Quality Assurance of Treatment Simulators

Author: Dr. Pál Zaránd

 
The radiation therapy uses even more modern imaging devices. From these, the most specific not diagnostic instrument is the simulator. The QA of the simulator is discussed in this chapter. Typical diagnostic or primarily diagnostic devices (CT, MR, US, and nowadays PET) and the CT option of the simulators are not discussed in this chapter. The CT simulator doesn’t use ionizing radiation and direct imaging during the simulation, so in a classical sense, it is not a simulator and that is not discussed in this chapter.
Simulators were made as crossing combination of the mechanism of a teletherapy equipment and the radiation source and imaging system of a C-arm image intensifier. Due to this the QA of the simulators shows many similarities to the QA of these devices. The main goal is simulating the high energy beam (Beam Eyes View) and checking the parameters of the planned radiation field (coordinates, field size, positions, etc.), that is why the elements of QA are focused on the mechanical parameters. This is understandable because till nowadays the image quality of the worst image intensifiers was better as for example the best port film or the EPID (Electronic Portal Imaging Device) systems (which use MV photons). Lately, the image quality improved dramatically by the appearance of the large surface, amorphous-silicon detectors.

The function of simulator determinates the mechanical requirements: these cannot be lower than prescribed for the simulated device, mostly the accelerators, moreover in some cases they must be stricter.
The simulator-related requirements are given in the appropriate IEC standard, but the BJR supplement 23 is also a good reference. However it is very important always to compare the actual measurement to the initial data (it is also required by the 31/2001 EüM Hungarian regulation).
It can be suggested to study the acceptance test document of the vendor, because some unnecessary information can be in this document, besides some deficiency can be discovered as well.
Hereinafter the most important tests are summarized, referred to the necessary methods as well. The tests and the tolerance values are summarized in a supplement of this chapter. The simulator must be used within the tolerance values. If there is a greater deviation than that, forthwith service must be called. In case of emergency the device can be used only with the written permission of the authorized person.
Important, that the QC program shall be always controlled by the medical physics expert.

Equipment required:
• Test patterns for optical and radiological test (Figure 1.-3.)
• Mounting system to fit and to rotate them for using at different gantry angles
• Mechanical pointer for 100 cm focus-table distance
• Tool for defining isocenter
• Films
• Image intensifier test phantom
• Densitometer
• Tape-measure
• Graph paper
• Spirit-level
• Weight for the loading test of the table

 

Mechanical controls

 
From the introduction it follows, that a part of the tests are very similar to those which are used by the treatment units (laser, field verification, etc.). At the same time simulators contain imaging device as well, so the results of the field verification can be seen on the image intensifier immediately. It can be used very well, if the field size is within the tolerance values (marked on the device in the principal planes), so on the image intensifier the conformance can be decided very quick and easily. Hereinafter some devices will be shown.
On Figure 1. there is a simple, even home-made tool. A cube with 4 cm edges contains those crosses on its sides which are necessary to adjust the lasers. The cube is set based on the cross-hairs of the devices. For the Theratron cobalt units, where the cross-hairs are St Andrew’s cross, diagonal signs must be on the cube as well (Figure 1. right side).
On Figure 2. there is a tool containing the central beam, the main planes, field margins and the tolerance values as well. This tool with the thick frame contains those signs (seen on cubes of figure 1 as well) which are necessary to adjust the lasers.

Figure 1. Cubes for laser adjustment

Figure 2. Tool for daily check of simulators (plan view)

 

Figure 3. shows a commercially available tool. It can check laser alignment and the field size, and also a film can be put between the two plates, so it can be used to check treatment devices. The plastic plates are rotatable, and can be fixed in specified angles. The base contains leveling mechanism. If the simulator is equipped with appropriate software (and it is on the network), than the image of the image intensifier can be captured, stored, evaluated or printed. For the more rigorous annual tests film should always be used.

 

Mechanical aspects

 
Crosswires
The location of the crosswires should be checked optically and on the image intensifier daily because of the importance of the determination of the central beam. In the daily check only the “default position” should be checked (gantry at 0°, collimator and table 0°, 10 x 10 cm2, and 100 cm focus to axis distance, FAD). If the institute uses machines with various FADs, than at times machines should be measured on all of the FADs. It is practical to check the crosswires less frequently but in more directions.

Optical distance indicator (range-finder)
Daily check is required at “default position”. There are two measurements: daily control is restricted on FAD: the coincidence of the optical distance indicator, the front pointer’s tip and the digital display value is compared. Less frequently – monthly – the optical distance indicator should be checked in various distances and in various angles.

