ASN Report 2017

273 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 09  - Medical uses of ionising radiation principle of helical image acquisitions obtained with computed tomography. A photon beam, emitted at a voltage of 6 MV and a dose-rate of 8 Gy/min, shaped by a multileaf collimator enabling the intensity of the radiation to be modulated, allows the irradiation of large volumes of complex shape as well as extremely localised lesions, which may be in anatomically independent regions. The system requires the acquisition of images under the treatment conditions of each session for comparison with reference computed tomography images in order to reposition the patient. Thirty-two systems of this type were installed in France (Radiotherapy observatory INCa 2016). 3.2.3 Stereotactic radiotherapy Stereotactic radiotherapy is a treatment method which aims to offer millimetre-precise, high-dose irradiation using multiple mini-beams converging in the centre of the target, for intra- or extra-cranial lesions. In stereotactic radiotherapy treatments, the total dose is delivered either in a single session or in a hypofractionated manner, depending on the disease being treated. The term radiosurgery is used to designate treatments carried out in a single session. This technique firstly requires great precision in defining the target volume to irradiate, and secondly that the treatment be as conformal as possible, that is to say that the irradiation beams follow the shape of the tumour as closely as possible. It was originally developed to treat surgically-inaccessible non-cancerous diseases in neurosurgery (artery or vein malformations, benign tumours) and uses specific positioning techniques to ensure very precise localisation of the lesion. It is more and more frequently used to treat cerebral metastases, but also for extra-cranial tumours. This therapeutic technique essentially uses three types of equipment: ཛྷ ཛྷ specific systems such as: -- Gamma Knife ® which directs the emissions from more than 190 cobalt-60 sources towards a single focal spot (five units are currently in service in four centres in France); -- CyberKnife ® which consists of a miniaturised linear accelerator mounted on a robotic arm (robotic stereotactic radiotherapy); ཛྷ ཛྷ multi-purpose linear accelerators equipped with additional collimation means (mini-collimators, localisers) that can produce mini-beams. Stereotactic radiotherapy with a robotic arm consists in using a small particle accelerator producing 6 MV photons, placed on an industrial-type robotic arm with six degrees of freedom, marketed under the name CyberKnife ® . Furthermore, the treatment table is also positioned on a robot of the same type. By combining the movement possibilities of the two robots, it is possible to use multiple, non-coplanar beams to irradiate small tumours that are difficult to access using surgery and conventional radiotherapy. This technique enables irradiation to be carried out under stereotactic conditions, and with respiratory tracking. Given the movement capabilities of the robot and its arm, the usual standards do not apply to the radiation protection of the treatment room and a specific study is therefore required. Twelve sites in France were equipped with this type of radiotherapy device (Radiotherapy observatory, INCa 2016). 3.2.4 Radiotherapy using a linear accelerator coupled to a magnetic resonance imaging system The installation of the first linear accelerator coupled to a Magnetic Resonance Imaging (MRI) system should take place in a French centre at the end of the first half of 2018. The recent combining of these two technologies (linear accelerator and MRI) raises new questions regarding its clinical use, in terms not only of measurement and calculation of the dose delivered to the patient but also the quality control of the complete machine comprising both the accelerator and the imaging device. Consequently, in July 2017, ASN asked the Institute of Radiation Protection and Nuclear Safety (IRSN) to conduct a study of the installation and the utilisation of this type of accelerator coupled with an MRI device. The results of this study are expected in summer 2018. MRIdian® Linac system (MRI-guided radiotherapy integrating a Linac system).