Verification of dose calculations in external beam treatment planning using a gel dosimetry system

In external radiation therapy, treatment planning is routinely performed on 3D Treatment Planning Systems (TPS). The absorbed dose distribution in the patient is calculated with the TPS and the treatment is administered according to the plan. Considering that the TPS-calculated dose has been shown to deviate from the actual dose distribution in various situations, there is doubtless a need for a realistic verification system. The dosimeter gel, based on ferrous sulphate and agarose and evaluated using Magnetic Resonance Imaging (MRI), can be used to measure the complete dose distribution in many situations. It is a mouldable tissue equivalent dosimetry system which has previously lacked a sufficient overall precision for verification purposes (5-10%). Using the same gel composition as previously with a new MRI system and a new MRI acquisition protocol, dose distributions from both photons and electrons were measured. The precision in the dose determination was evaluated. Comparison with TPS calculated results as well as diode measurements were performed. In this study the system's relative dose determination precision has been significantly improved and was found to be better than 3%. When examining the potential of the gel dosimetry system as a verification tool of TPS calculations, measured and calculated depthdose curves, dose profiles and isodose lines were compared for both photons and electrons. In the depthdose curves, only small variations were found (<2% at treatment depth). Larger deviations (>5%) were seen in the penumbra region of the electron beam when comparing measured and calculated profiles. The dosimeter gel seems to be a promising tool for verification of TPS calculations as measurements can be performed with high precision