AAPM‐RSS Medical Physics Practice Guideline 9.a. for SRS‐SBRT

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances. Approved by AAPM Professional Council 3‐31‐2017 and Executive Committee 4‐4‐2017.

practice setting, this Guideline has been developed to provide appropriate minimum standards for such services.

1.A | Scope
This MPPG's scope includes medical physics support for the entire treatment process including acceptance testing, commissioning, technical process development, treatment planning and delivery, and quality assurance related to linac-based SRS, SRT, and SBRT, hereafter referred to as SRS-SBRT. For ring-mounted helical tomotherapy linac delivery systems, this document applies to SBRT only. a This MPPG is not intended to address SRS-SBRT procedures based on gamma ray and particle beam (proton or heavier) sources as well as linac-magnetic resonance imaging (MRI) combination machines. 1.C | Definitions 1. End-to-end (E2E) testinga methodology used to test whether the flow of an application is performing as designed from start to finish. The purpose of carrying out E2E tests in radiation oncology is to identify system dependencies, to ensure that the intended information is correctly passed between various system components, to verify that clinical team members understand their tasks, and to assess overall treatment process accuracy. All aspects of the treatment process should be considered, including immobilization, simulation, respiratory-related motion management, treatment planning, and treatment delivery using a clinically relevant image guidance method. Each step in the E2E testing should be performed by the staff member who will perform the step when the program is clinically implemented.

2.
Medical dosimetrista person other than a radiation oncologist or medical physicist who participates in, performs, and/or assists in the procedures required to develop a radiotherapy treatment plan with related treatment delivery parameters, working under the supervision of a radiation oncologist and qualified medical physicist (QMP).
3. Quality Assurance (QA)as defined in the AAPM Task Group 100 report: 2 "QA confirms the desired level of quality by demonstrating that the quality goals for a task or parameter are met." In the context of this document, QA refers to the programmatic approach to ensuring quality and safety in SRS-SBRT treatments. 4. Qualified Medical Physicist (QMP)as defined by AAPM Professional Policy 1. 1 For this practice guideline, the applicable subfield is therapeutic medical physics. Task Group 100 report: 2 "QC encompasses procedures that force the desirable level of quality by evaluating the current status of a treatment parameter, comparing the parameter with the desired value, and acting on the difference to achieve the goal." In the context of this document, QC refers to specific tests performed as described in the QA program.

2.A | Supervision level
This document follows supervision levels defined in AAPM Professional Policy 18. 5 For the delivery of all radiation therapy services, the two responsible professionals are the radiation oncologist and QMP. All other team members work under the supervision of these professionalsclinical procedures supervised by the radiation oncologist and technical procedures supervised by the QMP.     "The medical physicist is responsible for the technical aspects of radiosurgery and must be available for consultation throughout the entire procedure: imaging, treatment planning, and dose delivery." The ACR-ASTRO Practice Parameter for SBRT 4 describes the medical physicist's responsibility similarly for SBRT.

2.
Perform acceptance testing and commissioning of the SRS-SBRT system, including validation of the treatment planning system accuracy with small fields and tissue heterogeneities (if relevant to the scope of SRS-SBRT services offered), accuracy of targeting through end-to-end (E2E) testing, and quality and precision of the image guidance system.
3. Implement and manage a QA program to ensure proper ongoing performance of the treatment delivery unit, immobilization and simulation devices, image guidance system, and treatment planning system.

4.
Work with other team members to develop standard operating procedures (SOPs) for major steps through the entire treatment process.

5.
Establish a comprehensive safety checklist to act as a guide for the entire treatment process, and determine appropriate methods for the clinic's quality assurance committee to monitor the SRS-SBRT program.
6. Facilitate and manage the clinic's participation in an incident learning system to ensure a transparent, structured evaluation of all "near miss" and actual deviations in the planning and treatment delivery process.

