# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 5 Discussion  
This study is not a comprehensive survey, but it has shown that kV CBCT is the imaging modality of choice for imaging immediately prior to treatment. The appropriate use of non-ionizing techniques may help to reduce numbers of X-ray exposures, where facilities are available. However, countries with lower HDI levels have fewer of these options available. Once the imaging modality has been decided, there remains a question as to whether the current use of advanced imaging techniques for IGRT is always appropriate. Beyond focusing on the PTV margin and geometric accuracy, the justification for daily imaging should also be on a patient-specific basis and take account of patient age and disposition during treatment, disease site and stage, radiotherapy dose and treatment delivery technique, as well as the availability of the imaging modality. In principle, the high targeting accuracy required for small PTV margins can only be conserved with a high imaging frequency (Figure 1). However, the clinical impact of imaging frequency on the potential targeting accuracy and cancer cure can only be determined on a patient-specific basis, as it depends on the tumour site and on the type of localization and positioning errors to be expected. Centres that only image once per week or once per course of treatment in this study were predominantly those without kV imaging facilities. The potential widening  
Figure 5: Arrangement being used for measurement of cumulative dose at the centre of a CT body phantom. The X-ray source and image receptor are typically mounted at 90\({}^{\circ}\) from the treatment head of a medical linear accelerator. The figure shows schema of the measurement set-up in (**a**) the axial plane and (**b**) the sagittal plane. Measurements would be made with in a CT standard phantom with (**c**) a 100 mm CTDI ionization chamber and/or (**d**) a 0.6 cc Farmer type ionization chamber. of the application of optical surface guidance has been suggested recently by an AAPM Task Group [33]. It is an approach that could help in reducing the frequency for X-ray imaging, but although optical surface guidance was employed in more high-income countries, only 11% of centres on average used it in countries with values of HDI less than 0.88. Since this is a less expensive alternative, it could assist in the optimisation of imaging in these countries. Globally, the frequency of imaging during treatment will be affected by the availability of appropriate equipment, resources and personnel, but more studies evaluating how imaging affects the accuracy of treatment delivery and treatment outcome are required to determine and support the optimal approaches. Methods for the checking of dose performance of imaging equipment and recording of patient doses from imaging need to be introduced generally in radiotherapy centres and awareness among radiotherapy staff raised through education and training. The recording of patient dose is the first step that needs to be taken before optimization of radiological protection can begin. The lack of information on imaging dose apparent in this study is likely to result in less attention being paid to optimization of radiological protection, as the impact of any changes cannot be evaluated. It is also probably a factor behind the use of standard adult imaging protocols supplied by the vendor for all patients in most countries, often with limited optimization for individual patients (Table 3). The majority of medical physicists working in radiotherapy do not have expertise in diagnostic imaging and will not be familiar with the dose quantities used. Therefore, introducing imaging dose measurements and surveys of patient imaging doses will require both involvement of diagnostic radiology medical physicists and training of radiotherapy imaging physicists. The survey showed that some centres did use diagnostic physicists, but the proportion in the different countries was variable (Table 3).