapy Camera light and skin colour settings: should be optimised for each patient if system allows Confirm that none of the SGRT camera pod(s) are blocked by gantry/couchrotation/panels for reference surface acquisition or during treatment (before the treatment start) and differentiate gantry-blocking variations from patient-variations (checklist to identify the cases, optimise the ROI) At first fraction, SGRT positioning must be verified by independent IGRT (kV and/or megavoltage (MV), 2D or 3D); document the workflow for the following fractions in the R&V system If a new reference surface is acquired, this should be done only with IGRT verification (kV, MV, or cone beam computed tomography (CBCT), according with the site-/application-protocol). In general, the generation of a new reference surface should be carefully considered, with minimal patient rotations (<1°). Ideally IGRT verification should be performed immediately before and/or after capturing the new reference.
Use motion monitoring: Treatment should be interrupted if the patient moves out of tolerance, ideally automatically. If a patient does not move back in the original position by themself, repositioning with SGRT/IGRT. -> define a decision-chart including staff roles to help RTTs to select the action to be taken (i.e. for motion during IGRT, motion after imaging, motion during irradiation. . .); the decision chart should clearly outline staff roles and decision-making authority (or define thresholds for requiring certain staff input, e.g. Notify physics if shifts > X mm) SRS/SBRT: After couch-rotation, establish a decision-process in case of outof-tolerance SGRT-vectors, e.g. repeat imaging and correct shifts. Documentation report of SGRT with additional indication of the gating windows and ROI should be produced. ESTRO-ACROP guideline on surface guided radiation therapy at treatment position [22]. Some vendors provide as a solution a hybrid platform where ceiling mounted cameras are used to facilitate patient setup outside the bore and reduce inter-fraction uncertainties. Inside the bore, miniaturised ring-mounted cameras are installed to monitor the patient for intra-fraction motion. This technology is critical for adaptive radiotherapy, breath hold treatments for breast and/or lung and liver SBRT [22,23].

Particle therapy treatment systems
The limited availability of 3D in-room imaging for patient positioning in particle therapy raises the need to improve the patient setup workflow. Therefore, SGRT has a role in clinical practice, both for pre-alignment and for treatment monitoring. The dose delivered by particle therapy is more sensitive to patient misalignment and prone to interplay effects of the scanned beam and respiratory motion, therefore a precise and reactive monitoring system is crucial to ensure the best dosimetric quality of the treatment for freebreathing and breath-hold treatments. The integration of SGRT in the particle therapy clinical workflow demonstrated a reduction in patient positioning times [24] and radiation exposures [25] during the setup phase.

Acceptance & commissioning Acceptance
The tests performed in the acceptance process (Table 2) should cover the relevant parameters and confirm that the delivered and installed system performs according to the agreed upon specifications. Therefore, the tests should cover the relevant parameters and they should be sensitive enough to confirm performance as specified. A recommended set of tests can be found in Table 2. Special attention should be paid to interconnectivity with other systems and if the installation differs in any way from a standard installation of a system of that type. The acceptance procedure is an integral part of the purchasing process to ensure that all tests required by the clinic are included. While usual system warranties will include performance guarantees, covering the important parameters during acceptance is most efficient. It allows on-the-spot corrections of any deviations found with the installation engineer still onsite.
It is important to schedule sufficient time for the acceptance procedure. Acceptance should be performed by one, or preferably two suitable members of the project team together with the vendor's installation engineer. Table 3 Suggested parameters to consider during the system commissioning. These can vary with the installation and with the SGRT-applications and product. Some tests might already have been performed during acceptance and are listed here for completeness in italic (#) -include other aspects not evaluated at the acceptance stage (x -mandatory, ooptional, pass -for a specific indication and application the system is accepted for clinical use). Different levels of testing might be required depending on the application (i.e. Basic, Advanced, Radiographic, . . .) consult the test description in Appendix III for details. Commissioning Commissioning is part of the QA program, and it includes testing the system capabilities and verifying its accuracy/precision in all clinically relevant scenarios. This set of tests (Table 3) should be outlined in a way that can be later reproduced, to assess the consistency of the system performance (e.g. after an upgrade, service-repair, etc.). This phase also includes identifying the system limitations, optimal performance, and documenting the initial system performance. Usually, commissioning is performed by a team of MPEs that consults with vendor representatives in case of inconsistencies or concerns.
One of the important aspects of this process is the end-to-end test, where the entire radiotherapy chain, related to the use of SGRT, should be checked for each unique workflow. This includes for a test phantom, the CT acquisition, import of scan to a treatment planning system (TPS), contouring, definition of the treatment plan, export to the SGRT system, import of plan to an R&V system, SGRT configuration, phantom setup using SGRT and relevant imaging and irradiation techniques (if applicable, breathhold techniques). Depending on the clinical usage of SGRT, additional tests have to be performed as indicated in Tables 2 and 3. The tolerances in Tables 2 and 3 have been adapted based on AAPM TG 147 [9] and AAPM TG 302 [8] for the tests described. Some tolerance values were tightened, and new tests were introduced based on the availability of new technologies and updated clinical needs.

Quality assurance
Quality assurance checks on SGRT should be performed at regular intervals which are generally annually, monthly, weekly, and daily with the understanding that daily means days on which the SGRT system is used. Additionally, SGRT QA needs to be performed following repairs and regular maintenance of the SGRT system or other peripheral systems that influence the SGRT system, which should be at least in an annual frequency.
For clinics new to SGRT or to the specific SGRT system it is reasonable to start with a higher frequency of tests, as well as with a larger number of tests and reduce either or both once the team is more comfortable with the equipment and based on test outcomes preferably including a failure modes and effects analysis (FMEA) specific to the clinic.
The recommended QA tests will generally be independent of the specific SGRT system, although most systems are technically integrated in either the CT, linac or particle-based systems which may require some slightly adapted dedicated test procedures. Table 4 lists the minimum recommended tests, which should be adapted and expanded on for newer installations and for specific treatment techniques requiring higher accuracies. Descriptions of the tests are provided in Appendix III and an overview of phantoms for the tests is provided in Appendix IV.
All tests using an anthropomorphic phantom should be performed as close to the clinical application as reasonable. This Table 4 Quality Assurance tests categorised by daily, weekly, monthly or annually periodicity. A -annually, M -monthly, W -weekly, D -daily, R -following repair or maintenance, Ooptional. ESTRO-ACROP guideline on surface guided radiation therapy means a relevant phantom should be used and the ROI of the SGRT system should be set as it would be in the clinical scenario. If there are multiple distinctly different scenarios (e.g., very large and very small ROIs), the tests should be done for each scenario separately. Additional detail can be found in [26].

Conclusion
This document presents the first European comprehensive guidelines on the commissioning, QA, and clinical use of SGRT systems. Its application will contribute to a consistent, safe, and efficient implementation and use of SGRT in clinics. The additional workload for training, implementation and QA that comes with SGRT should be reflected in hospital staffing and funding levels. SGRT and IGRT are complementary technologies and combined SGRT-IGRT workflows should be developed. The technology of SGRT is evolving rapidly and the current guidelines may need to be adapted to future SGRT solutions.