Isocentric Lasers
In the usual systems 3 or 4 lasers are used, which are fit on one point, on the isocentre. Two lasers are mounted on the sidewall projecting crosses and a third one a longitudinal laser-line. This can be completed with the fourth, mounted over the isocentre on the ceiling. Different laser lines are at right angle to each other. The goal of the daily check is to check the compliance of the fitting. The annual check should include the localization of the isocentre as well. It can also be necessary, if the routine-check indicates a displacement of the laser alignment system. The localization of the isocentre can be made in some different ways. Special tool can be used (Radac 2100), where the first step is to localize isocentre and the lasers should be fitted to this. There is another simpler method, using a ball or a pointer (marking the isocentre) and an appropriate precision moving mechanism. As a first step this is positioned (approximately) in the isocentre, than the real isocentre should be determined by successive approximation. Based on the initial value some extrapolation should be made from different angles on a film, and developing this results the “star shot”, and so the real isocentre (see accelerator QC).

Field defining wire
Besides the checking of the central beam and the field size it is very important to check the image of the field defining wires. The field size, the symmetry, the parallelism and the right angle of the wires should be checked. The measurement should be from the center to the center of the projected wires. These measures are in “default position”.
The check of the imaging on the simulated field sizes is part of the monthly QA check (by not 0° gantry angle as well).
Follows must be satisfied:
The field defining wires should be checked on the whole range of movement. The collimators can be moved asymmetrically on the modern simulators, so the fields must be checked in this mode as well. Generally the IEC recommendations are valid (field size tolerance ±1mm or 0.5% for one side, so for both side together ±2mm and 1%).
As part of the checking the reproducibility must be exanimated, too. It should be checked if there is a hysteresis of the field setting and if the field margin sings wobble. These errors should be periodically checked at every FAD and on 0°, 90°, 180°, 270° gantry angle. Easy and quick method is to check two horizontal opposed fields of the simulator if the other controls are passed.

 
Daily check
The daily check shall cover the measurement of the above four parameters. The method is: Simulator is on the “default position” (gantry at 0°, collimator 0°, field size 10 x 10 cm2 and 100 cm FAD). The tool from Figure 1. is enough to check the first three parameters, and a graph paper is adequate to check the field size optically. Using the tool shown on Figure 2. all four measurements can be made at the same time. If there is an appropriate lever mechanism the incidental margin under the lasers, the crosswires and the optical distance indicator can be measured numerically. The consistence of the crosswires location and orientation should be checked by rotating the collimator. The crosswires and the field defining wires provide shade, so the image intensifier shows the same results as the optical check (it can be controlled on image intensifier).
The above tests should be executed at 0°, 90°, 180° and 270°gantry angle in monthly QA check. The tool on Figure 3. is suitable for that.

 
Couch check
Couch isocentric rotation
It must be checked monthly. Accuracy and reproducibility mostly depends on the construction. The user has not got many opportunities because (on some designs) the couch and the image intensifier are fixed on the same frame. The check rather analyses the effect of the abrasion by usage. The digital and mechanical readouts should be checked monthly, detailed examination is necessary only annually.
Couch vertical movement
If during installation the couch did not differ from the vertical, it has no reason to differ later (except if only there is hard abrasion and wobble with it). Monthly it is enough to check the consistence of the localization of the crosswires during vertical movement of the couch. At annual test the effect of abrasion can be detected ostentatiously under load.
Couch positions readouts
It is enough to check only those mechanical and digital readouts which are really in use. Indication should be within 1 mm, it should have a simple check monthly.
Load tests
They should be executed at annual check.
Couch translation under load
Functionally the couch is an X-ray diagnostic coach, so that in diagnostic energy range it must be translucent adequately. Thus there cannot be expected the same mechanical stability as that of treatment coaches. Tilt and flexing can be expected extremely at maximal longitudinal and lateral shift under load that may be increased by attrition. The recommended tolerances are 0.5° in tilt and 5 mm in vertical flexing (as diagnostic couch).

Image intensifier carriage
The most significant aspect of this component is the indication of the image intensifier position along the axis parallel to the beam axis. This distance needs monthly check, as the calculation of the magnification factor and software correction is based on it. The annual check covers the complete range.
Gantry and diaphragm angle
These should have a simple check monthly (for example in every 90°), and a full, detailed check annually.
Alignment of shadow trays
The alignment of shadow trays should be checked at all diaphragm angles both at gantry 0°, 90° and 270°.

Imaging system

 
Fluoroscopy system

The quality of the fluoroscopic system can be quickly and easily checked in every half year. Extraordinary check is necessary if there is doubt about the performance of the system. The ETR1 tool, written in 31/2001 EüM regulation, used by OSSKI, made by Wellhöfer is suitable for this check. Another tools from the Leeds test objects the TOR 18FG (for fluoroscopy) and TOR CDR (for exposure) are suitable as well. More detailed examination can be performed by T0.10 tool (Figure 4.) or by the tool written in Supplement 23 of British J. of Radiology. This latter one has lead wires in itself for field size checking and has diagonally 1.5 mm holes in 2 mm aluminum stripes. For detailed check there are holes with different diameters in 1 mm aluminum stripes. These are completed by copper step wedge, 2mm thick lead stripe and the sign for the central beam. The results can be stored in file and printed in case of appropriate software and tool. The above test actually examines he stability of the resolving power and contrast resolve. The default values are determined during the core study after the acceptance test, as written in 31/2001 EüM regulation. The annual tests are the same as by every image intensifier (see there).

