ESTRO 2020 Abstract book

S970 ESTRO 2020

The sigmoid function was fitted to the reconstructed activity distal fall-offs and the difference between the reference position and the positions corresponding to proton range variation of +/-1, +/-2, +/-3 mm was computed.

Purpose or Objective The first aim of this study was to verify a quality of anthropomorphic phantom with hip prosthesis CT scans using cone beam CT and new reconstruction algorithm. The second aim was to verify the possibility of using CBCT scans for adaptive radiotherapy. Material and Methods The ability of new reconstruction algorithm iCBCT implemented by Varian in TrueBeam machine to improve quality of medical images gives a new opportunity for many clinically applications like adaptive radiotherapy. The new algorithm significantly reduces noise produced from scattered radiation during cone beam computed tomography (CBCT). Furthermore new algorithm has ability to reduce metal artifact coming from for example hip prosthesis. All measurements was performed with homemade anthropomorphic phantom of pelvic area with printed model of pelvic bone (Figure 1). First, scans of phantom on computed tomography (Somatom Definition AS, SIEMENS) was performed and a reference VMAT plan was prepared. To evaluate iCBCT algorithm as a solution for adaptive radiotherapy a set of scans with different reconstruction method and parameters was performed. Finally eight types of reconstructions were tested: standard, iCBCT, FAST iCBCT (with the lowest parameters of reconstruction) and High iCBCT with the highest parameters. Same four reconstructions was made for Pelvis and Pelvis Large scan parameters (which differ in parameters). For all scans a treatment plan created on reference scans from CT (Somatom Definition AS, SIEMENS) was transferred. A comparison of treatment plans, based on selected parameters, has been performed. For all dose calculation dedicated calibration curves was used.

Results The difference between the distal fall-off position for the reference proton beam energy and fall-offs corresponding to proton range shifts are listed in Table 1. Increasing/decreasing proton beam range is reflected in a shift of reconstructed activity profile distal fall-off. The preliminary results indicate that J-PET can detect relative range variation of proton pencil beam of about 2 mm. The presented results are burdened with statistical error of MC simulations and fit error which are not indicated.

Results All sets of phantom scans were analyzed. For water homogeneous region mean HU values were similar but significant differences were observed between maximal and minimal HU values. Scans reconstructed with standard algorithm had almost 200 HU difference for comparison CT and iCBCT scans had up to 90 HU difference. For dose calculation mean value of dose in PTV, bladder, rectum and femoral head also were similar. Significant differences were observed during dose distributions analysis performed in VeriSoft software (PTW) which showed point dose difference to a maximum 0.85 Gy (Figure 2). Calculations perform on images without any iCBCT reconstruction algorithm could led to overestimated calculated dose.

Conclusion The simulation results show that all presented configurations based on J-PET detector are feasible to acquire the β+ activity produced by therapeutic proton beams in the PMMA phantom which are sufficient for the 3D reconstruction of PET activity distributions using CASToR. Observed differences between the reconstructed activity profile distal fall-offs show feasibility of the J-PET technology to detect proton beam range variations of about 2 mm. The future activities include simulations of J- PET sensitivity to detect proton beam range variations in homogenous and heterogenous phantoms exposed to complex irradiation fields and in patients as well as experimental validation of the approach. PO-1672 Verification of the new reconstruction algorithm iCBCT for hip prosthesis adaptive planning B. Pawalowski 1,2 , H. Szweda 1 , A. Jodda 1 , U. Sobocka- Kurdyk 3 , D. Radomiak 1 , K. Matuszewski 1 , M. Kruszyna- Mochalska 1,4 , T. Piotrowski 1,4 1 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland ; 2 Poznan University of Technology, Faculty of Technical Physics, Poznan, Poland ; 3 Greater Poland Cancer Centre, Medical Physics Department, Kalisz, Poland ; 4 University of Medical Science, Department of Electroradiology, Poznan, Poland