Verification of IMRT beam delivery with a ferrous sulfate gel dosimeter and MRI

Intensity modulated photon beam radiation therapy often results in dynamically delivered beams with small field sizes and steep dose gradients. This defines a need for an integrating, tissue-equivalent, high resolution dosimeter. 3D ferrous sulfate gel based dosimetry involves the use of magnetic resonance (MR) images of radiosensitive paramagnetic gels. The goal of this work is to create a patient specific quality assurance (QA) procedure that links measured dosimetnc information to clinical goals.The gel dosimeter system is tested through a set of simple experiments which characterize and confirm the system as a valid QA tool for conformal and intensity modulated radiation therapy.For this work, dynamic photon beams are created on a commercially available inverse treatment planning system and the treatment is delivered to a gel filled acrylic mold. Software has been developed to quantify dose from the QA MR images, and to register this information to the planning computed tomography (CT) scan. The software displays the measured dose on the planning CT, and calculates dose-volume histograms for the registered measured data and contoured patient structures. This work reveals good agreement between planned and measured dose distributions, with less than 5% difference in the mean doses of the contoured patient structures