ESTRO 2021 Abstract Book

S1392

ESTRO 2021

pillow for patient comforts. Second, the patients were simulated with semi-immobilization setup (Group B) using All in Board (AIO) baseplate, Neutral neck rest (NNR), and knee rest. Third, the patients were simulated with immobilization devices (Group C) using AIO baseplate, and either vaclok or 4-Clamp TA cast, with NNR. In all three groups, the patients position was kept head first supine and arms above the head. As per the departmental image guided radiotherapy (IGRT) protocol for pre-treatment set-up verification, KV- cone beam computed tomography (CBCT) was acquired for the first three days consecutively. Once weekly kV-CBCT imaging was performed during the rest of treatment. The patient PTV position was matched with digitally reconstructed radiograph (DRR) either by auto-matching or by manual matching or both as per physician discretion. Mean translational displacements, systematic, and random errors of the study population for matching at three levels were calculated. Finally, CTV to PTV margins were calculated by a very famous margin recipe. Results A total of 75 pelvic radiotherapy patients and their 600 KV-CBCT images were considered for analysis. The Systematic error, random error and calculated CTV to PTV margins is tabulated in Table1. The calculated CTV to PTV margin was observed less for Group B patients in Anterior-posterior (AP) direction, in M

edio- lateral (ML) direction and in Cranio-caudal (CC) direction compared to Group A patients. However, Group B setup error was observed more in AP and ML direction compared to Group C patients. The largest CTV to PTV margin was calculated 12.46 mm in CC direction for Group C patients. Conclusion To balance the largest setup error in all three directions, and enhance the setup reproducibility as well as patient comfort, we recommend a semi-immobilization setup for Pelvic radiotherapy patients. The same is compatible to reduce the normal tissue complications, and it saves the patients setup time throughout the treatment. PO-1669 Commissionning of iterative model reconstruction (IMR) on Philips BigBore CT Scanner G. Bolard 1 1 Hôpital de La Tour, Radiation oncology, Meyrin, Switzerland Purpose or Objective Purpose of this work is to evaluate the benefit in the scope of radiation therapy planning of IMR, a model- based iterative reconstruction algorithm newly released by Philips for Big Bore CT version 4.8 with the promise of significative noise reduction. Image quality improvements were assessed for both standard and 4D reconstructions. Materials and Methods Spatial resolution, noise and low contrast detectability were measured at 120 KV using a Catphan 604 phantom for the three levels of noise reduction (1, 2, 3) and the two image definitions (Soft Tissue, Routine) offered by IMR (FOV 26cm, slice width 2mm, pitch 0.813). These metrics were compared to the current first generation iterative algorithm used clinically (iDose). Standard deviation a in 20mm diameter circular ROI was used as noise indicator while MTF at 50% and 10% were calculated for spatial resolution assessment. The largest low contrast rod (nominal 1.0%) was used for contrast calculation. HU to relative electron density stability was evaluated using a CIRS model 062 phantom. For 4D acquisitions (helical acquisition at low pitch), the same Catphan phantom was moving using a dynamic platform during X-ray following a periodic cos4 pattern (20mm amplitude, period 4s) in the superior-inferior direction. Results IMR exhibits higher spatial resolution and better low contrast detectability than iDose for the same exposure. Spatial resolution is nearly independent of noise level reduction and exposure and is respectively 1.35 and 1.1- times higher than iDose (level 4, filter B) for image definition routine and soft tissue. IMR1 show similar noise level than iDose (level 4) while IMR2 and 3 provide respectively 33% and 59% noise reduction in average leading to 1.5 and 2.4-times higher contrast. The benefit of this contrast restitution relative to iDose increases while slice thickness decreases (1.36 at 3mm slice thickness and 1.63 at 1mm).A similar low contrast detectability and an improved spatial resolution is achievable with a dose reduction factor of 4 with IMR2