Session Item

The motion compensation strategy in particle therapy depends on the anatomic region, motion amplitude and underlying beam delivery technology. The prerequisite for improving existing treatment concepts for moving targets is the quantification of the interplay effect between organ motion and beam delivery and its impact on the dose distribution and hence treatment delivery accuracy. While retrospective logfile based analysis gives insight into the patient’s breathing and beam delivery time structure, a prospective 4D dose prediction allows adaptation on a patient specific basis. 

Dose distributions of 3 pancreas and 3 liver cancer patients with motion amplitudes below 4mm were analysed. All patients were treated with scanned pulsed proton beams delivered by a synchrotron. Dose prescription was 5x7.5 Gy(RBE) for pancreas and 15x4.68 or 10x5 Gy(RBE) for liver. The treatment planning system RayStation8B (MCv4.2) (RaySearch) was used employing robust optimisation for mitigating different organ fillings. Treatment accuracy was determined using: (1) file based 4D dose tracking (f-4DDT) considering the time structure from accelerator logfiles and surface scanner breathing patterns (C-Rad) for each fraction; (2) parameter based 4D dose tracking tool (p-4DDT). Input parameters encompassed the averaged dose rate extracted from the accelerator logfiles over all fractions, scanning speed as well as constant breathing cycle length. The p-4DDT method was used additionally to investigate the influence of the starting phase and dose rate. Both methods considered the given time structures for tracking the static dose on 8 4DCT phases. This dose distribution was mapped onto the planning CT using deformable image registration and accumulated for all fractions. DVH parameters and γ-pass rates with a 2%/2mm criteria were used for dosimetric evaluation.

For the target structures 4DDT revealed a dose calculation accuracy of 1% for D_50% considering the interplay effect. D_98% of the static plan was disturbed by up to 7% for the PTV and up to 2% for the CTV for 5 out of 6 patients. For liver patients D_{33%_liver} showed deviations between the two tools up to 14%, while for pancreas patients D_{1%_stomach} varied up to 6.5%. The two 4DDT methods agreed within 2% for D_98% and D_2% for PTV and CTV for all patients, except for one liver patient where the deviation increased up to 7% (Fig1). The γ-pass rates analysis between the p-4DDT and the f-4DCT showed an agreement above 91% for all patients, confirming the usability of p-4DDT for dose prediction (Fig2). Variations in starting phase and dose rate were negligible for the 4DDT outcome. 

The p-4DDT could be used prospectively to determine the impact of beam and organ motion for pancreatic and liver cases in scanned proton therapy. The systematic uncertainties covered by the PTV margins compensated well for the motion effects of the investigated indications preserving an excellent CTV coverage when motion was considered.