Radiotherapy dosimetry assessment with optical projection tomography

Abstract

Recent advances in radiotherapy have created the need to develop novel methods for the accurate, three-dimensional assessment of the applied radiation dose during specific radiotherapy plans. Here we present a study based on the use of polymer gel dosimeters in combination with a novel Optical Projection Tomography system, which allows the association of optical properties, namely the attenuation coefficient, to the irradiation dose. Polymer gel dosimeters are polymerized after X-ray irradiation via free radical production during water radiolysis resulting to increased optical opacity as well as change of the nuclear magnetic resonance relaxation times, thus making it possible to study them with both optical and MRI techniques. The optical tomographic system employs a sensitive CCD camera, a rotation stage allowing full 360 degrees rotation and a homogeneous white light source transilluminating the samples. This setup allows the calculation of the optical attenuation coefficient which can then be directly related to the applied radiotherapy dose, as well as the definition of the surface of the sample in space. The experimental procedure involves the recording of transillumination images of the polymer samples in steps of 1 degree to get the desired resolution. Data analysis is performed by back propagating the photons using an inverse Radon transform resulting to the reconstruction of three dimensional images of the attenuation coefficient or equivalently the dose distribution. The sensitivity and dynamic range offered by the technique covers the range of radiotherapy doses in modern clinical practice and are compared with the corresponding achieved with MRI.