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Safety interlocks

As already mentioned the machines contain safety current circuits which are designed to protect staff and the patient against false operation. But these interlocks can be in fault and in some cases the patient could get injured or the equipment could be damaged when a fault is not detected. It is important to take into account of each particular device what is being monitored, and the specific error conditions can be simulated. Checks that can be applied in general are written below. Many interlocks are checked implicitly when the machine is used. Even in such cases a record of the check must be made at regular intervals.
In an ideal case a formal written analysis should be carried out of all the possible fault conditions. It is possible to select interlocks that must be checked explicitly and being convinced that a record is made of any implicit checks. In some cases physical intervention is necessary to check an interlocks's operation. The risk of such an intervention must be balanced with the possible damage that could be caused when an interlock fails to detect an error. For instance, the failure of a water flow relay can cause a considerable damage to the accelerator when the water pump breaks down.
It can be expected that interlocks will trigger from time to time in normal use. It is important that well considered procedures are in place for control of the resetting of such interlocks to make physics staff aware of the fault conditions. In rare cases it can be necessary to override particular interlocks. This practice is to be strongly discouraged, but in any event should only be carried out with the full knowledge of the Head of Physics who should normally discuss this with the Clinical Director and the Superintendent Radiographer. If it is necessary to override interlocks in order to carry out particular tests, it is essential that clear written procedures are in place to ensure that their function is restored, and these procedures must be carefully followed. At the accelerators made by Varian the override of interlocks could happen in sevice mode. When exit from this mode the override of interlocks are automatically stopped.

1.1. Room protection

It is essential that the operation of the interlocks on the maze entrance must be sufficient and it should be checked daily. The emergency switches must be checked monthly. All emergency-off switches should be tested monthly.

1.2. Movement interlocks

The most important movement interlock is the "deadman’s switch". The action of this must be checked at least weekly so that when it is released at least one of the machine movements is disabled. On some machines there may be several such switches connected to different movements. Monthly a regular check must be carried out that none of the machine movements move without the appropriate switch being pressed (except those which are designed such as the collimator jaws).
The touchguard should be checked weekly. When a check is carried out it must controlled that the touchguard operation can be cleared. Each electron applicator must have a touchguard. The operation of these should be checked monthly, ideally on a rota basis for a more frequent check. The operation of all limit switches should be checked monthly. Most accelerators have applicator and lead tray interlocks. These should be checked monthly.


1.3. Dosimetry interlocks

Before treatment starts it must be checked that the machine will stop when the set monitor units are reached, though this check usually must be carried out as part of the daily constancy checks. It is also required to check that the machine will switch off with the second dosimetry channel. To check this it will probably have to change the calibration of one of the dosimetry channels, therefore this check should be limited to an annual check. However, if the dosimetry has to be adjusted anyway it is worth to carry out this check at the same time. On computer controlled accelerators a temporary change in the calibration may be carried out and this will permit more frequent checks of the second channel. The backup timer interlock can be checked without intervention of the machine’s internal controls by setting a shorter time than it is necessary and checking that the treatment is interrupted at the set time. Therefore this can be tested monthly.
The accelerators should have a high dose rate interlock. Checking this could be tricky, but it is advisable to do it at least annually because it provides the last protection against serious dosimetry errors. The manufacturer’s advice should be considered that how to do this.


1.4. Beam steering

All modern accelerators have flatness and symmetry interlocks. These must be calibrated so that the indicated flatness and symmetry is approximately correct. It requires that the beam be steered away from the ideal, so this check can only be performed occasionally. If a significant change in the beam steering is due, such as when a monitor ionisation chamber is changed, the calibration of these devices and the operation of the interlocks should be checked.

1.5. Filter interlocks

Wedge filters have some interlocks to show that wedges are in place and in the case of machines with multiple wedges what wedge is in use. A way of operation of these interlocks should be ascertained and a systematic check carried out monthly to ensure that all possible faults are taken into account.
In the case of manually operated wedges this includes checking that the machine will not run:

  • if a wedge is selected and no wedge is in place;
  • if the wrong wedge or the wrong orientation is selected; or
  • if a wedge is in place and no wedge is selected.

Sometimes the wedge storage is interlocked so that a positive indication could be given that a wedge is in the store and not in the machine. This indication should also be checked. For automatic wedges it is necessary to examine the control system to identify possible errors in operation and then to establish a tests which are appropriate to the risk. Multi-energy accelerators have systems to state that the correct combination of filters or scattering foils are in place for the energy and modality in use. The advice of the manufacturer should be sought as how to examine these systems.

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