Last update in February 2012
The principal objective of the project is to develop a diamond dosimeter to measure the dose delivered in the minibeams used in stereotactic radiotherapy. This technique uses convergent beams of small size to very selectively irradiate a target of small dimensions (on the order of a square millimetre to a centimetre); they constitute a very-high-precision non-invasive treatment (unlike standard radiotherapy) useful for primitive or secondary tumours located near radiosensitive structures or difficult to access for surgery, such as the brain, spinal cord or lungs. These minibeams allow the total radiation dose distributed to the targeted tumours to be increased.
Context and issues
The issue in development of a detector adapted for dosimetry of minibeams is to increase the safety of the treatments and the radioprotection of patients.
Of 200,000 treatments annually in France, 2000 are performed under stereotactic conditions. These techniques involve risks related to evaluation of the doses delivered by these very precise but very dense beams, in particular overdosage and irradiation of healthy areas. In 2007, patients treated with stereotactic radiotherapy at the Toulouse UHC were accidentally overexposed. It has been shown that the accident resulted from an inappropriate choice of detector, with a sensitive volume too large given the dimensions of the beams to be calibrated. In fact, when the size of the detector becomes greater than the irradiation field, the measured dose is underestimated compared to the actual dose. The size of the beams thus constitutes the main obstacle to the precision of the measurements, as it calls for the use of detectors with volumes on the order of a millimetre. This also leads to consideration of the positioning and type of detector as well as its size. In addition, the IRSN has revealed significant variations in the assessment of doses by the various French radiotherapy centres, variations due to the use of different detectors. This is explained by the absence of a directive or standard on the best type of detector to use. Presently, no suitable detection system or dosimeter exists comparable to those existing in standard radiotherapy for beams of large size.
The Diadomi project is in line with two of the guiding themes of the medical technology, or TecSan, call for proposals launched by the French National Research Agency (Agence nationale pour la recherche) with the objective of promoting applications to health technologies and autonomy: the development of new instrumentation for cancer treatments by radiotherapy using advanced methods, and the development of new products fulfilling a market need in terms of health.
Experimentations on a radiotherapy installation delivering minibeams
In the light of these observations, the challenge is to develop an operational detection system composed of a device of very small dimensions (volume <1 mm3, which is less than the smallest of the presently existing detectors), with high sensitivity, for which the reaction to ionising radiation represents that of human tissues. Diamond, a highly stable substance, can be used as an equivalent of human soft tissues with regard to ionising radiation, as it has a similar density (the atomic number of diamond is Z=6, with encapsulation in water-equivalent material). To develop the detector, it will be simulated and used in context in dosimetric measurements, with the radiation source and the detector simulated by a Monte Carlo method, in order to study the parameters that affect measurement of the dose. The objective is in particular to adapt the support of the diamond, its electrical connections and its encapsulation in radiotherapy apparatus in order to reduce as much as possible its influence on dose measurement. The simulation and measurements will allow the characteristics of the ideal dosimeter to be approached. The challenge in the long term is to establish a precise measurement protocol for minibeams with a diamond dosimeter sufficiently user-friendly for systematic use in hospitals.
Task organisation and project planning
The project, coordinated by the CEA-LIST (Laboratory for Integration of Systems and Technologies) began on 1 April 2011 for a period of three years. The program is organised into five main tasks:
analysis of the needs and definition of the specifications that the detector developed must fulfil (task coordinator: La Pitié Salpétrière Hospital);
design, optimisation and production of a prototype diamond dosimeter and characterisation of the dosimeter under reference conditions (task coordinator: CEA-LIST);
obtaining dosimetric data in facilities providing minibeams (task coordinator: IRSN). This will involve modelling the existing detectors and making measurements under clinical conditions with the various types of dosimeters;
testing the dosimeter under clinical conditions (task coordinator: IRSN). Comparisons will be made between experimental results with the prototype synthetic diamond dosimeter and models of the facilities.