Radiation hygiene requirements for IGRT (image guided radiotherapy)

In recent years, image guided radiotherapy (IGRT) has increasingly provided the opportunity to determine the position of the target volume and organs at risk immediately prior to an irradiation fraction. In general, this involves procedures which are based on the application of ionising radiation. In this context, the Commission on Radiological Protection adopted the following recommendations in its 242nd meeting on 1/2 July 2010:

• Planar imaging, for which an x-ray source producing energies in the kV or MV range is integrated into the moving part of the irradiation device (gantry), can be a reliable tool for quick and dependable imaging if the examination is focused exclusively on bony structures. On availability imaging with a standard x-ray tube (kV imaging) is preferable over a solely control of the isocentre with the treatment beam (MV imaging) because of better image quality and lower dose. In future, using the treatment beam for imaging should be limited to the purpose of monitoring the radiation field, which is required under radiation protection legislation.
• Three-dimensional imaging can be realised directly at the accelerator on the basis of helical CT or cone beam CT (CBCT) with kV or MV photon radiation. These techniques provide a higher information yield (soft tissue contrast, 3D images), but they also require higher doses for all irradiated volumes as compared to planar kV imaging, and the volume at risk of stochastic radiation effects is greater. Particularly in those clinical situations where a risk of a secondary tumour development due to high imaging doses must be assumed, the potential benefits and risks of these procedures must be weighed against each other in a differentiated approach.
• Even in case of optimal imaging, high priority must be given to the quality of reproducible patient positioning, which must be ensured through appropriate positioning aids or procedures.
• The potential benefit of daily positioning control with imaging techniques should be weighed against the benefits of a protocol with a lower control frequency, taking into account the parameters relevant for secondary tumour formation, such as age of the patient, disease aggressiveness, organs at risk near the target volume etc. As a rule, in case of curative therapy protocols with high doses per fraction (e.g. stereotaxis) control for each fraction is preferable. Regardless of this, the frequency of imaging must always be based on the principles of justifying indication.
• Among the available IGRT methods, preference should be given to methods which ensure sufficient image quality for positioning control while entailing the lowest dose deposition from imaging. Volume imaging is to be chosen if dose application is problematic due to a high treatmentdose required near organs at risk.
• Quality control standards, as laid down for diagnostic systems in the X-ray Ordinance, must be applied for all imaging systems used in the context of IGRT. In particular, technical processes must be established which document the imaging dose or allow the user to estimate the applied dose.
• Manufacturers must be urged to advance technological developments with a view to reduce the dose for imaging while maintaining appropriate image quality and develop systems for IGRT without using ionising radiation.
• Moreover, suitable measures taken by manufacturers and additional quality controls must ensure that the isocentres of imaging and irradiation are identical in order to avoid new systemic errors occurring in the treatment chain. Due to the increasingly complex structure of modern radiotherapeutic processes, the provisions governing the quality assurance inspection no longer reflect all the key physical-technical aspects of the treatment procedure. This is both because of ambiguities in determining what is actually included in the radiotherapy system and hence covered by the inspections, and because of unclear or nonexistent regulations regarding which provisions should be applied to which components in the inspections. Furthermore, some standards no longer reflect the state of the art. For this reason, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) requested the Commission on Radiological Protection (SSK) to identify existing gaps in the technical inspections by professional experts and formulate new quality control requirements.