7.
Perform or supervise the dosimetric treatment planning process, providing supervision levels as appropriate to each task (e.g., direct supervision at the initial and final phases of the treatment planning process).
8. Review the final treatment plan for accuracy and deliverability, consulting with the radiation oncologist to ensure that both professionals are confident of the acceptability of the chosen treatment plan.

9.
Validate the chosen treatment delivery parameters via an independent dose calculation. When deemed appropriate, a phantom measurement or treatment delivery "dry run" may also be performed.

10.
For the first treatment session, a QMP with relevant SRS-SBRT training must provide personal supervision of the entire session. b For any subsequent treatment sessions, direct supervision must be provided by either a QMP or a medical physicist who was present during the initial treatment session.

2.C.2 | Responsibilities
As stated in the ACR-ASTRO Practice Parameter for SRS 3          3.C | Simulation, planning, and treatment resources 1. Appropriate devices for patient setup and immobilization.

2.
Appropriate devices for proper motion management.  7. Capability to calculate, display, and evaluate composite dose for patients who have received prior radiation therapy.
8. Linac-based treatment delivery system with appropriate mechanical accuracy, field-aperture size, and resolution for small-target conformality, and image-guidance devices for target localization and verification including motion management technology relevant to the scope of SRS-SBRT services to be offered. 6,7 3.D | Administrative support 1. Commitment to support the delineation of duties, procedure-specific QA, and staff authority required for safe delivery of SRS-SBRT services, as defined in SOPs developed by the institution's QMP and medical director of radiation oncology consistent with the institution's credentialing process.

4.A | Acceptance testing
The QMP must be involved with the process of facility design, equipment selection and specifications, and provide direct supervision during the acceptance testing process. 9 Customer acceptance test procedures are intended to ensure that the equipment satisfies the performance requirements stated in the purchase agreement, including that the equipment is safe to operate. In some cases, measurements completed as part of the acceptance procedures may also serve as components in establishing the routine quality assurance program. The vendor must demonstrate acceptable system performance.

4.B | Commissioning
To determine the scope of SRS-SBRT commissioning, the QMP must understand the scope of procedures/services to be offered. Commissioning encompasses the overall process of validating the planning and delivery system for the services to be offered, and developing appropriate QC and technical procedures to support these services.
The scope of commissioning must therefore be commensurate with the scope of clinical services to be offered.

4.B.1 | Equipment commissioning
Commissioning of a linac-based treatment delivery system is performed after acceptance testing. Commissioning tests should be developed by the institution's medical physics team to explore in detail every aspect of the system with the goal of developing a comprehensive baseline characterization of the performance of the system, identifying any limitations relative to clinical use, and developing procedures for QA and clinical use. 10,11 A variety of task group reports are referenced in this document to provide guidance on best practice for performing commissioning and QA of delivery devices. However, SRS-SBRT intent requires special consideration.
Each SRS-SBRT system is highly specialized with fixed cones and/or multileaf collimators (MLCs). Specific validation should be considered based on manufacturer recommendations and the determined scope of the practice. Commissioning of such systems includes, but is not limited to, a safety and geometric accuracy evaluation of the treatment and imaging components, comprehensive small-field data measurement with appropriate stereotactic detectors and careful equipment setup, evaluation of treatment planning system capabilities including multimodality image processing and calculation accuracy for small fields, and the development of a comprehensive QA program for each of the following critical components: 1. Treatment delivery machine 2. Immobilization devices 12 3. Ancillary systems for imaging 13 and motion management

