Fricke Gel Dosimetry: Translation into Routine Radiotherapy Practice

Technological advances in Radiotherapy treatment delivery improve the ability to shape dose distributions to tumours in 3D and reduce side effects. Healthy organs can be spared and tumours given higher doses, which improves the potential for cancer cure. However, technology to deliver these treatments is complex and therefore increase risk of treatment error. Consequently, there is a need for more sophisticated measurement methods to verify the accuracy of radiotherapy delivery. Radiation-sensitive gels are increasingly used by research groups for the verification of proposed radiotherapy techniques; this is known as 3D chemical dosimetry. These gels can be irradiated with a radiotherapy plan and analysed using Magnetic Resonance Imaging (MRI). This provides quantifiable images of the delivered radiation dose distribution in three dimensions.

Although, extensive research has been carried out in the use of poly-acrylamide gels those present some drawbacks for routine use (complex chemistry, toxicity). Fricke gel dosimetry involves the radiation-induced oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+). The ferrous ions in the form of ferrous ammonium sulphate are embedded within a gelatin matrix to form a 3D radiation detector. Compared with other types of gel detector they are simple to prepare requiring only very basic laboratory facilities, involve chemicals with a reduced toxicity and do not suffer from oxygen inhibition. Irradiated gel detectors are readout with 3D imaging techniques such as magnetic resonance imaging (MRI) and optical-CT scanning.

The aim of our research is to develop a chemical gel detector into a routine dosimetric tool within a clinical radiotherapy department. We set requirements that the detector should be simple to manufacture with only basic laboratory facilities, the irradiated detector should be readout using existing imaging equipment and that the system should have a proven dosimetric performance with quantified measurement uncertainties over a clinically relevant dose range. Magnetic resonance (MR) analysis methods were selected for this work as MR technology is more commonly available to clinical radiotherapy departments including ours.

In the first stages of the project we have focussed our effort on the chemical formulation and properties of the Fricke gel using small volume and a bench top NMR spectrometer to quantify the doses. Experiments to characterise dosimetric properties of the selected detector were carried out including precision, dose response, chemical stability and detector response versus dose rate and energy. The dose response and basic measurement precision of the detector were quantified over a clinically relevant dose range. The results from these experiments demonstrated the potential of this detector for higher dose radiotherapy techniques (>5Gy), such as stereotactic radiotherapy.

We then investigated larger volumes and in particular the post-irradiation diffusion of ferric ions, the detector homogeneity, the volume dependence and the calibration methods using a clinical MR scanner. It was demonstrated that the detector responded uniformly to radiation and that there was negligible difference in response between different detector volumes. It is known that the ferric ions diffuse through the gelatin matrix following irradiation which causes a blurring of the measured dose distribution. This was quantified and it was shown that irradiated detectors must be scanned within 2 hours of irradiation. In our current work, we are looking at increasingly complex clinical radiotherapy plans.

Dose map
Dose map of detector irradiated with simple plan (left) and comparison with treatment planning software right)

Funding:

Monk J, McDougall ND, Miquel ME , 2009, Translating Polymer Gel Dosimetry Research into Clinical Routine Use in Radiotherapy, Barts and the London Charity, £145,000

Spectrometer for relaxation measurements of polyacrylamide gels used in radiotherapy dosimetry, Capital Equipment 2006/2007, Barts and The London Charity, £36,000