Determination of complex dose distributions in inhomogenious phantoms, irratiated by high-energy photons

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In our preceding project, we investigated the accuracy of measurements of dose distributions in water for high-energy Photon beams. We investigated fixed fields only. Such fields are used for both, the generation of the physical parameters for dose calculation software and for verification of dose calculation with such software. For different detector types, we studied and quantified the response inregions where secondary charged particle equilibrium does not exist. Besides fields with radii smaller than the lateral electron range, weconsidered field edge and surface regions. Over that, fields with central blocking were investigated in order to identify the Response effects to pure scattered radiation. Thus and with knowledge of the ratio of primary to scatter radiation detector response may be calculated. In a next step, we want to measure more realistic dose distributions, as applied to patients, in phantoms similar to the anatomy: In general, the accuracy of the dose calculation of a treatment planning system is tested for simple fixed beams only. However, in clinical practice complex sequences of partly very small and irregularly formed fields are delivered. Verification of such complex treatment plans, e.g. spinal vertebrae plans, by measurement in homogeneous plastic phantoms, often shows large deviations between measured and planned dose distribution. This may have various reasons: the collimation system of the Treatment unit is modeled approximately only, the dynamic collimation control is not exactly as specified in the plan or the dose calculation is defective. However, also the measurement is afflicted with errors. Too large detectors average too heavily across their volumes, while nonwater- equivalent probes lead to perturbations of the field. The latter mainly depend on the spectrum of the secondary particles at the measurement point. Therefore, it is a major goal to improve the accuracy for the measurement of complex field configurations. For that, it is necessary to determine correction factors, depending on the particular field configuration, at every measurement point in a phantom. Sensibly, pertinent guidelines recommend testing calculated dose distributions randomly, also in phantoms with inhomogeneities. Despite of that, simple clinically usable procedures are lacking. To develop such procedures is a further focus of the project. Critical regions are air cavities, lung lesions, bones, teeth and implants. Weplan to design phantoms and produce them with a 3D printer in order to test treatment plans realistically. Here the challenge will be again to determine the response at every measurement point with sufficient accuracy in order to recognize delivery errors. The treatment plans shall be made with different treatment planning systems. This will enable to quantify the influence of individual parameters of the calculation algorithms.