COMP report: CPQR technical quality control guideline for medical linear accelerators and multileaf collimators

Abstract The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR) has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control testing for medical linear accelerators and multileaf collimators.

nes have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control testing for medical linear accelerators and multileaf collimators. The development of the individual TQC guidelines is spearheaded by expert reviewers and involves broad stakeholder input from the medical physics and radiation oncology community. 1 Refer to the overarching document Technical Quality Control Guidelines for Canadian Radiation Treatment Centres 2 for a programmatic overview of technical quality control, and a description of how the performance objectives and criteria listed in this document should be interpreted.
All information contained in this document is intended to be used at the discretion of each individual center to help guide quality and safety program improvement. There are no legal standards supporting this document; specific federal or provincial regulations and license conditions take precedence over the content of this document.

| SYSTEM DE SCRIPTION
Medical linear accelerators (linacs) are cyclic accelerators which accelerate electrons to kinetic energies from 4 MeV to 25 MeV, using nonconservative microwave radio frequency (RF) fields in the frequency range from 10 3 MHz (L band) to~10 4 MHz (X band), with the vast majority running at 2856 MHz (S band). [3][4][5][6] In a linear accelerator the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides. Electrons follow a linear path through the same, relatively low potential difference several times; hence, linacs also fall into the class of cyclic accelerators just like the other cyclic machines that provide curved paths for the accelerated particles (e.g., betatrons). The high power RF fields used for electron acceleration in the accelerating waveguides are produced through the process of decelerating electrons in retarding potentials in special evacuated devices called magnetrons or klystrons.
Various types of linacs are available for clinical use. Some provide X-rays only in the low megavoltage range (4 MV or 6 MV) while others provide both X-rays and electrons at various megavoltage energies. A typical modern high-energy linac will provide two or three photon energies (usually a combination of a low [4 to 10 MV] and a high [12 to 25 MV] photon beam) and several electron energies (ranging from 4 to 22 MeV).
Included in the scope of this document are multileaf collimators (MLCs); computer-controlled devices capable of providing photon beam shielding for linear accelerators using high density leaves (typically tungsten alloy) which are projected into the radiation field. [7][8][9] In addition to static beam shaping, beam intensity modulation can also be achieved by adjusting the position of the MLC in the radiation field between treatment fields (step and shoot, or static intensity-modulated radiation therapy [IMRT]), by moving the leaves across the field with varying velocities during the beam-on time (dynamic IMRT), or by varying the dose rate, gantry speed, and MLC leaf positions during arc delivery (volumetric modulated arc therapy [VMAT]). By doing this, a desired fluence pattern can be approximated within certain physical limits.
Current MLC systems vary with respect to design, location, and use. They may be installed as a tertiary device below the secondary collimators, or they may comprise a total or partial replacement of the secondary collimators. The leaves must provide an acceptable degree of beam attenuation, provide a large enough field coverage, and must be well integrated with the rest of the collimator shaping system. In order to minimize penumbra, various design considerations have been devised by manufacturers to provide focused field shaping.
Computer control is a key component of the MLC, particularly during the delivery of dynamic treatments. There must be feedback on the leaf position and beam interlock capabilities when leaf misplacement is detected. In addition, there must be interlock capabilities to detect leaf carriage positions that could lead to unintentional irradiation outside the shielded area. Other safety interlocks must recognize the unintentional use of the MLC in electron mode and incorporate the use of the MLC in port-film mode (Tables 1-3

| TE ST TABLES
To ensure a safe and acceptable level of radiation treatment quality the performance of medical linear accelerators and their associated multileaf collimators must be assessed and monitored as a part of a comprehensive quality control program. Tables one through three, below, along with their associated notes, summarize the tests, frequencies, tolerances and action levels recommended for this equipment within such a program. Gantry angle 0°, 100 cm source-axis distance (SAD). This test demonstrates the field edges are accurately defined by jaws and/or MLC leaves. It is sufficient to confirm a predefined field shape using the projected light field at isocenter. Tolerance and action levels apply to each edge of a rectangular field at isocenter as defined by the jaws/MLC leaves. Note that systems with a tertiary collimation MLC system will require both jaw and MLC leaf positions to be verified DL8 Output constancy must be verified for all photon energies in use on the particular treatment day. Measurement is to be conducted using standard local geometry using a dosimetry system calibrated against the local secondary standard system DL9 Output constancy must be verified for all electron energies in use on the particular treatment day. Measurement is to be conducted using standard local geometry using a dosimetry system calibrated against the local secondary standard system Measurements are made to confirm that the depth dose has not changed since commissioning the unit. Tolerance and action levels are specified in percentages for photon beams and in millimetres for electron beams. A single ratio of doses taken at clinically relevant depths is sufficient for these measurements. Alternatively, a tissue-phantom ratio (TPR) measurement or a check of profile constancy at a shallow depth could be used, and the tolerance and action levels adjusted appropriately

ML12
This test replaces testing of flatness and symmetry and is intended to be consistent with the testing suggested in American Association of Physicists in Medicine (AAPM) protocol TG-142. 14 The goal is to ensure that profiles are delivered in a manner consistent with that modeled in the associated treatment planning system. Tolerance and action levels refer to differences from commissioning (or baseline) profiles as defined in the AAPM protocol TG-142. 14

ML13
Geometric alignment of the radiation and optical field edges must be established over a range of field sizes. Tolerance and action levels apply to each edge of a rectangular field

ML14
Accuracy of the radiation field edge of the jaw must be established over a range of jaw positions. The number of positions tested shall be determined from the jaw calibration method. In conjunction with this test it is important to establish acceptable dose profiles for abutting fields at the 0 position. Here, the 2 mm action level for each jaw is generally not sufficient since in principle, abutting fields could have a difference of up to 4 mm between field edges, which can lead to unacceptable peaks or valleys in dose distributions. A tolerance of 5% and an action level of 10% in dose profile deviations for abutting fields are suggested

AL13
This test determines the diameter of the radiation isocenter defined by couch rotation through its full clinical range of motion. The diameter must be within specifications

AL14
The coincidence of radiation and mechanical isocenters is established for the collimator, gantry and couch. Coincidence must meet the specified limits

AL15
The three axes of rotation (the collimator/MLC, the couch, and the gantry) must meet within a sphere of the specified diameter. 20 The average and maximum leakage between abutting closed MLC leaves is verified in this test for all photon energies and compared with the values established at the time of commissioning or the values adopted in the treatment planning system. Tolerance and action levels refer to changes from the commissioning measurements AL20 Use a leaf pattern where one leaf from each leaf bank protrudes well into the field. Confirm the leaf edge parallelism with the collimator or solid jaw edge AL21 A dynamic leaf gap test (sometimes referred to as a dosimetric leaf gap test) is performed to confirm consistency with baseline measurements. The minimum standard is to establish this using a single detector (e.g., an ion chamber) method, although methods that calculate separate factors for each leaf pair may be employed. The value must be consistent within tolerance for all four cardinal gantry angles

AL22
To ensure redundancy and adequate monitoring, a second qualified medical physicist must independently verify the implementation, analysis, and interpretation of the quality control tests at least annually

ACKNOWLEDG MENTS
We thank the many people who participated in the production of this guideline.

CONFLI CT OF INTEREST
The authors have no conflicts of interest to declare.