(Apr 2012 – Jan 2015)
Dosimetric traceability and potential RBE changes in MRI-accelerator combinations
In 2006 the incidence of cancer in the European Union was about 2.3 million people. A yearly increase of about 1.5 % in cancer cases is to be expected in the next two decades. Roughly half of these patients are treated by radiotherapy (RT), i.e. irradiation of the tumour with ionising radiation, such as high-energetic (MeV range) photon beams produced by linear electron accelerators (linac beams).
The unmatched soft-tissue contrast of Magnetic Resonance Imaging (MRI) make it the ideal candidate for the recent development of image guided radiotherapy. MRI-linacs are an exciting new development fusing both MRI and RT modalities and providing precise, soft-tissue based, on-line position verification and treatment monitoring for radiotherapy. MRI integrated with a radiotherapy system provides better targeting of the radiotherapy at tumours, while sparing healthy surrounding tissues, which can lead to improved tumour control and lower toxicity. Both the clinical and commercial expectations of such clinical systems are high as real-time MRI feed-back on the target position is a long-standing goal of the radiotherapy community.
Safe clinical introduction of new radiotherapy systems is highly dependent on accurate dosimetry. However, radiation dosimetry inside an MRI-scanner is a new challenge and a traceable metrologically sound dosimetry system is not yet existing for this application. Traditionally ionization chambers are used as secondary standards for calibration of photon beams in linear accelerators. In addition, little investigations have been done on the biological response to the photon radiation in the presence of a magnetic field. This WP aims to increase patient safety in MRI guided radiotherapy, by developing a traceable method for measuring the absorbed dose to water for high energy photon beams in an MRI environment and by assessing a potential change in RBE for linac photon beams under the conditions of a magnetic field.
To achieve these goals, a primary standard for measurement of absorbed dose to water in the presence of a strong magnetic field based on water calorimetry will be developed. Additionally, a dosimetry method will be developed for the newly emerging application of MRI in image guided radiotherapy. This will comprise of a system for traceably measuring the absorbed dose to water for high energy photon beams in an MRI-linac combination. Additionally potential changes in relative biological effectiveness of accelerator photon beams due to the magnetic field will be assessed.