Abstract Book

ESTRO 37

S471

3 Northwestern Medicine Chicago Proton Center, Medical Physics, Warrenville- Illinois- 60555, USA 4 Northern Illinois University, Department of Physics, DeKalb- Illinois- 60115, USA 5 University of California- Santa Cruz, Physics Department, Santa Cruz- California- 95064, USA 6 Loma Linda University, Division of Bioengineering Sciences- Department of Basic Sciences, Loma Linda- California- 92350, USA Purpose or Objective The goal of this ongoing work is to compare proton treatment planning based on proton CT (pCT) and x-ray- CT. Proton CT is mostly immune to artifacts from metal dental implants often present in patients with head and neck cancer. Here we report on initial results of the planning accuracy in the presence of titanium (Ti) dental implants using an anthropomorphic head phantom. Material and Methods Two dental implants manufactured from Ti were inserted into premolar and molar teeth of an Alderson head phantom (Alderson®, Radiology Support Devices, Long Beach, USA). The phantom was scanned with the pCT Phase II scanner developed by the pCT collaboration [1] and the standard x-ray CT used for proton treatment planning at the Northwestern Medicine Proton Center (NMCPC). These scans were imported into the clinical version of the RayStation® (RaySearch Laboratories, Stockholm, Sweden) proton treatment planning system (TPS), which uses a Monte-Carlo (MC)-based algorithm. Implant-generated artefacts visible in the x-ray CT with Ti implants but not in the pCT, were corrected by the treatment planner. The TPS conversion process of Hounsfield unites (HU) to materials and material densities was adapted for relative stopping power values provided by pCT. A planning target volume was created, simulating a tumour containing the two implants close to isocenter. A single-field- uniform-dose (SFUD) plan was calculated on the x-ray CT resulting in an x-ray CT-based dose distribution (xCT-DD). The fluence map of the plan was imported into the pCT study, resulting in a pCT-based dose distribution (pCT-DD). The proton fluence map of the plan was then delivered to the phantom at NMCPC. The delivered dose distribution (dDD) was measured with a Gafchromic EBT2 film (Ashland Inc., New York, USA) inserted between the phantom layers containing the Ti implants. The dDD was compared to the planned pCT-DD and xCT-DD by performing a gamma-analysis. Results Fig. 1 shows comparison of delivered dDD (black lines) to the pCT-DD and xCT-DD (red lines), respectively. Table 1 reports the results of the gamma analysis for different isodose lines. The pCT plan had a pass rate between 79% and 94%, whereas the x-ray CT plan had a lower pass rate between 42% and 84%. The pass rates were particularly small in the region of the distal fall-off of the SFUD.

Figure 1. Comparison of x-ray CT and pCT plans with the delivered dose distribution. Conclusion The artifact-free image generated by the pCT method combined with an MC based TPS resulted in a SFUD DD that was more accurate than the DD planned with x-ray CT, despite an attempt to correct for CT artefacts. Proton CT has the potential to reduce range uncertainty in the presence of high-density dental implants. [1] R.P. Johnson, et al., A Fast Experimental Scanner for Proton CT: Technical Performance and First Experience with Phantom Scans, IEEE Trans. Nucl. Sci. 63-1 (2015) PO-0889 Validation of transit EPID and application for Head & Neck adaptive radiotherapy N. DELABY 1 , J. Bouvier 1 , S. Sorel 1 , F. Jouyaux 1 , E. Chajon 1 , J. Castelli 1,2,3 , A. Barateau 2,3 , C. Lafond 1,2,3 1 Centre Eugène Marquis, Ille et Vilaine, Rennes CEDEX, France 2 Inserm, U1099, Rennes, France 3 Université Rennes 1, LTSI, Rennes, France Purpose or Objective EPID based transit dosimetry is an efficient method to assure safe dose delivery during radiation therapy. It could be a useful adaptive radiotherapy (ART) tool to help in treatment replanning decision. The aim of this study was to evaluate machine and patient’s error detection capability of the new Elekta’s transit dosimetry system and to evaluate its performance for Head and The study was performed on a VersaHD LINAC equipped with Agility collimator and EPID iView (Elekta). iViewDose v1.0.1 (Elekta) software reconstructs delivered dose in the total volume of the planning CT scan using EPID frames. VMAT treatment plans were calculated with Pinnacle v9.10 (Philips). Detector validation focused on : i) Intrinsic characteristics evaluation (repeatability, reproducibility, linearity, dose rate dependence) ; ii) Evaluation of error detectability for H&N plans on an anthropomorphic phantom with several leaves shift (1 to 5 mm), collimator rotation (1 to 5°) and MU increase (1 to 5%). The in vivo agreement between planned and reconstructed dose was investigated for 26 H&N patients (4 patients enrolled in a weekly re-planning ART process, and 22 patients without ART), for a total of 470 fractions. According to the results, a parallel qualitative analysis on CBCT images was performed to trigger or not a re- planning. Agreement between planned and reconstructed dose was evaluated with gamma index analysis in 3D (γ 3D , criteria: 3% global /3mm, threshold 50%). Neck (H&N) treatments. Material and Methods

Table 1. Gamma-analysis (3mm/3%). Isodose% 30 50 70 80 90 >90 x-ray CT 59.6 42.2 53.6 73.0 83.5 62.0 pCT 93.7 80.5 90.9 80.4 79.8 78.7