ESTRO 38 Abstract book

S1015 ESTRO 38

Material and Methods Gammex RMI®465 CT Electron Density Phantom (Gammex Inc.) with titanium insert was used to get SEMAR-off and SEMAR-on HU-Electron Density (HU-ED) calibration curves for both voltages: 120 kVp and 135 kVp. 8 patients with metallic prosthesis, divided in 2 groups (limbs, prostate), were selected to the compare following dose calculation conditions (Anisotropic Analytical Algorithm 13.5, VMS): Condition 1 : SEMAR off CT // SEMAR off UH-ED (clinical use) Condition 2 : SEMAR on CT // SEMAR off UH-ED Condition 3 : SEMAR on CT // SEMAR on UH-ED Monitor Units (MU) resulting from the condition 1 were used for each calculation. Mean, minimal and maximal PTV dose were recorded. Results were expressed as prescription percentage. HU-ED curve impact Dose Comparison between conditions 2 and 3. UH modification impact Dose Comparison between conditions 1 and 2. For VMAT prostate patients, results from inverse planning (Photon Optimizer 13.5,VMS) with SEMAR on CT // SEMAR off UH-ED were also compared to the reference plan (clinical use). For identical PTV coverage, MUs and global Gamma index passing rates with [2% / 2 mm criteria] (Portal Dose Image Prediction 13.5, VMS) were compared. Results The difference between SEMAR off and SEMAR on is smaller than the difference between 120 kV and 135 kV regarding the UH value of Titanium. No differences were recorded around pelvis densities. HU-ED curve impact Adapting the UH-ED did not modify the average mean dose calculated over the 8 patients. UH modification impact Mean PTV dose increased by 0.5% for the 8 patients. Concerning limbs group, the minimal dose was strongly impacted (+1.9%). Prostate cases for which beams do not enter through metal, did not show such differences even if higher dose indices were recorded comparing to clinical use. Results from optimizing on SEMAR on followed the same trend found with dose indices, with fewer MUs needed to get identical coverage. Gamma passing rates remained in our tolerance.

Malpighi Hospital, Medical Physics Unit, Bologna, Italy ; 4 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S. Orsola- Malpighi Hospital, Radiation Oncology Department, Bologna, Italy ; 5 Fondazione Policlinico Universitario A. Gemelli - Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy Purpose or Objective Treatment plans for head-neck cancer are highly complex due to large irregular shaped target volumes, multiple dose prescription levels and to several OARs close to the target. We assessed the performance of the Auto-Planning module present in the Pinnacle TPS (version 16.0), comparing automatically generated VMAT plans (AP) with the historically clinically accepted manually-generated ones (MP) for head-neck cancer patients. Material and Methods Twelve consecutive patients treated with VMAT-SIB for bilateral head-neck cancer were re-planned with the Auto-Planning engine. The PTV1 included the primary tumor, PTV2 and PTV3 included the high-risk and low-risk lymph-nodal areas, respectively. PTV1, PTV2 and PTV3 were simultaneously irradiated over 30 daily fractions at 67.5Gy, 60.0Gy and 55.5 Gy, respectively. All manually (MP) and automatically (AP) generated plans were created by means of the 'dual arc” feature. For the MP plans, additional non-anatomical structures needed to be delineated in order to interactively guide the optimization process. For AP plans, a progressive optimization algorithm is used to continually adjust initial targets/OARs objectives; tuning structures are automatically added during optimization to increase the dose fall-off outside targets and improve the dose conformity. Various dose and dose-volume metrics (D98%, D95%, D50%, D2%, Dmean, V95% for target volumes; Dmean, Dmax and various Vx% for OARs), as well as conformity (CI) indexes and healthy- tissue integral dose (ID) were evaluated. A Wilcoxon paired test was performed for plan comparison with statistical significance set at p<0.05. Results Differences in all dose coverage metrics (in terms of V95%, D98%, D50%, D2% and Dmean) for all PTVs were not statistically significant (p<0.05). AP plans reported a better CI for PTV3 (MP:1.43 vs. AP:1.35, p=0.01). No significant differences in maximum doses were found for eyes, lens and optic chiasm. AP plans reduced maximum doses to PRV spinal cord and brainstem by 1.3Gy (p=0.04) and 4.3Gy (p=0.02), respectively, and mean dose for parotids by 3.4Gy (p=0.02). In addition, AP plans provided a significant decrease in Integral Dose of 6.6%. The mean number of MUs was higher for AP (586 vs. 451, p=0.01), suggesting an increased degree of fluence modulation. Conclusion The Pinnacle Auto-Planning module was able to produce highly consistent treatment plans for this complex anatomical sites. The working time was substantially reduced with Auto-Planning. EP-1870 Dosimetric assessment of metal artefact corrected CT images use for pelvis treatment planning E. Jaegle 1 , E. Cordier 2 , M. Alayrach 1 , A. Badey 1 , V. Bodez 1 , C. Khamphan 1 , P. Martinez 1 , R. Garcia 1 1 Institut Sainte Catherine, Physique, Avignon, France ; 2 Institut National des Sciences et Techniques Nucléaires, Medical physics, Gif sur Yvette, France Purpose or Objective Single Energy Metal Artefact Reduction algorithm (SEMAR, Canon Medical Systems) provides modified Hounsfield Unit (UH) Computed Tomography (CT) images by correcting artefacts related to photon starvation induced by metallic prostheses. The aim of this study is to assess the dosimetric impact of using such images for treatment planning with Eclipse™ (Varian Medical Systems VMS).