Selection of checks and check frequencies

The Table 1. at the end of this chapter lists the recommended tests for linear accelerators with photon beams. It is not stated that the checks and frequencies on this list are either necessary or sufficient for proper quality control of each and every machine, but they represent a level considered appropriate as the basis of a quality control system. It is the individual physicists responsibility to change the tests and frequencies to suit the requirements of their particular equipment. It is necessary to balance the complexity and difficulty of performing a test against the consequences of a fault occurring and perhaps it can cause a treatment error. In choosing to differ from these recommendations it is wise to record the reasons for the change both for medico-legal purposes and as an indication of the thinking about the applied quality control system. For example low energy accelerators with a standing wave guide do not have a bending system and flatness and symmetry are unlikely to change. Otherwise a machine with servo controlled bending that is known to be unstable therefore it may require checks at very frequent intervals.
The frequency data of the tests have been compared with those in IPSM Report 54 (IPSM 1988), the WHO publication, Quality Assurance in Radiotherapy (WHO 1988), the survey of UK practice (IPSM 1992) and the report of AAPM Task Group 40 (AAPM 1994). It is assumed throughout in this publication that a test which is required to be carried out daily should also be carried out at the other frequencies except where some more strict test is prescribed. The tolerances given for daily checks are mostly wider than would be appropriate for more extended monthly checks and in such cases the check has been explicitly repeated. It may be appropriate for the simpler test to be performed at the same time as the more thorough test to provide a baseline. Daily checks are often carried out by the radiographers and this is to be encouraged as it gives them a feeling of the accuracy of the equipment. Other checks can be carried out by technicians or physicists occasionally.

Table 1. Check frequencies for linear accelerators

Test and frequency	Reference	Tolerance
Daily
Output constancy	12.3	±5%
Lightfield size (10 cm square field)	6.1	2 mm
Crosswires and laser axis lights coincident	4.3	2 mm
Optical distance indicator (compared to lasers)	4.3	3 mm
Maze entrance interlock	1.1	Functional
Audio visual system		Functional
Machine log	15.1
Weekly
Deadmen’s switch	1.2	Functional
Touchguard	1.2	Functional
Distance indication at different SSDs	4.4	3 mm
Pointers (if used)	4.5	2 mm
Calibration	12.2	±2%
Wedge factor constancy (machines with adjustable wedge factors)	12.5	±2% of expected
Two-weekly
Flatness 0º-os gantry-nél	11.2	±3%of expected
Monthly
Secondary access interlocks	1.1	Functional
Emergency-off switches	1.1	Functional
Movement interlocks (full check)	1.2	Functional
Coded lead tray interlocks	1.2	Functional
Backup timer interlock (where set by user)	1.3	Functional
Gantry and collimator rotation scale	4.6	±0.5º
Optical field size variation for different field sizes	6.2	2 mm (small sizes)
Isocentre quick check	4.3	2 mm diameter
Shadow tray alignment	7	1 mm from centre
Distance indication at different SSDs	4.4	2 mm
Couch movement calibration	8	2 mm relative
Couch vertical movement	8	2 mm
Gantry angle indication	4.6	1º
Radiation field versus light field (one field size)	9.1	2 mm
Calibration in water	12.2	±2%
Energy check using dose ratio	13	Ratio ±2%
Arc therapy (if used)	14	Dose ±2%
Three-monthly
Indicator lights	2	Functional
Filter interlocks	1.5	Működik
Backup dosemeter and timer interlocks (computer controlled)	1.3	Functional
Field size indication at extended SSD	6.2	3 mm at 150 cm
Output calibration variation with gantry angle	12.4	Calibration±2%
Wedge factor variation with gantry angle	12.5	±3%
Dose check for non standard field size and SSD	12.1	±2%
Optical field versus radiation field (small and large fields)	9.1	2 mm
Flatness and symmetry full scans at all gantry angles	11.1	3%
Six-monthly
Radiation isocentre	9.3	2 mm diameter
Flatness interlocks	1.4	2% of intended
Radiation check of shadow tray alignment	7	1 mm (at isocentre)
Calibration of constancy check device (if used)	12.3	Device dependent
Linearity of dosimetry system	12.8	Within 1%
Annual
Verification of water flow and other interlocks		Functional
Couch deflection under load	3.1	5 mm
High dose rate interlock	1.3	Functional
Definitive isocentre check	4.1	2 mm diameter
Definitive calibration	12.1	±2%
Backup dosemeter and timer interlocks (not computer conrolled)	1.3	Functional
Plotting tank measurements of depth dose and profiles	12.7	> 2% change
Calibration of flatness monitor	1.4	2% of intended

 
The frequency data of the tests have been compared with those in IPSM Report 54 (IPSM 1988), the WHO publication, Quality Assurance in Radiotherapy (WHO 1988), the survey of UK practice (IPSM 1992) and the report of AAPM Task Group 40 (AAPM 1994). It is assumed throughout in this publication that a test which is required to be carried out daily should also be carried out at the other frequencies except where some more strict test is prescribed. The tolerances given for daily checks are mostly wider than would be appropriate for more extended monthly checks and in such cases the check has been explicitly repeated. It may be appropriate for the simpler test to be performed at the same time as the more thorough test to provide a baseline. Daily checks are often carried out by the radiographers and this is to be encouraged as it gives them a feeling of the accuracy of the equipment. Other checks can be carried out by technicians or physicists occasionally.

15.1. Documentation

 
It is important that appropriate records are kept both of the checks carried out and of any fault conditions that may occur (NRPB 1988). These records should be analysed regularly and required action taken where necessary. It is important that records made by radiographers should immediately be accessible to the physics and technical groups and vice versa.

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