ESTRO 2020 Abstract book

S136 ESTRO 2020

PBS pulsed proton beams and a dynamic lung phantom were employed for the dosimetry. A treatment was planned on a CT with a static phantom (TPS RayStation6.99, MC4.1) and modified into different delivery scenarios: with varied prescribed physical dose (2 & 4Gy), and particle fluence degraded to 10, 20, and 50% of the nominal beam transmission. All plans were delivered to the static and moving (2cm cranio-caudal target motion) phantom. The start of phantom motion was synchronized with the beam-on signal. The dose was measured inside and outside of the target using 5 pinpoint IC (TM30013,PTW) PP1…PP5. Time-resolved values with 0.5s step were recorded by communicating with an electrometer via RS232 serial port using an in-house written Python script. Retrospective 4D-DC was enabled by a scripting interface integrated in the TPS. Input data included beam delivery details (logfiles) and 4D-CT reflected the target motion. Each proton spot was reassigned to a certain breathing phase and the single phase sub-doses were transferred to and summed up on the planning CT. Absolute dose differences (DD) between measurements and 4D calculations were analyzed for 15 time points t i of each irradiation (fig.1a). At each t i dose gradients were calculated in a sphere of 3mm radius around each PP measurement point. Results For the point of interest outside the target (PP4) the DD were below 0.1Gy for both static and moving deliveries over the full irradiation time. At the end of the plan delivery, the DD for the target PPs summed up to 0.1Gy for static scenarios and to 0.3Gy -for moving scenarios. These DD of static and moving cases increased over irradiation time until a certain moment, after which they stabilized at a plateau (fig.2). A strong correlation between DD and corresponding dose gradients was observed. The peaks of static DD were associated with certain proton energies and, therefore, with dose gradients due to the beam scanning in depth (fig.1b). In the moving phantom steep dose gradients occurred additionally in the direction of target motion. Measurement points of PP2 and PP3 moving in-and-out of high dose area were most affected by this interplay process, while PP1 and PP5 stayed either in or out of the irradiation volume. Neither the dose per fraction nor the beam intensity had an influence on the DD. Conclusion The agreement between 4D dose measurements in the moving phantom and retrospective 4D-DC was found to be comparable to the static DD for all delivery scenarios. For both, static and moving, cases accurate measurements were challenging at steep dose gradients originating from the beam scanning and tumor motion. [1] ESTRO38 PO-977