Abstract book - ESTRO meets Asia

S30 ESTRO meets Asia 2018

as a ~6% dose variation for radiation fields employing a 60° physical wedge. Three affected clinics conducted two- year historical reviews of patient treatments. An investigation was conducted into under-dose behind low density material related to the anisotropic analytic algorithm (AAA) implemented by a specific manufacturer. Clinics can refer to the national data when decision making about a technological upgrade. For example; the replacement of wedged fields with more advance techniques, or in upgrading from the AAA algorithm to a newer calculation engine such as ACUROS (National data for 4 cases in the Level III end-to-end audit are shown in Figure 1).

0.05 deviations in relative ED and a dose difference of less than 2%. Three CBCT image sets captured consecutively also showed excellent short-term reproducibility with maximum deviation of 6 HU and relative ED difference of 0.01. The CBCT also exhibited good long-term stability over 31 days with maximum deviation of 18 HU and maximum relative ED difference of 0.02. Conclusion The CBCTs investigated showed acceptable agreement with conventional CT simulator, low variability between different units of CBCT of the same model, and excellent short-term reproducibility and long-term stability. Therefore, these CBCTs were found to be suitable for dose calculation. OC-078 Re-irradiation with SBRT for pancreatic cancer: dose summation and toxicity X. Zhu 1 , Y. Cao 1 , F. Cao 1 , S. Qing 1 , Y. Shen 1 , L. Gu 1 , F. Fang 1 , Z. Jia 1 , X. Ju 1 , H. Zhang 1 1 Changhai Hospital, Radiation Oncology, Shanghai, China Purpose or Objective A significant number of patients with pancreatic cancer may develop local recurrence after treatment. These patients may not be amenable for surgery due to its high complication rates. In addition to second-line chemotherapy regimens, re-irradiation with stereotactic body radiation therapy (SBRT) may be an alternative, which has been used in few studies. However, the techniques were conventional radiotherapy prior to re- irradiation with SBRT in previous studies and the dose constraints of organs at risk (OAR) were never reported. Therefore, the goal of the study was to determine the cumulative dose-volume parameters to OAR after two courses of SBRT. Material and Methods All patients received two courses of SBRT for the same region with a partial or complete overlap of two previous dose distributions were enrolled. The maximum dose of OAR was calculated as 50% more than the normal constraint. Secondly, we allowed a dose reduction of 50% for a re-irradiation 12 months after the last radiation. A dose reduction of 25% was allowed for a re-irradiation after 6–12 months. No dose reduction was used when re- irradiation was done within 6 months. Due to different doses to target regions and OAR and fractionation schemes, all treatment schedules were recalculated to an Equivalent Dose of 2 Gy per fraction (EQD 2 ). An α/β value of 10 Gy (Gy 10 ) was employed for the tumor dose and acute effects, and the value determined as 3 Gy (Gy 3 ) concerning late effects. Dose distributions, structures sets and CT scans of two treatment plans were extracted from Multiplan ® System (version: 4.0.2) and sent to MIM ® System (version: 6.6.8) for analysis. Firstly, two CT scans were aligned rigidly via automatic bone matches (translation and rotations). Therefore, for each plan before summation, each of the contoured OARs was registered rigidly. Subsequently, a non-rigid registration was followed in dose summations. After non-rigid registration, the dose distributions of the first plan were projected to the second treatment with both of doses converted to EQD 2 , which were summed up finally. Results A total of 41 patients were identified. The median accumulated D max , D 1 and V 10 of the stomach, duodenum and the bowel were 36.75Gy 3 , 27.87 Gy 3 and 66.41cc; 30.36 Gy 3 , 22.13 Gy 3 and 18.66cc; 35.76 Gy 3 , 28.06 Gy 3 and 119.48cc, respectively. The median accumulated D max of the spinal cord was 6.42 Gy 3 . The median cumulative D mean and D 2/3 of the left and right kidney were 4.62 Gy 3 and 3.03 Gy 3 ; 2.67 Gy 3 and 2.10 Gy 3 , respectively. The median cumulative D mean and D 50% of the liver was 4.28 Gy 3 and 3.03 Gy 3 , respectively. The median summed dose to the radiation field was 93.38 Gy 3 . No grade 3-4 toxicity

Figure 1 An example comparison of a clinic using the AAA algorithm verses a clinic using the ACUROS algorithm against the national data in the ACDS level III end-to-end audit. Four 3DCRT treatments are shown using the ACDS Thorax phantom.

Conclusion The ACDS has found that this presentation of national benchmarking is a highly effective way to communicate, visualise and contextualise audit results. OC-077 Calibration and characterization of CBCT for dose calculation Y.I. Tan 1 , Y.F. Leong 1 , J.F. Loria Cruz 1 , S. Baggarley 1 1 National Cancer Institute of Singapore, Radiation Therapy Centre, Singapore, Singapore Purpose or Objective Cone beam computed tomography (CBCT) is routinely used for patient positioning in radiation therapy and could potentially be utilised for dose calculation. The aim of this study is to calibrate and characterize CBCT for dose calculations. Material and Methods To calibrate the CBCT, a phantom containing rods of different densities (air to Teflon) was used to establish CT number to relative electron density (CTED) curves for CBCT (CTED CBCT ) and compared to simulation CT (CTED CT ). For characterisation, we investigated the: (i) variability of different CBCT units, (ii) short-term reproducibility of CBCT, and (iii) long-term stability of CBCT. Results Agreement between CTED CBCT and CTED CT were good with a maximum deviation of 67 HU and a maximum relative electron density (ED) deviation of 0.04. Comparisons between volumetric arc therapy (VMAT) prostate plan calculated on CBCT image set assigned with CTED CBCT and CTED CT showed dose differences within 1% for target (PTV D95% ) and organs at risk (Rectum V40Gy , Bladder V30Gy ). Variability between different CBCT units was acceptable. The maximum difference was 224 HU, which translates to