18F-FET-PET-Based Dose Painting by Numbers with Protons

Purpose: To investigate the potential of F-18-fluoroethyltyrosine-positron emission tomography-(F-18-FET-PET-)based dose painting by numbers with protons. Material and Methods: Due to its high specificity to brain tumor cells, FET has a high potential to serve as a target for dose painting by numbers. Biological image-based dose painting might lead to an inhomogeneous dose prescription. For precise treatment planning of such a prescribed dose, an intensity-modulated radiotherapy (IMRT) algorithm including a Monte Carlo dose-calculation algorithm for spot-scanning protons was used. A linear tracer uptake to dose model was used to derive a dose prescription from the F-18-FET-PET. As a first investigation, a modified modulation transfer function (MTF) of protons was evaluated and compared to the MTF of photons. In a clinically adapted planning study, the feasibility of F-18-FET-PET-based dose painting with protons was demonstrated using three patients with glioblastome multiforme. The resulting dose distributions were evaluated by means of dose-difference and dose-volume histograms and compared to IMRT data. Results: The MTF for protons was constantly above that for photons. The standard deviations of the dose differences between the prescribed and the optimized dose were smaller in case of protons compared to photons. Furthermore, the escalation study showed that the doses within the subvolumes identified by biological imaging techniques could be escalated remarkably while the dose within the organs at risk was kept at a constant level. Conclusion: The presented investigation fortifies the feasibility of F-18-FET-PET-based dose painting with protons.