Cancer patients are treated with radiotherapy in whicha high dose of ionizing radiation is used to target and kill cancerous cells. MR-guided radiotherapy, the simultaneous use of Magnetic Resonance (MR)-imaging and Megavolt (MV) photon irradiation allows staff to see what is being treated, and this will lead to a step change in radiotherapy over the coming years. Improved metrology in dosimetry and imaging is required for the safe clinical implementation of MR guided radiotherapy and to support future innovations in MR guided radiotherapy, which is of great importance.
In 2012, cancer incidence in the European Union was approximately 2.6 million people (approx. 0.5 % of the population) per year. The devastating consequences of this disease therefore affect the daily life of a large proportion of the European population with roughly half of these patients beeing treated using radiotherapy. In Europe and worldwide several manufacturers and academic hospitals are developing MR guided radiotherapy facilities. The first patients were successfully treated in 2014 and through this the feasibility of MR-guided radiotherapy was demonstrated. The benefits of MR-guided radiotherapy is:
- an increased accuracy in defining the contours of the tumour, organs and other healthy tissue.
- avoidance of additional exposure to harmful radiation from diagnostic imaging modalities (e.g. CT), which are currently used to verify the setup of the patient at the treatment device.
- the ability to image motion, caused by internal movements of the patient (e.g. breathing, swallowing), during treatment. This allows adaptation and optimisation of the dose during the treatment based on the actual patient anatomy and previously delivered dose distributions.
The magnetic field cannot easily be switched off for these MR-guided radiotherapy modalities; therefore measurement of the radiation dose (dosimetry) needs to be performed in the presence of the constant magnetic field. Under this condition, both the detectors used for dosimetry and the dose distributions are highly influenced by the magnetic field. Since the underlying physical mechanisms are not well understood,
traceability for radiation
dosimetry and adequate
knowledge of detector characteristics is lacking and no
dosimetry protocols or Codes of Practice (
CoP) are available for reference dosimetry and measurements of the
radiation field characteristics. Therefore medical physicists are not able to calibrate the radiation field and to characterise the radiation fields in MRgRT for treatment planning with a known accuracy. Furthermore
the accuracy of the Monte Carlo based algorithms for the calculation of detector response and dose distributions in the presence of magnetic fields needs to be improved. To guarantee that the dose distribution is delivered to the patient as intended in treatment planning, the medical physicists need
QA procedures which include QA of the individual steps (MR imaging, dose delivery) as well as end-to-end tests of the complete treatment workflow. This adaptive treatment workflow poses higher demands on both the geometrical and dosimetrical accuracy than before. To assess these accuracies and to verify the dose delivery under static and dynamic conditions medical physicists and clinicians need
MR compatible dynamic phantoms, and
methods to determine appropriate safety margins around the tumour.
This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the Project.
