Gel dosimetry for the dose verification of intensity modulated radiotherapy treatments

Abstract

To meet the requirements for 3D dose verification of high-precision radiotherapy treatments (such as in intensity modulated radiotherapy; IMRT), two different kinds of gel dosimeters were developed. In the Fricke gel, an oxidation of ferrous-ions into ferric-ions occurs upon irradiation. As the ferrous and ferric ions possess different magnetic moments, the dose distribution can be read out by use of magnetic resonance imaging (MRI). However, this method is susceptible to spatial instabilities as the ions are able to diffuse through the gel matrix. Attempts have been made to reduce the diffusion by changing the gelling substance. In monomer/polymer gel dosimetry, the spin-spin relaxation rate, R2, is related to the radiation dose that was delivered to the phantom, as ionizing irradiation causes the formation of polymer aggregates. The intended target figure of accuracy in gel dosimetry for IMRT is approximately 3% to 5% of the maximum dose. However, in a conventional MR scanner several imaging artefacts may cause dose inaccuracies. Studies of these different artefacts have resulted in different compensation strategies, otherwise the dose maps will also contain stochastic noise. To minimize the stochastic noise in the images, the imaging sequence parameters should be optimized. Monomer/polymer gels are proven to be valuable dosimeters for IMRT applications. They are able to provide a three-dimensional dose distribution that is integrated over all beams delivered in the treatment. The gels can be molded in humanoid shapes. As a result, gel dosimeters are able to simulate the complete treatment. Gel dosimetry can therefore be regarded as a dosimetric tool to verify dose distributions calculated by the computer planning. Furthermore, gel dosimetry has also been very useful in quality control and investigation of extreme beam configurations possibly occurring in IMRT, in dose verification after implementation of additional devices, and in investigating dose disturbances by low-density structures. The search for new chemical compositions may result in dosimeter gels that are easier to fabricate and possess higher sensitivity. Besides MRI, other imaging techniques are investigated to read out the dosimeters.