Towards Safer Radiotherapy

known discrepancies between clinicians 9–13 such reviews should be incorporated into clinical practice
where time can be identified in job plans.
Note 2: This is a task which requires clinical judgement and is usually within the role of the clinical
oncologist. Other radiotherapy professionals might be authorised to carry out such checking against
departmental protocols on the basis of specific training and assessment of competence. Such
training and competence will be documented within departmental training records.
Note 3: Normally, the planning target volume (PTV) should conform to ICRU 50 and 62 and be
within the range of 95–107% of the prescribed dose. This should be checked by the prescriber both
by inspection of the dose-volume histogram (DVH) and also by checking the isodose coverage of
the PTV on each CT slice in turn.
Note 4: This includes checking that normal tissue dose constraints required by treatment site
protocols are met. Again, the prescriber should inspect the dose distribution within these vulnerable
organs visually, and not rely solely on the DVHs.
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Recommendation
Each radiotherapy centre should have protocols within its quality system which define what data
are to be checked by planners and prescribers along the radiotherapy pathway and how the results
of these checks are to be recorded.
5.7 Monitor unit calculation checks
The calculation of monitor units is a critical element for a safe treatment planning process as
covered in the above section. It is essential that checks of the monitor unit calculation are carried
out before treatment commences and that this check goes back to the prescription in Gy. It is
critically important to check both that the correct calculation method has been followed and that
the arithmetic result of the calculation is correct.
5.7.1 Independent checks
In England, independent recalculation is mandated under the Cancer Peer Review measures 61 and
defines this as: ‘A method which is independent of the planning computer and independent of the
person producing the computer generated plan, should be in place for checking the monitor unit
calculation, based on the following criteria:
• The method should be the responsibility of a Medical Physics Expert, as defined previously
• The method should only be carried out by staff approved by the Medical Physics Expert
• The data used should be independent of the planning computer
• The result of the check using independent data should be within a defined tolerance of the
computer’.
For plans generated by treatment planning computers (producing isodose distributions resulting
from the combination of two or more beams), calculation of the monitor units and dose to the
reference point must be independently checked either by hand using tabulated data or by using
another computer program.
Towards Safer Radiotherapy