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Investigations into small-field dosimetry in magnetic fields for MRI-guided radiation therapy


In Germany, almost 500,000 people are diagnosed with cancer every year [1]. About half of the treatments involve irradiation of the tumor with ionizing radiation. An innovative development known as the MRlinac combines a linear accelerator (linac) with an MRI scanner. This enables high‑contrast imaging in soft tissues during and immediately before irradiation. In this way, the location of the tumor and the organs at risk, which may vary from one day to another, can be determined, and the initial irradiation plan can be adapted in real time as required. In addition, organ or tumor movements during therapy caused, for example, by the patient’s breathing, can be taken into account. As a result, the irradiated volume and thus also the side effects can be reduced. [2]

Successful radiation therapy treatment requires exact knowledge of the dose distribution in the patient. For this purpose, usual clinical practice is to measure the dose distribution in a water phantom under precisely specified reference conditions and then transfer the measurement data to a so‑called irradiation planning system to create an accurate accelerator model. The planning system can then use a CT scan of the patient to calculate the exact dose distribution in the patient and, ultimately, the applied dose.

For dosimetry in magnetic fields it is important to take into account the changed behavior of dosimeters in magnetic fields. For example, in air‑filled ionization chambers, the secondary electrons generated by high‑energy photons are more strongly deflected by the Lorentz force than they are in water. This effect and others have an impact on the measurement signal and have to be considered. [2, 3].

Dosimetry in magnetic fields for radiation fields of 10 x 10 cm2 (reference field) has already been the subject of numerous publications [4]. In order to be able to transfer the techniques that have become standard practice in conventional radiation therapy, like intensity‑modulated radiation therapy (IMRT/VMAT), to the MRlinac, dosimetry under non‑reference conditions (especially small‑field dosimetry) using such devices is an important next step. For this purpose, not only detectors with smaller sensitive measuring volumes are used but also another correction factor correction factor is added to the international code of practice IAEA TRS‑483 [5]. This factor corrects the measurement signal in the magnetic field from the reference field to smaller fields:

correction factor formula

PTB’s Working Group 6.21, Dosimetry for Radiation Therapy, performs investigations in this field as part of the EU‑funded EMPIR project “MRgRT-DOS” (https://mrgrtmetrology.com/) in collaboration with several international partners. Various detectors, including ionization chambers, radiochromic film or alanine dosimeters are to be examined in terms of their suitability for dose measurement under non‑reference conditions in magnetic fields and the required correction factors are to be determined. Two medical linac, several measuring devices and a mobile electromagnet with a variable magnetic flux density of up to 1.5 T are available for this purpose. A typical measuring setup is shown in Figure 1.

electromagnet and ionization chamber

Figure 1: Measuring setup for determining output factors in magnetic fields. Left: the Bruker ER0173W mobile electromagnet in front of a medical linac. Right: a cylindrical ionization chamber (Farmer Ionization Chamber 30013, PTW) in a water phantom between the pole shoes of the electromagnet.

As several field geometries are to be examined, the first step is to determine the measurement signal of the detector as a function of the field size (so‑called output factors). As a second step, these output factors have to be correlated with the “true” dose. To this end, Monte Carlo simulations with detailed accelerator and detector models were used. Step three is then to determine the correction factor using equation (1). In Figure 2, on the left, you can see the measurement signal of a cylindrical ionization chamber with a small sensitive volume (Semiflex 31021; PTW), positioned perpendicular to the radiation and magnetic field, with various magnetic flux densities and field sizes. For each field size, the signal has been normalized to the value measured without the magnetic field. What is striking here is that in large fields (field size > 3 x 3 cm²), the curve progression is initially asymmetric but becomes increasingly symmetric for smaller field sizes. On the right, the output factors are shown: For small field sizes, the measurement signal in the magnetic field shows a stronger decrease than in the measurement without magnetic field. During the next few months, Monte Carlo simulations will be done among other things to work out what causes these effects.

diagram response and diagram output factors for the magnetix flux densities

Figure 2: Relative response (left) and output factors of a Semiflex 31021 (right). On the left, the relative response of the Semiflex 31021 ionization chamber is shown as a function of the field size. The measurement values were normalized to the value measured without the magnetic field (B = 0 T) for each field size. On the right, the output factors for the magnetic flux densities ±1.5 T and 0 T are plotted against the field size.


[1]        Zentrum für Krebsregisterdaten im Robert Koch‑Institut: Datenbankabfrage mit Schätzung der Inzidenz, Prävalenz und des Überlebens von Krebs in Deutschland auf Basis der epidemiologischen Landeskrebsregisterdaten, DOI: 10.18444/

[2]        Blum et al, The dose response of PTW microdiamond and microSilicon in transverse magnetic field under small field conditions, Physics in Medicine & Biology 66, 2021, doi.org/10.1088/1361-6560/ac0f2e

[3]        S. Pojtinger, Dosimetry of Ionizing Radiation in Magnetic Fields, Dissertation, Eberhard Karls Universität Tübingen, 2021

[4]        Pooter et al, Reference dosimetry in MRI‑linacs: evaluation of available protocols and data to establish a Code of Practice, Physics in Medicine & Biology 66, 2021, DOI: 10.1088/1361-6560/ab9efe

[5]        International Atomic Energy Agency, Dosimetry of Small Static Fields Used in External Beam Radiotherapy – An International Code of Practice for Reference and Relative Dose Determination, Technical Reports Series No. 483, 2017, aufrufbar unter: www-pub.iaea.org/MTCD/Publications/PDF/D483_web.pdf


Opens local program for sending emailS. Frick, Department 6.2, Working Group 6.21