Dose measurement in magnetic fields: from the lab to the hospital

Radiation of tumoral tissue with high‑energy photons is a widely used treatment method. Around one in ten cancer patients undergoes treatment of this kind. Several hospitals now have MR‑linacs – innovative special‑purpose devices that allow radiation to be directed to the afflicted tissue with even greater precision. PTB has investigated methods designed to ensure the traceable measurement of the absorbed dose to water for these devices; these methods have now been tested at different clinical sites.

When treating an illness, two important questions emerge: How can the treatment be used to achieve the greatest possible healing effect, and how can undesirable side effects be prevented? In radiation oncology (the treatment of cancer by means of ionizing radiation), these questions are quantitatively described by the therapeutic index. Up to a certain radiation dose, the damage to the tumoral tissue outweighs the damage to the surrounding healthy tissue; however, if the radiation dose is too large, the healthy tissue is increasingly negatively affected. For this reason, it is especially important to characterize the radiation fields used as well as possible. To this end, in clinical settings, regular measurements are taken of the absorbed dose to water by means of ionization chambers. The absorbed dose to water is a quantity that indicates how much energy a radiation field transfers to a certain amount of water; it is of central importance for prescribing radiation oncology. However, the exact position and shape of the tumor in the patient is also of central importance; after all, the photons must be directed to exactly the correct spot. To achieve this, initial studies have irradiated patients inside magnetic resonance imaging (MRI) systems that have been complemented by linear accelerators designed to generate high‑energy photons (MR‑linacs). This allows the tumor to be identified in the MR image and the photons to be directed to the correct spot. However, the characterization of a radiation field inside a magnetic resonance imaging system entails certain particularities. The strong magnetic field required for the imaging cannot simply be switched off.

PTB is researching how to adapt the available dose‑measurement methods for application in MR‑linacs. This pertains specifically to the use of ionization chambers and alanine probes for measuring the absorbed dose to water. In a series of measurements at Christie Hospital in Manchester (UK), at Tübingen University Hospital (Germany) and at Hôpital Riviera‑Chablais in Rennaz (Switzerland), the previously obtained findings have now been tested under clinical conditions.

At these sites, different detectors were placed inside a water phantom at a water‑equivalent depth of 10 cm. Then, they were irradiated in different orientations with 10 x 10 cm² photon fields (Figure 1). Here, the magnetic flux density of the MR‑linac used (Elekta Unity) was 1.5 T. At the Manchester and Rennaz sites, it was also possible to perform measurements with the magnetic field disabled.

The measurements confirmed the correction factors determined under laboratory conditions for the influence of the magnetic field on ionization chambers as well as the theoretical observations and computer simulations performed by PTB.

However, knowing the exact absorbed dose to water is only the first step. Via MR imaging, exact knowledge of the position of the tumor allows further therapeutic methods such as stereotaxy to be applied. The exact characterization of these especially small radiation fields is a special challenge that PTB will take on in the future.

Figure 1:   Measurement setup at Christie Hospital in Manchester (UK). A Farmer ionization chamber has been placed inside a water phantom in the MR‑linac. In addition, a smaller semiflex ionization chamber is used to control the beam. The photon beam enters the MR tube from the right‑hand side through the plastic sheathing. The water phantom was specially developed by the National Physical Laboratory (London).