The role of dosimetry audit in achieving high quality radiotherapy.

Introduction

Dosimetry audit is a key component in quality management programmes in radiotherapy, playing an important role in the safe implementation of new treatment modalities and techniques [1], [2], [3], [4]. National and large scale audits provide data which can help to create, sustain and increase standards as well as have the potential to identify issues which may cause harm to patients, thus improving both quality and safety [1], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. They can also help to reduce variability in dose delivered to the patient both nationally, internationally and within multi-institutional trials [3], [13], [14], [15], [16]. At an institutional level, an external dosimetry audit provides an independent check of the local approaches and thus supports the implementation of novel and complex techniques [6], [7], [13], [15], [17], [18], [19], [20]. Where multiple centres have been included in the audit, the process of comparison with other centres facilitates awareness and understanding of issues which may exist and which may not be identified by a single centre alone [5], [6], [21]. Furthermore this sharing of experience allows benchmarking of centres with similar equipment and thus increases the knowledge of what is achievable with a particular combination of equipment [22], [23].

There are multiple challenges in designing and running effective dosimetry audits, including an ever expanding horizon, with new techniques and new equipment combined with increasingly high expectations from both patients and professionals in outcomes and safety [24]. This leads us to question whether the same things still need auditing or whether assumptions can be made on previously validated techniques [25]. However, the design must still consider what needs to be tested, where the highest risks lie, how the auditing equipment will work, e.g. if the detector will work with the chosen phantoms, as well as whether the process is appropriate.

There are different types of audit from postal to an on-site visit and from basic measurements in reference conditions through to a full end-to-end audit where an anthropomorphic phantom takes the place of the patient and follows the full pathway from imaging, through planning and to complex dose distribution delivery [1]. The development of techniques for dosimetry audit can include the development of new materials [26], development of anthropomorphic phantoms [27], [28] as well as characterisation of detectors prior to audit use [29], [30].

The special issue available online at https://www.sciencedirect.com/journal/physics-and-imaging-in-radiation-oncology/special-issue/10S8T6FN296, presents an extensive range of dosimetry auditing activity and highlights the different approaches, including whether an external measurement is needed or whether data measured by the local centre can be assessed externally for a dosimetry audit [31], [32], [33], [34]. The papers in the issue represent efforts on regional, national and international levels, each of which has been designed to focus on a specific problem or technique where variation in implementation or practice is known or issues with beam modelling capabilities may exist, thus creating a comprehensive set of publications in a single location.

Beam output

Measurement of beam output is the most fundamental measurement which confirms whether the machine has been correctly calibrated [8]. Existing errors will create a systemic error for every patient treated on the machine and therefore have the potential to create systematic differences in treatment outcomes. Worldwide dosimetry audits are organised in different ways, often for geographical, economic or political reasons [35] but fundamentally are checking the same thing. Over recent decades the variability between centres has generally reduced [9], [36], however globally there are still findings outside tolerance (e.g. of ±5%) [35] that can be attributed to a wide range of causes, including equipment failures, errors in setup, and incorrect implementation of the calibration protocol [37], [38], [39]. Several of these could create a systematic error for every patient treatment on a given machine. However data from centres that strictly follow a code of practice (such as the Belgian-Dutch NCS-18 CoP [40]) can give very high standards of compliance [10].

Advanced techniques

One of the strengths of external dosimetry audit is the ability to independently verify, and thus give confidence in, a new technique which has been recently commissioned in a centre. Ideally this should take place between the end of commissioning and the beginning of clinical practice. Recent advances in radiotherapy have posed many challenges for radiotherapy centres to use techniques which use extreme conditions which no longer fit comfortably into the routine protocols and codes of practice. An example of this is the expanding use of small fields which are too small to adhere to the standard codes of practice. This has been recently addressed by the publication of the IAEA code of practice [41] but the accurate modelling of small fields and how the correct detector is used to collect the commissioning data is still challenging. Several groups [11], [12], [21] conducted national and multi-national audits respectively, of calculated small field output factors by comparing with standardised data published by the IROC-Houston QA center at MD Anderson [22], [23], [42] and found that not all centres were modelling these fields well. The audits found that the deviations increased with decreasing field size and the treatment planning system commonly overestimated in comparison with the reference data. Several of the centres adjusted their calculation data following these audits, thus improving their beam models for small fields.

Another technique which is rapidly expanding, due to the increased interest in hypo-fractionated regimens, is that of stereotactic ablative body radiotherapy (SBRT). This is a higher risk technique as greater doses are given in fewer fractions and therefore there is less room for error correction. The most common site for SBRT is in the lung and several trials have been run to assess the effect on treatment outcome of this approach. Tsang et al. [14] found that although the clinical trial plans pre-treatment verifications were within the tolerance of +/3%, there were certain scenarios of basic beam tests using single beams which failed, indicating that care must be taken in commissioning as although results may be good for some beam combinations, this may not be the case for all scenarios, depending on the algorithm used [13], [15], [16], [17], [43].

There are also more specialised techniques being implemented which may only be used by a few centres. The safety issue here is that there is little data about what can and should be achieved with these systems. An example is rectal contact brachytherapy where no specific code of practice exists and hence a kV code is often used. A national audit for rectal contact brachytherapy was carried out in the UK [44] to assist users to optimize their own practice, thus providing reassurance that the implementation had been performed within the standards stated in previously published audit work and recommendations for kV and electronic brachytherapy units. However, it was recommended that optimised and standardised quality assurance testing could be achieved by reducing some methodological differences observed between the centres.

Future directions

As radiotherapy techniques become more complex and less intuitive, there is a potentially greater risk for errors to be missed. This is particularly true in places which may not have an established culture of peer-to-peer review. Thus there is a need for wider access to audit in a cost effective and efficient manner. Several groups have investigated methods for remote auditing including the use of local EPID data which can be analysed centrally [45]. Further approaches have suggested the use of locally measured QA data, however there is still a debate as to the sensitivity and specificity of the local detection of errors in comparison with independent measurements [31], [32]. Alternative approaches could include collection of log-files for continued assessment of quality [25], which could be of particular value in clinical trials. Alternative methods for efficiency of audit include the use of complexity metrics to pre-determine which plans should be investigated, however no metric appears to yet exist which gives a robust response across all planning and/or delivery systems [46].

The role for ESTRO in dosimetry audit

Dosimetry audit has been identified in the ESTRO physics strategy as being a topic of high importance which can support quality improvement through standardisation of radiotherapy practice across Europe. Two workshops have been held during 2017 to address this subject to identify how existing groups, including IAEA and clinical trials QA groups, can work together to develop methods to address specific issues as well as to encourage new national groups to start running regional dosimetry audits. The combined body of data from dosimetry audits published in this special issue and elsewhere, represents an opportunity to share protocols and best practice, with examples of how to start a regional audit system, thus augmenting the potential for increased quality of radiotherapy. Furthermore the dosimetry audit data can be combined to create datasets for meta-analysis which can identify issues for investigation into beam modelling and measurement methodologies that need further research, which would not be seen in smaller studies.

Conflict of interest

There is no conflict of interest.