# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 5 Discussion
More training courses in diagnostic imaging are required for both radiotherapy physicists and diagnostic imaging specialists who can also be involved in imaging for radiotherapy. The attention to dose levels should be associated with assessments of image quality, not only as a constancy check on imaging performance, but also to determining the level of image quality required for making the alignments necessary to ensure accurate treatment delivery. If more assessment of imaging doses is carried out, there may be other simple steps that can be taken to reduce doses in the early stages. For example, similar protocols are generally used throughout the treatment of a patient, and since imaging of the same region around the tumour is often repeated at each treatment fraction, further adaptation of exposure factors to optimize radiological protection should be possible based on knowledge derived from the initial exposures. Another aspect of optimization is the size of the radiation field used and the adjustment to suit the size of the patient. If a standard field size is used for patients of varying height, this will include a greater proportion of organs and tissues on the periphery in smaller patients. The size-specific effective dose that will be received by a person who is 5 cm shorter than the standard height will be between 3% and 10% greater [34], but use of standard field sizes for patients of all heights treated for a particular type of malignancy could result in much higher increases in dose than this. For example, upgrades in treatment planning software or radiation oncology information systems may include changes in imaging protocols, and associated increases in field sizes or exposure factors that could lead to gradual increases in imaging dose [35]. Such changes should be monitored as part of the evaluation process to check effects on imaging doses and ensure that any changes are reasonable in terms of potential improvements in treatment delivery. Conventional CT scanners enable the fields to be included in a scan to be determined from scan projection radiographs and include automatic tube current modulation to reduce dose levels both for smaller patients and in parts of the scan where attenuation is lower. Facilities are included in some CBCT systems to enable the exposure arcs to be limited to protect radiosensitive organs, but the field size is not based on recorded images and is determined either by simple manual adjustment of collimation or selected from small, medium, and large options and there is no automatic exposure control. Tools to allow field sizes to be set based on an initial radiographic image from a single projection and radiation exposure levels to be adapted automatically to sizes of patients being imaged might be included by CBCT vendors to function in a similar manner to devices used in CT scanners. This type of development could be used together with appropriate selection of beam direction and bow-tie filters [20] to enable further optimization not only of dose but also image quality, and vendors are encouraged to consider potential use of an initial image for both adjustment of field size and exposure factors. One aim of the Task Group is to provide guidance on the recording of imaging doses linked to programmes for dose optimization that will be applicable in all countries. However, the resources for CBCT CTDI dose measurement may not be readily available in all institutions in low- and middle-income countries, in terms of the phantom, measurement tools, and expertise. It should not be an issue for large teaching hospitals in most countries to perform such measurements, particularly for hospitals where diagnostic imaging medical physicists are available. While the radiation oncology medical physicists can be trained to perform the CTDI measurement, it can be a challenge for these institutions to justify the procurement of the CTDI phantom required for such measurements. Inclusion of the equipment in the procurement budgets may help to improve availability.