State of the art of "transit" dosimetry for In Vivo Dosimetry (IVD) in radiotherapy in France
Publication date: 15/06/2018
Below an extraction of the study “Etat de l’art de la dosimétrie dite « de transit » pour un contrôle de la dose in vivo en radiothérapie en France” translated into English, you will find a link to the original study in French at the bottom of the page.
In Vivo Dosimetry (IVD) has been a regulatory requirement in France since 2008 (INCa approval criterion No. 15 for the practice of external radiotherapy). It concerns any "technically measurable" beam. Historically based on point measurement techniques adapted to conventional radiotherapy, the applicability of this method of IVD has greatly decreased with the dissemination of modulated intensity radiotherapy techniques (IMRT, VMAT).
Most of the French radiotherapy centers practice these advanced techniques (nearly 70% of them in 2013, INCa), which in some centers represent almost 100% of treatments. The proportion of "non-technically measurable" beams is increasing significantly, and alternative methods to those based on point measurements should be considered to ensure a satisfactory level of treatment control.
From the observation of practices during inspections, Nuclear Safety Authority (ASN) wanted to know about available or in development solutions of IVD - in particular "transit" ones based on the use of the portal imager (EPID) -, the technical and organizational constraints of "transit" IVD and its interaction with other types of Quality Assurance systems.
IRSN (Institut de Radioprotection et de Sûreté Nucléaire) met with leading providers of IVD solutions or comparable control devices, and visited radiotherapy departments with experience in the field. The point of view of the French Society of Medical Physicists (SFPM) position has been gathered.The study identified the main "transit" IVD solutions currently proposed and to come, as well as alternative or complementary solutions, e.g transmission detectors. These transmission detectors - of the matrix type of diodes or ionisation chambers - which are fixed at the gantry head of the accelerator, measure the fluency of the beam of radiation at the output of the collimator. From the measurement, some of these systems offer the possibility of recalculating, on the initial scanner, the dose to the patient. The total independence of the measurement, the possible use in all treatment configurations and the monitoring of the dose delivered in real time are part of the arguments announced in favor of this type of system, also presented as IVD solutions, these devices however, rely on a dose measurement upstream of the patient.
Interviews with medical physicists revealed that IVD is part of an overall process of patient dose verification and can hardly replace the controls performed in the absence of the patient. The offer of IVD solutions in the most advanced radiotherapy techniques is progressing and seems to be maturing. The EPID-based IVD is the most widely used but, routinely, is often confined to conventional techniques. The exploitation and interpretability of the results is the main difficulty encountered by the teams, leading them to develop a treatment verification strategy in which the IVD is a one possibility of verification but not the ultimate and global solution. Thus, the controls carried out before the treatment (evaluation of treatment plans, pre-treatment controls) and the new control devices (transmission detectors, log files ...) must be included in this overall process of verification, and formalized.
Testimonials from clinics in France which have used the transmission detector Delta4 Discover.
“The Delta4 Discover system allows a monitoring of the machine performances and can lead to anticipate the deteriorations of quality of the treatment”
Mathieu Gonod, Physicien Médical, Centre Georges François Leclerc, Dijon, France who was part of this study
Publication : Poster SFPM Discover VA
“Major advantage is 2D measurements of all LINAC parameters (energy, gantry and collimator angles, leaves positions, field size…) that can be associated to a laser check of patient positioning. Whatever the technique used, especially intensity-modulated one, there is no need to define an appropriate measurement point. Delta4 Discover is an integrated part of the verification of treatment quality, it is a step towards security of treatment”
Aurélia MANA, Radiophysicienne, Centre Antoine Lacassagne, Nice, France
Link to the original French study: IRSN-PSE-SANTE-SER-2018-00003_Dosimetrie-de-transit