4D PT-PET simulation for dose monitoring in moving targets treated with scanned ion beams

Ion beams have the advantage of providing steep dose gradients and therefore they allow high tumour conformality and sparing of nearby organs at risk. Precise knowledge of the actual ion range is highly desired since anatomical changes along the beam path could lead to serious misdosage. Especially for intra-fractionally moving targets the continuous density changes and complex dose delivery techniques increase the risk of an inaccurate dose deposition. The applied dose distribution can be monitored by means of particle therapy positron emission tomography (PT-PET), because ion beams generate positron emitters along their path. Since the delivered dose cannot be extracted directly from the PET scan the validation is usually done by means of a comparison between the reconstructed activities from a PT-PET measurement and a PT-PET simulation. Thus, the simulation software for generating predicted PET data from the treatment planning is an essential part of the dose verification routine. For intra-fractionally moving target volumes the PET data simulation has been upgraded by using time-resolved (4D) algorithms to account correctly for the motion dependent displacement of the positron emitters. Moreover, it considers the time dependent relative movement between target volume and scanned beam to simulate the accurate positron emitter distribution generated during irradiation. We will present the revised and extended version of the simulation software which proceeds with motion compensated dose delivery by scanned ion beams to intra-fractionally moving targets. By means of a dedicated preclinical phantom simulation it is demonstrated that even the most ambitious motion-mitigated beam delivery technique of range compensated target tracking can be handled correctly by the simulation code. Results of the 4D PT-PET simulation will be discussed in detail and compared to a reference simulation and a simulation without consideration of relative movement and range compensation.