ESTRO 2021 Abstract Book

S1479

ESTRO 2021

the dose was split in two subsequent fractions simulating volumetric rescanning. The treatment planning system RayStation8B (Monte Carlo v4.2) (RaySearch, Sweden) was used for clinical treatment plan creation employing robust optimisation strategies for mitigating different organ fillings. Phase-based 4DCT data was acquired at the time of planning and for some patients also during treatment. The delivery time structure was extracted from the accelerator logfiles for each fraction. In combination with the breathing time structure, the time-resolved dose distribution was calculated on the 4DCT phases. This dose distribution was mapped onto the planning CT using deformable image registration tools and subsequently the dose distribution from each fraction was summed to obtain the total accumulated dose. Results The analysis of the first three patients confirmed the robustness of the treatment plans with respect to the interplay of beam and organ motion. The maximum amplitudes observed were 1.3, 1.9 and 3mm, while rescanning was applied for the 3 mm amplitude. The retrospective 4D dose tracking for these three patients revealed that the CTV D 50% did never vary more than 1.6%, even when the beam-on time varied by up to 10% for the different fractions. Changes in the dose statistics of up to 20% were observed for OARs depending on their location with respect to relevant dose gradients. Rescanning did not show any relevant effect on the interplay. Conclusion The hypofractioned proton treatment concept for pancreas patients including the robust optimisation methods and the selection of optimal beam entrance directions showed to be robust against intra-fractional movements. Within this study the necessary tools to investigate the behaviour of carbon ions for pancreas patient treatment at a synchrotron based ion beam facility were developed. PO-1754 Planning study investigating intrafraction motion for prostate SBRT with focal lesion boost M. Trainer 1 , B. Nailon 1 , L. Carruthers 1 , M. Kirby 2 1 Edinburgh Cancer Centre, Oncology Physics, Edinburgh, United Kingdom; 2 Liverpool University, Directorate of Radiotherapy, Liverpool, United Kingdom Purpose or Objective To investigate whether clinically acceptable dose escalated SBRT plans could be created for patients enrolled in the PRINToUT trial (https://clinicaltrials.gov/ct2/show/NCT04081428) and whether the RayPilot tracking system could be used in their treatment. Materials and Methods A retrospective planning study (arm A) was carried out on seven patients treated within the PRINToUT clinical trial (reference). Plans were created on the Eclipse treatment planning system (v13.6), aiming to deliver 36.25Gy to the prostate and to boost the dose to the dominant intraprostatic lesion (DIL) to 45Gy. Positional displacement data for each patient using CBCT images[1] and the RayPilot tracking system[2] respectively were available from previous studies. Additional planning calculations were carried out to simulate the dosimetric impact of intrafraction motion based on CBCT imaging data (Arm B) and the RayPilot positional data (Arm C). This was done by creating an individual plan for each fraction and displacing the isocentre using the positional data from the respective studies and summing the dose on all the fractions. Results In arm A, the PTV, CTV and PTV(boost) dose targets were within the trial objectives for all plans. The bladder dose constraints were met for all patients in the study. The rectum V36Gy exceeded the tolerance of 2cc for three out of the seven patients and only two patients met all of the rectum dose constraints. In arm B the displacement from the CBCT data led to the PTV dose targets for only four out of seven patients being met, CTV dose targets for six out of seven patients and the PTV(boost) targets for only one out of seven patients. In arm C implementing the displacements from the RayPilot data led to the target and OAR doses being comparable to the original plans for all patients. Table 1: Dose statistics for Arm C of the planning study

Fig 1: Graph comparing absolute difference in percentage dose to Arm A and Arm C