Exposure mode

The whole imaging system should be checked in this mode. This includes naturally the film processing. It refers to the standards of the X-ray workplace in this context. Some requirements weigh more by simulators than normal diagnostic devices. Particularly the follows should be watched.

Figure 4a.: T0.10 Leeds test object – detail layout.

Figure 4b.: T0.10 Leeds test object – detail layout

 
The appropriate setting of the focus together with the coarse adjustment and fine tuning of the focus are very important. The fine tuning is used in the fluoroscopy mode and the mostly in the exposure mode, so the settings must be made by fine tuning. After tube change the maximum accuracy is expected. The spatial localization of the two different focus is not the same, so the image can be different made by variant focus. The difference cannot be greater than 0.5 mm (on 100 cm FAD). This must be checked in every 6 months and after tube change. The crosswires and the field defining wires are fitted 40 cm from the tube-focus, so there is a four time magnification on the imaging system. It follows that the thinness of the crosswires is very important and in case of tube change the tube must be selected carefully that the focus is on its nominal place. The usual focus size is 1 mm x 1mm (nominal value).

Security, safety devices

 
The daily QA checks must be completed with the safety checks. These concern mostly the mechanical fixation of the movable, changeable parts (block tray, cassette tray, grid, etc.). These must be checked daily and must be repaired immediately in case of failure. The so called “else safety devices” (emergency stop, interlocks, grounding, etc.) must be registered into the preventive – maintainer program, and as a part of this program (in Hungary usually in every 3 months or annual) they must be checked regularly.

Literature

 
1. BIR (British Institute of Radiology) 1985 Criteria and Methods for Quality Assurance in Medical X-ray Diagnosis British Journal of Radiology Supplement 18 (London: British Institute of Radiology)
2. BIR (British Institute of Radiology) 1989 Treatment Simulators British Journal of Radiology Supplement 23 (London: British Institute of Radiology)
3. Horton PW, Deaville JL, Gamble JM and Gerard-Martin S 1987 Quality control tools for simulators Proceedings of the Fifth Varian European Clinac Users Meeting pp 133– 136 (Zug, Switzerland: Varian Associates)
4. HPA (Hospital Physicists Association) 1980 TGR 32 – Measurement of the Performance Characteristics of Diagnostic X-ray Systems Used in Medicine Hospital Physicists’ Association (York: IPEMB)
5. IAEA TECDOC 1040: Design and implementation of a radiotherapy programme: Clinical, medical physics, radiation protection and safety aspects. I.A.E.A., Vienna, 1998.
6. ICRU (International Commission on Radiological Units) 1992 Phantoms and Computational Models in Therapy, Diagnosis And Protection International Commission on Radiation Units and Measurements Report 48 (ICRU Publications, 7910 Woodmont Avenue, Suite 1016 Bethesda, Maryland 20814, USA).
7. IEC (International Electrotechnical Commission) 601-2-29. Medical electrical equipment. Part 2: Particular requirements for the safety of raditherapy simulators. Ed. 2. Geneva: IEC 1998.
8. IEC (International Electrotechnical Commission) 1993 TR 1170 Radiotherapy Simulators – Guidelines for Functional Performance Characteristics (Geneva: International Electrotechnical Commission)
9. IPEM 81 Physics Aspects of Quality Control in Radiotherapy (Edts.: W.P.M. Mayles, R. Lake, A. McKenzie, E.M. Macaulay, H.M. Morgan, T.J. Jordan and S.K. Powley) The Institute of Physics and Engineering in Medicine. York, 1999 ISBN 0 904181 91 X
10. IPSM (Institute of Physical Sciences in Medicine) 1981 Measurement of the Performance characteristics of Diagnostic X-ray Systems used in Medicine. Part III. The Physical Specification of Computed Tomography X-ray Scanners Institute of Physical Sciences in Medicine, Topic Group Report 32 (York: IPEM)
11. McCullough EC and Earle JD 1979 The selection, acceptance testing and quality control of radiotherapy treatment simulators Radiology 131 221–320
12. MDD (Medical Devices Directorate) 1994 The Testing of X-ray Image Intensifier– Television Systems Medical Devices Directorate Evaluation Report 94/07 (London: HMSO)
13. WHO (World Health Organisation) (1988) Quality Assurance in Radiotherapy (Geneva: World Health Organisation)

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