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One of the goals of modern radiotherapy is to generate a dose distribution which corresponds best to the shape of the tumour and with which the undesirable effects of the irradiation on healthy tissue can be prevented as far as possible. Such dose distributions are usually characterized by small dimensions (in the cm or mm range) and large dose gradients. Thus, they differ considerably from the dose distributions that are recommended in the dosimetry protocols DIN 6800-2 or IAEA TRS-398 for the measurement of the dose at reference conditions.

With regard to quality assurance and patients' safety, it is necessary to measure the dose reliably and with low uncertainty - even in small and complex radiation fields with the above-mentioned properties. For this purpose, the procedures and data recommended in the dosimetry protocols DIN 6800-2 or IAEA TRS-398 are, however, suited only to a limited extent. Certain parameters - such as, e.g. the finite dimensions of ionization chambers which lead to the measured dose being averaged (whereas it would be better to measure a punctual dose), the potential lack of lateral secondary electron equilibrium, or the change in the spectral photon fluence, compared to dosimetry at reference conditions - cause problems.

Within the scope of an international cooperation with other national metrology institutes, PTB is actively involved in the development of dosimetric processes to be used under such conditions.

An emerging field in radiotherapy (RT) is the combination of photon irradiations with magnetic resonance (MR) imaging in hybrid systems, enabling the adaptation of treatment plans in the case of inter- or intrafractional changes. Dose measurements for quality assurance in this MR-guided RT have to be taken in the presence of a static magnetic field. Ionization chambers (ICs) are widely used in dosimetry, e.g. due to the convenient way that they are handled. However, the IC responses have shown deviations of up to 11 % in magnetic fields mainly due to the curved trajectories of secondary electrons (electron return effect, ERE) [Raaijmakers2005, Meijsing2009]. Hence, the application of ICs in MR-guided RT in accordance with dosimetry protocols like IAEA TRS-398 or DIN 6800-2 involves correction factors compensating for the changes in the response of ICs in magnetic fields. The determination of these correction factors with small uncertainties requires a well-defined secondary standard for the dose to water, with a response possibly unaffected by the magnetic field.

The dosimetry system of  alanine and electron spin resonance (alanine/ESR) is a well-established method for dose measurements at the PTB. We investigate, within a PhD project, the suitability of this dosimetry system as a secondary standard for dosimetry in MR-guided RT. Subsequently, we are going to use alanine/ESR for determining the correction factors for ICs in magnetic fields experimentally.

An electromagnet (Bruker ER0173W) is available for investigating the properties of alanine/ESR in static magnetic fields. This electromagnet generates magnetic fields with flux densities up to 1.1 T in the 100 mm wide gap between its pole shoes (diameter about 25 cm). The magnetic flux density can be increased to 1.4 T by mounting additional pole tips, which reduces the gap width available for measurements to 72 mm. The following table summarizes the characteristics of the electromagnet:

The electromagnet can be positioned in front of our linear accelerators in such a way that the accelerator's isocenter is located between the pole shoes of the electromagnet in the area of high magnetic flux densities. This allows for measurements in water phantom in presence of high-energetic photon fields and concurrent high static magnetic fields. The characteristics of the photon radiation field as well as the flux density of the magnetic fields can be varied in order to study the properties not only of alanine/ESR but also of dosimetric detectors in general.

The Bruker ER0173W electromagnet in front of the clinical accelerator.

Literature

• [Raaijmakers2005]
  Raaijmakers, A. J. E., B. W. Raaymakers, and J. J. W. Lagendijk:
  Integrating a MRI scanner with a 6 MV radiotherapy accelerator: dose increase at tissue–air interfaces in a lateral magnetic field due to returning electrons.
  Physics in medicine and biology 50.7 (2005): 1363.
• [Meijsing2009]
  Meijsing, I., et al.:
  Dosimetry for the MRI accelerator: the impact of a magnetic field on the response of a Farmer NE2571 ionization chamber.
  Physics in medicine and biology 54.10 (2009): 2993.