Treatment planning systems 14
Special consideration: Small-field measurements Small-field dosimetry as used in SRS-SBRT is challenging due to many factors including source size, detector size, and response. 15 As a generalization, even micro-ion-chambers are large relative to the field sizes used in SRS-SBRT due to violation of cavity theory. 16,17 Generalized approaches to the lack of lateral equilibrium and violation of cavity theory have been addressed in the literature. [18][19][20][21] Newer solid-state microdetectors have become available such as diode, plastic scintillators, and synthetic microdiamonds that have shown appropriate characteristics for small-field dosimetry. Evaluations of many commercially available detectors have been published with correction factors for small-field dosimetry. 22 A practical measurement methodology for validating small-field beam data using multiple detectors has also been reported. 23 A newly published code of practice from the International Atomic Energy Agency 24 is also a useful guideline. An important characteristic of any detector used for commissioning is that the detector's active area be of a small size compared to the field size range to be characterized. A daisy-chain method is recommended, using two independent detectors suitable for measuring small fields. 14,22 Upon completion of beam data measurements, key data points (such as percent depth dose at 10 cm depth and output factors for field sizes ≤2.0 cm) should be compared to other machines of identical design, whether in the same institution or from other centers, to guard against gross errors which could arise from inappropriate detector selection or misaligned equipment setup.

Immobilization equipment
Immobilization equipment should be evaluated for its effectiveness in targeting accuracy and precision (e.g., through analysis of shifts after pretreatment imaging for a representative sample of patients for each treatment site), and should be evaluated for its beam attenuation and surface dose characteristics. 12 The effect on surface dose should be clearly articulated to the clinical team prior to implementation of the clinical service. Specific clinical implementation guidance is found in Section VII of the AAPM TG 101 report. 10 These components described in the TG 101 report are also applicable to SRS procedures. The following section is consistent with the TG 101 recommendations, providing additional details deemed relevant to a clinical SRS-SBRT program.

Standard operating procedures
Site-specific SOPs should address the components essential to the patient review, simulation, planning, treatment, and follow-up (see the Appendix for a sample SOP document). Patient safety should be the primary consideration when developing any SOP. c. The SOP should establish certain process expectations for safe implementation such as appropriate time intervals from simulation to treatment with critical points along the path allowing for reconsideration or rescheduling.
d. Every team member has the right and responsibility to halt a case and/or a particular procedure based on safety imperatives.

Patient selection
a. Patient selection criteria should initially be determined using data available from clinical protocols or published guidelines. HALVORSEN ET AL.

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Maximum target size should be documented along with standard prescription dose and fractionation schemes. b. Where possible, a multidisciplinary review or a peer review of proposed cases should be completed prior to simulation.
If the patient is enrolled in a clinical trial, the rules and guidelines of the clinical trial must be followed.   e. In cases of re-irradiation, the cumulative dose should be evaluated by the treating physician. A description of the method used and the outcome of the evaluation should be documented.
f. The use of an isotropic calculation grid size of 2 mm or finer is recommended. The use of a grid size >3 mm is discouraged. 10 For very small targets, a 1 mm calculation grid size may be necessary.
g. Target dose coverage, dose fall-off beyond the target, dose conformity metrics, and compliance with critical structure dose objectives 32 should be clearly reported and signed by the radiation oncologist to confirm that the chosen treatment technique is clinically acceptable.
h. An independent dose calculation check must be performed prior to treatment. b. The SOP should clearly describe the professional supervision requirements for each SRS-SBRT treatment type. [3][4][5]10,28 c. The SOP should clearly describe the image guidance method to be used, including target anatomy, critical organ avoidance, and localization tolerance. Pretreatment verification of target localization should always be performed; the criteria for intratreatment image guidance should be clearly described. 34 d. If motion management is used, the SOP should clearly describe the process, tolerances, and professional supervision.

End-to-End (E2E) testing
To assess the clinical team's readiness and to validate the SOP, the team should conduct "dry runs" of the entire process, observe and take notes, edit the SOP as needed, and repeat the E2E testing until the process is clear to all participants. The pre-implementation E2E tests and findings should be described in the commissioning report.
Each step in the E2E testing should be performed by the staff member who will perform the step when the program is clinically implemented. E2E process "dry runs" should be performed for each category of SRS-SBRT service, and when a key aspect of the process is changed.
When developing the E2E tests, all aspects of the treatment process should be considered, including immobilization, simulation, respiratory management, treatment planning, and treatment delivery using a clinically relevant image guidance method.

