EPR Dosimetry for Measuring Very Small Fields in Modern Radiotherapy

Dr Christopher Noble,
Honorary Fellow, Centre for Advanced Imaging
The University of Queensland, presents:

EPR Dosimetry for Measuring Very Small Fields in Modern Radiotherapy

Talk abstract: 

Christopher Noble^1,2, Jeffrey Harmer², Benjamin Perrett¹, Emma Horgan¹, Paul Charles¹

¹ Radiation Oncology, Princess Alexandra Hospital, Woolloongabba, Brisbane

² Centre for Advanced Imaging, University of Queensland, St. Lucia, Brisbane

Advances in stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) have increased the need for accurate small field dosimetry. As the size of radiotherapy fields are reduced the detector density begins to play an increasingly dominant role necessitating the use of correction factors[1]. An ideal detector for small fields would have a density equivalent to that of human tissue.  The alanine/electron paramagnetic resonance (EPR) system was proposed as a tool for radiation dosimetry in the 60s [2]. It can be used for absolute dosimetry, has good dose linearity, is energy independent over the range of treatment energies and has good tissue equivalence with a density of 1.42 g/cm³.

More recently Irganox 1076, a phenol antioxidant used in polymer production, has been proposed as a novel material for EPR dosimetry [3].  Reports in the literature have used Irganox mixed with binders such as paraffin wax to produce dosimeter pellets [4]. However, Irganox has a density of 1.012 g/cm³ and as such it could be an ideal material for small field dosimetry without the need for correction factors. Preliminary results for solid pellets of pure Irganox irradiated with 6MV photons are presented here for the first time. The EPR spectrum of irradiated Irganox, the identity and stability of the radical created are characterized and the dose linearity, reproducibility and detection limits are measured.  The possibility of using EPR imaging to obtain a 3D dose map is also explored.