Addressing this, researchers in the US have shown that three chemical dosimeters – originally developed for use in non-MRI guided radiotherapy – can measure dose in strong magnetic fields without a correction factor. Used in block form enabling 3D measurements, the tissue-equivalent dosimeters are "ideally suited" for use in MRI-guided therapy, says first author Hannah Lee of the University of Texas MD Anderson Cancer Center (Radiother. Oncol. doi: 10.1016/j.radonc.2017.08.027).

In each dosimeter, radiolysis by the treatment beam changes the material's magnetic or optical properties, or both. These changes are then quantified using an MRI scanner or spectrophotometer, respectively. For example, in FOX, an iron-based radiochromic gel, ferrous ions convert to ferric ions upon irradiation. This transformation causes the yellow gel to turn purple and alters the relaxation of water protons in the dosimeter.

Dosimeter testing

Lee, together with senior author Geoffrey Ibbott and co-authors, tested FOX, PRESAGE radiochromic plastic and BANG polymer gel, irradiating them in the presence and absence of a strong magnetic field provided by a bench-top electromagnet. Held in transparent containers, the dosimeters were positioned between the poles of the magnet in phantoms that provided build-up and backscatter.

The researchers irradiated FOX and PRESAGE with a cobalt-60 source, with the electromagnet set to generate a field strength of 1.5 T when switched on. A 6 MV commercial linac was used to irradiate BANG, with the electromagnet set to provide a 1.0 T field. Irradiations were carried out on the same day to minimize day-to-day and inter-batch variations. Measurements between the poles of the magnet demonstrated the B0 field to be stable.

The electromagnet was aligned with its field perpendicular to the central axis of the radiation beam, resulting in a Lorentz force and deflection of secondary electrons when the electromagnet was switched on. However, the beam area was significantly larger than the small regions in the dosimeters used to read out the dose. Consequently, the volumes were exposed to a uniform radiation field, meaning that the delivered dose could be assumed to be constant, whether the magnet was on or off. < mpu />

FOX and PRESAGE were read out optically, while BANG was read out using a commercial 3T MRI scanner. In FOX, the researchers observed differences in optical signals of 1.6% and 0.5%, at spectral peaks of 440 nm and 585 nm, between when the magnet was on and off. In PRESAGE, the difference was 1.5% at the 632 nm spectral peak, while in BANG, the difference in R2 MRI was 0.7%. Linear dose responses were also observed in each.

The authors see the differences as "minimal," concluding that the dosimeters are suitable for measuring dose in an MRI-guided treatment system without any correction factors at the field strengths investigated.

Further work is needed, however, to make 3D dosimeters a more feasible option in the clinic. The dosimeters are not commonly used as they are difficult to make in-house, Ibbott told medicalphysicsweb.

Addressing this, the authors are working on a new 3D dosimeter. "We have developed a reusable FOX gel and are completing our characterization of this gel for evaluation of the dose delivery and image-guided radiotherapy capabilities of the MRI-Linac," said Ibbott.

"We believe that our FOX gel and other Fricke gels represent the ideal dosimeters because they respond linearly with dose and the dose information can be acquired with the MRI component of the MRI-guided radiotherapy system," added Lee. In ongoing work, the group is also developing techniques that use real-time read-outs of FOX gel, either before or following treatment, to verify MRI-guided compensations for patient motion.