Towards Safer Radiotherapy

Recommendation
Each radiotherapy centre should have protocols for in vivo dosimetry monitoring. In vivo
dosimetry should be used at the beginning of treatment for most patients. Patients should
only be excluded from this procedure according to clear departmental protocols.
5.12.2 The issues to be considered in providing an efficient and cost-effective service have been
considered in detail elsewhere. 83,86,87 The technical details of dosimeter placement are critical,
particularly in breast patients. A departmental database should be established to capture the
data and permit analysis of trends. This will also assist in setting appropriate action levels as it
is important to ensure that small measurement discrepancies do not precipitate needless
investigation. The details of implementation and the action to be taken by different staff
groups are the key to avoiding unnecessary disruption of patients’ treatment. It is essential
that any anomalies which exceed agreed action levels are investigated promptly following a
departmental protocol.
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Because of daily variation in set-up and the precision of the method, in vivo dosimetry is not
considered a more accurate approach to patient dosimetry than standard planning and
calculation: it is a final check of the calculation pathway. It is common to set an action level
of 5% for most sites. For glancing breast and chest-wall fields, difficulties in reproducible
diode placement and the intrinsic imprecision of diode measurements may lead to a greater
action level, for instance, 10% at such sites. The planned implementation of an in vivo
dosimetry programme can lead to progressively lower action levels. 88
It is imperative that the calculation of the dose expected at the position of the dosimeter is
based on verified and appropriate information. In some circumstances, it is possible to obtain
the expected reading even though the dose delivered was not as intended: it certainly will
not detect an erroneous prescription and other errors are also possible unless the calculation
pathway is carefully considered.
Recommendation
Each radiotherapy centre’s protocols for in vivo dosimetry should specify action levels and
the procedures to be followed for results outside the tolerance range.
5.12.3 It is now possible to use electronic portal imaging devices (EPIDs) to measure the dose
absorbed by the patient (transit dosimetry). This technology is still being developed and is
not routinely available. It may have particular application in the verification of IMRT dose
delivery. In the future, these technologies will also permit verification of the total dose
delivered to the planning target volume, even allowing for changes in the shape of the
patient and the tumour over the course of treatment. Research into these technologies
should be encouraged and supported.
5.13 Clinical review during treatment and its role in error detection
In the practice of radical radiotherapy, it has been estimated that, in some circumstances,
observation (for example, of the skin reaction) can detect those differences of 10% and in some
settings 5%. 89 Clinical observation in ‘on-treatment review clinics’ thus has an important role in
monitoring the treatment of patients. It will not detect underdose, 26 but can detect a systematic
Towards Safer Radiotherapy