Towards Safer Radiotherapy

However, at the present time and in the absence of simple independent calculation methods, best
practice is to carry out measurements of dose from the individual modulated beams as a minimum.
An IPEM report Guidance for the Clinical Implementation of Intensity Modulated Radiation Therapy
is due to be published in 2008 and will provide a comprehensive discussion of this emerging
technology including the requirements for checking. 76
5.7.2 Calculation of monitor units for prescribed dose
There must be a clear pathway extending from the prescription in Gy initiated by the
prescriber through to the delivered monitor units. It is essential that checks cover the whole
pathway and not just a part.
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Some centres have a policy for the treatment planning system to calculate the monitor units
required to deliver a normalised dose (for example, 1 Gy) to the International Commission on
Radiation Units and Measurements (ICRU) reference point. This system has been implicated
in high-profile errors. 5 It is recommended that this additional step be eliminated to remove
the requirement for a manual calculation to translate from the normalised to the prescribed
dose. However, any change will bring its own risks, and a thorough risk analysis must be
undertaken before embarking upon such a project.
Recommendation
Protocols should stipulate the calculation of monitor units for the actual dose to be delivered,
rather than a normalised dose, to eliminate the need for additional manual calculations. It
should be possible to check this as a single procedure.
5.7.3 Standardised treatment protocols
For some commonly delivered treatments, such as those for prostate and breast cancer, the
range of monitor units per fraction falls within a predictable range for the majority of
patients. Centres should consider which kinds of treatment fit into such categories and draw
up lists of ranges of expected monitor units for certain beam configurations to assist staff in
establishing familiarity with standard protocols. This is one of the benefits of experience.
Recommendation
The use of standardised treatment protocols allows the definition of an expected range of
monitor units, which provides an additional safeguard.
5.8 Data transfer
The output from computerised treatment planning systems includes beam settings and monitor
units and so on. These data should be transferred electronically to the treatment machine to avoid
transcription errors. If this is not possible because of equipment incompatibility this should be
highlighted as a risk and an action plan should be developed to remedy it. Meanwhile, additional
verification procedures should be established. It should be recognised that the replacement of
equipment provides an opportunity to improve safety in this respect.
When data transfer systems are installed, it is absolutely essential to test their integrity and accuracy
during acceptance testing and commissioning. 71
Towards Safer Radiotherapy