COMP Report: CPQR technical quality control guidelines for conventional radiotherapy simulators

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 of conventional radiotherapy simulators.

appropriate. The radiation production systems, however, are very different for simulators and accelerators, the former being low dose and low energy imaging systems, the latter being high dose and high energy treatment systems.
Radiotherapy simulators have two roles in the preparation of patients for radiotherapy. The first is localization in which the high contrast and resolution achievable with kilovoltage x rays are used to guide the oncologist in the determination of the anatomical volumes to receive therapeutic radiation doses and those to be avoided.
The information obtained during localization can be used as the input to two-dimensional dose computation. The second role is that of true simulation. Beams, which may have been designed during a three-dimensional treatment planning process, are set on the patient and the oncologist confirms that the geometric aspects of the treatment intent are being met.
Basic simulator design varies little across manufacturers. Detailed descriptions of the conventional radiotherapy simulators have been reported in the literature. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] A rotatable gantry c-arm is mounted on a stand. The source end of the c-arm supports an x ray head consisting of a shielded x ray tube, field delineation, and collimation systems; the opposing end supports an image receptor and film cassette holder. The x ray head is translatable to enable different focus-to-axis distances (FAD). A treatment couch capable of translation, elevation, and full rotation on a turntable is used to position the patient. A control console is located in a shielded area adjacent to the simulator room. Some of the mechanical and optical systems may also be operated using controls inside the simulator room, for example, a hand pendant.
Traditionally, the image receptor used most often has been an image intensifier. A permanent record of the x ray image has been acquired either through digitally capturing the image as presented on the TV monitor connected to the camera viewing the output phosphor of the image intensifier, or through the use of film. More recently, large area flat panel detectors have become widely available and these are finding increasing use in radiotherapy simulation.
A major difference between conventional radiography and therapy simulation is the large distance (typically 120-170 cm) between the x ray focal spot and the image receptor. Since the simulator has geometric flexibility (to rotate around the patient), the image receptor is further away from the patient. Furthermore, simulation often requires beam-patient geometries not normally used in conventional radiography/fluoroscopy, such as lateral or oblique views through large body thicknesses. These requirements result in higher skin exposures than would be encountered in diagnostic radiography. The requirement for geometric flexibility also limits the amount of shielding that can be attached to the x ray image intensifier and precludes restrictions on x ray beam size.

| RELATED TECHNICAL QUALITY CONTROL GUIDELINE S
In order to comprehensively assess conventional radiotherapy simulators performance, additional guideline tests, as outlined in related CPQR Technical Quality Control (TQC) guidelines must also be completed and documented, as applicable. Related TQC guidelines, available at cpqr.ca, include: Notes for daily tests

DS1
The configuration of this test will depend on the design of the facility and equipment. Safety is the concern and tests should be designed accordingly. As a minimum, manufacturer's recommendations and applicable regulations shall be followed.

DS4
Coincidence of crosswires and/or reticle and/or block tray axes for collimator angle 0°, gantry angle 0°at isocenter.

DS5
Coincidence of the x ray and optical images of the field defining wires for a 10 9 10 cm 2 field with a gantry angle 0°, collimator angle 0°, and source-to-surface distance (SSD) 100 cm. The tolerance and action levels apply to each field border. With an appropriate tool the test should be performed using the real time imaging device.

DS6
Both the optical and x ray images of the field defining wires for each field border should agree with the electronically indicated field size within the specified tolerance and action levels and for the geometry in DS5 above. With a verified reticle these tests can be performed with the aid of the realtime imaging device.

MS2
After determination of the 0°collimator position, which is then used as a reference, mechanical and digital collimator angle readouts shall be verified using millimeter paper for at least 0°, 90°, and 270°.

MS3
This test refers to the field wires orthogonality and to their perpendicularity to the crosswires. This test should be performed on both the optical and radiation image.

MS4
Automatic setting of the focus-axis-distance shall be checked, if relevant, using mechanical devices.

MS5
The possibility to move the amplifier to limits (determined at commissioning) in three cardinal axes should be verified.

