It is known that random and systematic errors may occur. There are several methods of avoiding and detecting these errors in order to prevent adverse outcomes.
Record and Verify System
A record and verify system ensures that the planned treatment is delivered in a similar manner (within tolerance) every day. It includes daily measurements of:
- Monitor Units
- Beam Energy
- Beam Mode (electrons/photons)
- Jaw positions
- MLC positions
- Collimator angle
- Gantry angle
- Couch angle
- Source-surface distance
Unfortunately, if a systematic error is introduced on the first day of treatment, the record and verify system will 'verify' the incorrect position every day. It is standard that patients are set up according to the treatment plan (rather than the record / verify values), which is then checked with the record / verify system to ensure it lies within tolerance.
Tolerance refers to the acceptance of random error in setup. It varies for different measurements and different setups. More accurate immobilisation practices (eg. thermoplastic mask) have less tolerance as values should remain much closer together.
Typical treatments and their tolerances include:
- Head and Neck (in mask)
Select and Confirm Procedure
Select and Confirm means that when an option is 'selected' by the staff member - for example, a change from photon beam to electron beam - the treatment process will unable to continue unless the mechanical changes in the linear accelerator are confirmed to have occured (eg. scattering foil in position, change in electron gun energy, changes in accelerating waveguide).
Select and Confirm also checks that the planned field size, energy and so on correlate with the mechanical positions of the field, the selected energy etc. If there is a discrepancy then the computer highlights this to staff members, who must correct the error or (hopefully not) override the check. Overriding may be needed in the event of a small change in patient contour that does not require eplanning of the case.
Interlocks are emergency shut off mechanisms that are triggered when by certain events. A microwave oven has an interlock which turns off microwave generation if the door is opened (in fact there are two), and prevent activation of the microwave generator if the door is opened.
Linear accelerators (being more dangerous than microwaves) have the following safety interlocks:
- Last man out button: Prevents activation of the linear accelerator until triggered
- Emergency button: Ceases treatment immediately if triggered (manual)
- Positioning interlocks: Prevents beam 'on' unless the gantry, collimator and couch are in the correct positions
- Beam interlocks: Prevents beam 'on' unless the independent jaws and MLC positions are in the correct positions (as per record/verify and/or treatment plan paramaters). These can be overridden if they are within tolerance by treating staff.
11: Treatment Planning And Delivery
- 11.01 - Simulation
- 11.02 - ICRU Reports 50 and 62
- 11.03 - 2D And 3D Planning
- 11.04 - Principles Of IMRT
- 11.05 - Patient Data Acquisition
- 11.06 - Choice of beam and modifiers
- 11.07 - Field Junctioning
- 11.08 - Calculation Of Monitor Units
- 11.09 - Dose Calculation Algorithms
11.10 - Accuracy Of Treatment Planning And Delivery
- 11.10.1 - Patient Immobilisation And Monitoring
- 11.10.2 - Image Guided Radiotherapy
- 11.10.3 - Consistency Of Contours During Treatment
- 11.10.4 - Accuracy And Tolerance
- 11.10.5 - Determination Of Accuracy
- 11.10.6 - Types Of Errors
- 11.10.7 - Avoidance And Detection Of Dose Delivery Errors
- 11.10.8 - Errors Due To Computer Control
- 11.10.9 - In Vivo Dosimetry