Dosimetry of a 68 MeV proton beam using normoxic polymer gels and 3 T MR scanners

Purpose Polymer gels are used for the verification of dose distributions in intensity modulated radiotherapy IRMT , stereotatic radiotherapy and brachytherapy. First experiments in proton therapy of eye tumours were done by us with BANG polymer gels. We analysed patient collimators and wedge angles. The BANG gel handling is very complicated and they show a pronounced quenching of the Bragg Peak. The resolution of the MR images 0.3 mm Pixel should be improved. Methods and Materials Now we use two different polymer gels MAGIC gel proposed by S. Scheib et al. and THPC gel proposed by C. Bayreder et al. . Both are normoxic gels and can be easily produced by ourselves within two hours. The gels were filled in poly propylene PP or Barex containers and were irradiated with a 68 MeV proton beam at the Hahn Meitner Institut Berlin. In order to achieve a better resolution and a better signal to noise ratio we used two different 3 T MR scanners, a clinical MR scanner at the Charit in Berlin and a research MR scanner with high field gradients at Medical University of Vienna. Results In the PP containers we observed an area of reduced polymerisation near the wall of the containers. Both gels show a linear dose response THPC gel up to 4 Gy; MAGIC gel up to 30 Gy. The Quenching of the Bragg Peak can be equally parametrised but it is not as prominent as in the BANG gels. Discussion and Conclusion With the 3 T MR scanners we achieve the needed resolution for gel dosismetry in eye tumour therapy. Due to its great linear dose range the MAGIC gel should be preferred