MS6
The couch isocentricity shall be checked over a range of couch angles from 90°to 270°. The tolerance and action levels refer to the maximum displacement of crosshair projection from the initial position in the isocenter plane.

MS7
With a couch angle 0°, couch motions shall be parallel the cardinal axes of the simulator geometry over an appropriate clinical range.

MS8
The couch rotation angle shall be verified over an appropriate clinical range. Deviation between the true 0°a nd the mechanical and numerical scale should be determined. (Continues)

MS9
Mechanical and digital couch position readouts shall be verified over an appropriate clinical range in the directions of the three cardinal axes, if relevant.

MS10
Measurement of couch relative displacement in all three cardinal axes should be verified against digital readouts.

MS11
The radiation isocenter is established radiologically using the real time imaging device. Alignment of the crosswire and lasers at the isocenter is then confirmed for gantry angles of 0°, 90°, and 270°. The tolerance and action levels refer to deviation between the measured system and the isocenter.

MS12
A mechanical device, calibrated against the true radiation isocenter, is used to provide the base reading for the check of the optical distance indicator. The standards stated in Table 2 apply at the isocenter. The optical distance indicator should be checked over a clinically relevant range of SSD and gantry angle. The tolerance and action levels may be twice as large (i.e., 2 and 4 mm) at the clinical limits of the optical distance indicator's range.

MS13
The coincidence of both the optical and radiological images of the crosswires are measured with respect to radiological isocenter at 100 cm SSD for collimator angles of 0°, 90°, and 270°. The tolerance and action levels refer to the coincidence with the radiation isocenter.

MS14
Geometric alignment of the x ray and optical images of the field defining wires shall be established over a range of field sizes from 5 9 5 cm 2 to 35 9 35 cm 2 at gantry angles 0°, 90°, and 270°. Representative half-blocked fields shall be included. A minimum of six field sizes will be required for this test. The tolerance and action levels apply to each edge of a rectangular field.

MS15
Compliance of the x ray and optical images of the field defining wires with the indicated dimensions shall be established over a range of field sizes from 5 9 5 cm 2 to 35 9 35 cm 2 at gantry angles 0°, 90°, and 270°. Representative half-blocked fields shall be included. A minimum of six field sizes will be required for this test. Different field sizes should be examined at different gantry angles if appropriate and efficient. The tolerance and action levels apply to each edge of a rectangular field.

MS16
Documentation relating to the daily quality control checks, preventive maintenance, service calls, and subsequent checks shall be complete, legible, and the operator identified. Notes for semiannually tests SS1 Any available lead aprons, gloves, and other protective wear should be visually and radiologically inspected for cracks and appropriate action taken should cracks be found.
SS2À4 A variety of equipment is available for performing these tests. The tolerance and action levels will need to be developed locally depending on the equipment available and the performance variability in the observers. Routine monitoring of these parameters should be based on performance at installation.

SS5
The limit on fluoroscopy time is verified. The mechanical, optical, and radiation isocenter should be redefined and optical and mechanical systems realigned. Coincidence between gantry, collimator, and couch isocenters shall be verified.

AS3
Couch deflection is measured with 70 kg at the end with the couch extended to the isocenter.

AS4
Typical exposure factors are used.
AS5 kVp should be measured at least three settings over the range from 60À120 kVp. When measured non-invasively, tolerances and action levels should refer to baseline values established at acceptance and referenced to invasive measurements.

AS6
Tolerance and action levels refer to the coefficient of variation in 10 measurements of relative exposure at a typical set of operating parameters. These tests should be performed with and without automatic exposure control. (Continues)

AS7
Half-value layer (HVL) is to be compared at three kVp values with the baseline values established at acceptance.

AS8
Where more than one detector can be used for automatic exposure control, consistency between the exposures delivered should be established.

AS9
To ensure redundancy and adequate monitoring, a second qualified medical physicist shall 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 Cancer.

CONFLI CT OF INTEREST
The author has no conflict of interest to disclose.