Draft report: Radiological Protection Aspects of Imaging in Radiotherapy
Abstract
Dramatic improvements in delivery of patient radiation therapy enable radiation treatment fields to be conformed to any shape of tumour, a trend that began in the latter stages of the twentieth century. External beam radiotherapy linear accelerators (linacs) can potentially limit irradiation induced cell death to the tumour and spare surrounding normal tissue. However, this can only be achieved if the patient is in a position on the treatment couch that corresponds precisely to the treatment plan. This can often only be accomplished through imaging at many, if not all, of the fractions in which treatment is delivered. This process is often referred to as image guided radiation therapy (IGRT). Treatments are given to patients on the basis of clinically approved radiation distributions calculated on planning computers using computed tomography (CT) x-ray images. When the patient is set-up for treatment, further images, planar or cone-beam CT, are taken and compared to the planning images. The comparison ensures that differences in patient position, patient anatomy and tumour location between the planning images and those taken on the day of treatment, are clinically insignificant. Image guidance enables changes in patient anatomy to be monitored and modifications made to treatment plans daily. Imaging during treatment planning and delivery can also be used to account for motion, with the recording of multiple images through breathing or other motion cycles. However, increased x-ray imaging exposes patients to radiation doses that carry a risk of inducing second primary cancers in tissues surrounding the target volume. This is important because of improvement in long term patient survival with the success of modern therapies and is crucial for paediatric patients. Therefore, reductions in treatment margins and alignment errors that can be realised from IGRT need to be balanced against detriments from higher doses from more frequent imaging. Less effort has been put into optimisation of imaging doses in radiotherapy as they are much lower than those from therapeutic radiation, but imaging irradiates more normal tissues than the treatment beams and the frequency of scanning is much higher than in diagnostic radiology. This report considers all aspects of optimisation for imaging, starting with options available for both planning and delivery, including alternatives using non-ionising radiations, and the frequency with which imaging is carried out during treatment. The optimisation of radiological protection requires teams comprising radiation oncologists, therapy radiographers / radiation technologists and medical physicists and vendors to work together on improving imaging protocols. Consulting colleagues from Diagnostic Radiology departments can be beneficial for reducing dose and improving image quality. Considerable progress has been made in optimisation of radiological protection for diagnostic imaging, based on surveys of patient doses, but few radiotherapy centres record imaging doses. The awareness of imaging doses needs to be raised, and improvements made in the display of the dose quantities on imaging equipment in radiotherapy to allow calibration and assessment to be performed more readily. Recommendations are included for users, managers, equipment vendors, professional bodies and regulators to facilitate improvements in the application and optimisation of imaging in radiotherapy.