5.A | Introduction
A comprehensive QA program for SRS-SBRT is critical to ensure the correct dose is delivered to the target, given the very small target volumes and rapid dose fall-off associated with SRS-SBRT. QA processes and procedures related to SRS-SBRT should be designed to cover the follow-ing aspects of the SRS-SBRT program: equipment-specific QA, patientspecific QA, and procedure-specific QA. Safety and QA recommendations have been extensively described in several publications. 3,4,10,33 When equipment performance is found to be out of tolerance, the affected module(s) of the delivery system should be promptly adjusted, and the QMP should verify proper performance before clinical SRS-SBRT services resume. In the event of a significant service interruption, the QMP should coordinate closely with treating physicians to evaluate the impact on patients' treatment schedules given the importance of completing SRS-SBRT treatment courses in a short overall time interval (generally 14 days or less). 35 Patient safety should be the primary consideration in determining when to resume clinical services.

5.B | Minimum equipment-specific QA
The AAPM has published task group reports with recommendations for QA related to SRS-SBRT. TG-142 describes the linear accelerator QA for both conventional radiation therapy procedures and for SRS-SBRT procedures. 36 MPPG 2.a provides recommendations for commissioning and quality assurance of X-ray-based image-guided radiotherapy systems. 29 TG-135 provides specific guidance for QA of robotic radiosurgery systems, 28 and TG-148 provides specific guidance for QA of helical tomotherapy systems. 37

5.C.3 | Instrumentation for PSQA
The QMP determines the instrumentation appropriate to the SRS-SBRT technique to be verified. Common instrumentation includes radiochromic film, small-volume ion chamber (for relatively larger treatment fields), diode detector, portal imaging device calibrated for dose response, detector arrays, and, less commonly, polymer gel dosimetry. The institution must provide appropriate instrumentation to conduct PSQA as deemed necessary by the QMP. The clinical service should not be initiated if appropriate instrumentation is not available for the QMP's use.

5.D | Procedure-specific QA
Procedure-specific QA addresses issues related to operational tasks, such as checking whether: 1. The workflows to perform SRS-SBRT as defined in the SOP documents are consistently followed.

2.
Staffing level is appropriate.

Frequency
Test Tolerance

Daily
Laser localizationonly if using SRS techniques relying on lasers for target localization (e.g., frame-based SRS without X-ray IGRT) Ongoing competency assessment is necessary given the rapid evolution of technology and treatment methods for SRS-SBRT.
These activities should be properly documented.

5.E | QA program supervision
The QA program should be designed by a QMP who has specific training in SRS-SBRT, and should be reviewed by another QMP with SRS-SBRT experience. The daily QA procedure can be performed by a physicist or radiation therapist and be reviewed by the QMP prior to any SRS-SBRT treatment. Other routine QA or patient-specific QA may be performed by an appropriately trained medical physicist, and reviewed and co-signed by the QMP.

5.F | QA program review
When the SRS-SBRT program is in its initial phase, the QA program should be reviewed bi-annually as the clinical practice and utilization evolves. The frequency can be reduced to annual reviews once the clinical practice and utilization stabilizes.

N O T E S
a SRS and SRT using helical tomotherapy were excluded from the scope of this document due to the infrequent use of this technology for SRS and SRT. Exclusion from the scope of this document does not imply any AAPM position regarding the appropriateness of delivering such treatments using helical tomotherapy. b All treatments must occur under supervision of a QMP. In addition, a QMP must provide personal supervision at the first treatment, and as needed for subsequent treatments. The personal supervision should include participation in a time-out checklist, assessment of patient immobilization, assessment of adequate imaging parameters, accuracy of respiratory management (if applicable), consultation on excessive or unusual patient shift requirements during treatment not clearly caused by patient motion on the treatment couch, as well as other patient-or plan-specific needs.
T A B L E 3 Minimum SBRT relevant equipment QA and tolerances for helical tomotherapy systems.

Frequency
Test Tolerance