Gel Dosimetry / MRI

Modern radiotherapy treatments are increasingly sophisticated allowing, the delivery of extremely conformal doses with the aim of improving prognosis by specifically targeting tumours with high doses and avoiding normal tissues. However, the technology for checking and measuring dose distributions is becoming increasingly redundant in these situations. Currently only one-dimensional (using ionisation chambers) and two-dimensional (e.g. film) verification is possible. This is extremely time consuming and impractical and is holding back the acceptance of state-of-the-art treatments.

Gel dosimetry offers a step-change in dose verification. Following irradiation, the magnetic properties of the gel are changed and the gel dosimeter can be imaged using MRI making three-dimensional dose information available to radiotherapists.

Early types of gel suffered from post-irradiation diffusion effects, which blurred the spatial information. Second generation gels based on polymers were more successful, albeit their toxicity and oxygen inhibition effects limited widespread clinical use. From 2001, a new type of gel was introduced that remedied previous problems. Imagel’s product is based on this formulation, and with improvements in the manufacturing process, Imagel now offers a reliable and uniform product making it a robust dosimetric tool.

Irradiation of the gel changes its magnetic properties. Specifically, the transverse relaxation rate is increased causing the gel to darken in regions that have received dose. This makes it a highly sensitive tool for relative dosimetry of even the most complicated distributions, allowing radiotherapists to verify the delivered intent in patient plans. The gel exhibits a linear dependence between the dose delivered to it and the relaxation rate. This means that if this relaxation rate is measured using MRI the gel can be calibrated to obtain absolute dose measurements. The high field systems that Imagel use allow these plans to be imaged at very high isotropic resolution ideal for modern external beam deliveries as well as for brachytherapy.